- Open Access
Expression of the disease on female carriers of X-linked lysosomal disorders: a brief review
© Pinto et al; licensee BioMed Central Ltd. 2010
Received: 10 August 2009
Accepted: 28 May 2010
Published: 28 May 2010
Most lysosomal diseases (LD) are inherited as autosomal recessive traits, but two important conditions have X-linked inheritance: Fabry disease and Mucopolysaccharidosis II (MPS II). These two diseases show a very different pattern regarding expression on heterozygotes, which does not seem to be explained by the X-inactivation mechanism only. While MPS II heterozygotes are asymptomatic in most instances, in Fabry disease most of female carriers show some disease manifestation, which is sometimes severe. It is known that there is a major difference among X-linked diseases depending on the cell autonomy of the gene product involved and, therefore, on the occurrence of cross-correction. Since lysosomal enzymes are usually secreted and uptaken by neighbor cells, the different findings between MPS II and Fabry disease heterozygotes can also be due to different efficiency of cross-correction (higher in MPS II and lower in Fabry disease). In this paper, we review these two X-linked LD in order to discuss the mechanisms that could explain the different rates of penetrance and expressivity observed in the heterozygotes; this could be helpful to better understand the expression of X-linked traits.
Classification of LSDs (adapted from Platt and Walkley, 2004 )
Primary lysosomal hydrolase defect
Post-translational processing defect of lysosomal enzymes
Multiple sulphatase deficiency
Multiple sulphatase deficiency
Trafficking defect for lysosomal enzymes
Mucolipidosis type II
Defect in lysosomal enzyme protection
β-Galactosidase and Neuraminidase deficiency
Defect insoluble non-enzymatic lysosomal proteins
Hexosaminidase activator deficiency
GM2 type AB
(Tay-Sachs disease variant AB)
Transmembrane (non-enzyme) protein defect
Lysosomal-associated membrane protein 2
Intracellular accumulation of autofluorescent of lipopigments storage material
Neuronal ceroid lipofuscinoses (CLN4)
Among the over 300 human X-linked diseases described so far, only three are LD: Fabry disease (MIM 301500), MPSII II (MIM 30900), and Danon disease (MIM 300257). This review will focus on the clinical heterogeneity found among heterozygotes of the two most frequent conditions: Fabry disease and MPS II.
Fabry disease is a rare X-linked lysosomal inborn error of glycosphingolipid catabolism which results from the deficient activity of lysosmal hidrolase α galactosidase A (α-GAL; EC 220.127.116.11). The estimated incidence of this rare disease is 1:40,000-117,000 live male births [4, 7, 8]. This figure, however, may be underestimated, as screening performed in newborn males in a Northwestern Italian region showed an incidence of 1 in ~4,000 males . The α-galactosidase gene (GLA 300644) is located in Xq22.1 and spans 12 kb of DNA comprising 7 exons . More than 400 mutations in the GLA gene have been described in Fabry patients, most of them private [10–12]. The enzyme deficiency leads to a progressive accumulation of globotriaosylceramide (Gb3) and deacylated globotriaosylsphingosine (lyso-Gb3).
The prevalence of females carrying the Fabry mutation was estimated as 1:339,000 in the UK . Heterozygote females were usually described as asymptomatic or as exhibiting intermittent and mild symptoms of the disease [7, 8]. The most common signs and symptoms in Fabry disease are shown in Table S1, Additional file 1[8, 14–19]. Deegan et al.  showed that 70% of the heterozygote females may display signs and/or symptoms. The disease in females can be as severe as in male patients [7, 20, 21]; however, a slow rate of progression and a high phenotypic variability is more common [14, 22]. Life expectancy is reduced in about 20 years in heterozygous females .
The measurement of the α-GAL activity does not always allow the identification of the heterozygotes [8, 23], and it is believed that there is no correlation between α-GAL activity (in plasma or leukocytes), genotype and the severity of the clinical picture presented by heterozygotes . However, some authors stated that females with classic Fabry disease usually have lower α-GAL activity and increased urinary Gb3 levels . On women suspected of presenting Fabry disease due to symptomatology or findings on a tissue biopsy but with a negative family history, DNA analysis is often required for the definitive diagnosis [15, 25]. On the other hand, Wilcox et al.  showed that leukocyte α-GAL activity can point to diagnosis in 88% of females; this suggests that used in conjunction with urinary Gb3 measurement  this test may be an efficient way to screen females at high risk to be carriers.
Mucopolysaccharidosis type II
The incidence of MPS II is estimated between 1:110,000 and 132,500 live births [27, 28]. MPS II is an X-linked recessive inborn error of metabolism that results from the deficiency of lysosomal iduronate-2-sulfatase (IDS; EC 18.104.22.168). The impaired IDS function leads to the storage of glycosaminoglicans (GAGs) in many organs and tissues. The partially degraded GAGs are heparan sulfate and dermatan sulfate, which are present in increased concentrations in the urine of patients. The IDS gene was mapped to Xq28 and contains 9 exons that spread over 24 kb [29, 30]. More than 300 mutations have been described . And there is also a pseudogene located 20 kb from the active gene .
In MPS II, the clinical disease in females is very rare, and most of the cases reported in the literature have the severe phenotype [35–43] (Table S2, Additional file 2). At least ten female cases have already been described .
In most cases, the identification of heterozygote females with MPS II by the measurement of the IDS activity is not reliable [32, 44, 45]. It is suggested that the lower the value of IDS, the higher the probability that these women will be a heterozygote . Unfortunately, GAGs measurement in the urine is not helpful to identify carriers either . Usually, the only way to reach the definitive diagnosis is through genetic analysis .
X-linked inheritance was first described by Morgan [48, 49], and colorblindness was one of the first diseases to be recognized as having this pattern of inheritance . The classic X-linked disorder usually shows a vertical transmission in which heterozygote females transmit the allele down to her sons and daughters; daughters of affected males are always heterozygotes; sons of affected males are always normal. Traditionally, these diseases are described as dominant if they are expressed both in heterozygotes and hemizygotes, or as recessive if they are expressed predominantly or almost exclusively in males. A subgroup of X-linked dominant disorders includes those characterized by male lethality or reduced male-viability (X-linked dominant male-lethal disorders) while semidominant X-linked diseases would occur when the heterozygotes present a phenotype intermediate to the phenotype of affected hemyzygotes and normal homozygotes .
It is true that the vast majority of mutations in X-linked genes will lead to diseases only in males . However, many of the X-linked diseases show different rates of penetrance and expressivity in both genders. The following situations can be observed: 1) for some conditions, such as Fabry disease, heterozygotes are usually affected but tend to have a milder and more variable phenotype than hemizygotes (e.g., semidominant inheritance); 2) for other diseases, as MPS II, penetrance in heterozygotes is very low and, consequently, very few heterozygotes are affected (e.g., recessive inheritance); 3) on the other hand, as in Vitamin D resistant rickets, penetrance and severity are high in both genders (e.g., dominant inheritance); 4) finally, as in Rett syndrome, penetrance is high in both genders, but males usually die very early (intrauterus), and only females are usually affected at birth (e.g., lethal dominant inheritance) [21, 48, 52].
Classification of X-linked disorders
Comparison between Drosophila and humans regarding the compensation-dose mechanism (based on Dobyns et al., 2004 )
6A+XX and 6A+XY
46, XX and XY
Dosage compensation in X chromosome
Transcribing rate of the X chromosome
Type of gene product
Positive cellular selection (gene product causing early cell death)
Clinical Phenotype in Heterozygotes
Probably X-inactivation Pattern
Random or skewed favorable to the normal allele
Random or skewed unfavorable to the normal allele4
Non-cell-autonomous1 or functionally non-autonomous2
Random or skewed favorable to the normal allele
Skewed favorable to the abnormal allele
Twinning and X-Linked Diseases
Discordant phenotypes in monozygous twin females (MZF), who were heterozygous for X-linked disorders, have already been reported for many diseases, including Duchenne Muscular Dystrophy (DMD), G6PD deficiency, Haemophilia B, Fragile-X syndrome, Green color deficiency, MPS II and Fabry disease [39, 54–60]. Clinical expression of the X-linked diseases in one of the MZF seems to be proportionally more frequent than the expression in heterozygote females. Also, it seems that concordant expression between MZF heterozygotes has never been reported .
Three female Fabry twin pairs with discordant phenotype were described in the literature. Only one MZF pair was confirmed regarding monozigosity. At the age of 26 years, the affected MZF presented severe signs and symptoms: renal proteinuria, angiokeratotic lesions in the pelvic region and thighs, verticillate cornea, and acroparesthesia in all four limbs. Both MZF were heterozygous for the G10182A mutation (exon 5). The phenotype was attributed to the skewed X-inactivation favoring the GLA allele mutant in the affected MZF, showing an opposite direction in the normal MZF [54–56]. For female MZF MPS II twins, only one case has been described in the literature, and the affected twin had a normal karyotype and skewed X-inactivation also favoring the IDS mutant allele .
Tiberio  reviewed some characteristics common to the X-linked diseases in MZF as follows: 1) karyotypes are normal; 2) one twin is severely affected and the other is spared; 3) intermediate phenotypes have not been described; 4) the X-inactivation pattern is opposite to skewing (abnormal allele inactivated in most cells of the normal twin, and the normal allele inactivated in most cells of the affected twin), and 5) the disease maps into regions Xq27-28, Xp21, Xq22 and Xp22. Due to these observations three hypotheses came up to explain the discordant phenotype in these cases: 1) casual aggregation of cells carrying the same inactive chromosome that predisposes twining ; 2) a "sampling effect" through early symmetric or asymmetric splitting of the inner cell mass occurring after the X-inactivation [61, 62], and 3) early mitotic crossing over, segregation of one crossed and one uncrossed chromatid in the daughter's cells, and obligate inactivation of the uncrossed chromatid . Unfortunately, it seems that none of these hypotheses can completely explain the MFZ discordant for X-linked diseases .
The X-inactivation occurs very early in embryonic life in the late blastocyst stage of embryogenesis [41, 63], and one possible explanation of the variable expression of an X-linked abnormality in MZF lies on the fact that X-inactivation precedes the twinning event [56, 57, 61, 64–66]. It was also observed that in MZF who are dichorionic, twinning occurs prior or around the onset of X-inactivation and, in monochorionic twins, twinning occurs later . So it might be expected that discordant phenotype should be more frequent in twins dichorionic; however, anedoctal cases of discordant X-linked diseases in dichorionic MZF have already been reported [39, 58]. A new explanation proposed by Monteiro et al.  is that monochorionic MZF is, in fact, a heterogeneous group that differs in the timing of the twinning event after the onset of X-inactivation .
There are many other possible explanations for discordant phenotypes between monozygotic twins: environmental causes, several embryologic mechanisms, vascular abnormalities, defect of midline structures and postzygotic changes in the genetic material, including somatic mutations .
X-Inactivation, Fabry Disease and MPS II
The main question to be answered is if skewed X-inactivation is the main mechanism responsible for the different rates of penetrance and expressivity presented by heterozygotes for Fabry and MPS II diseases (Table 3).
The X-inactivation was first described by Lyon [68, 69], who suggested that the X-inactivation was based on the pattern of patches in heterozygote female mice. This author proposed some general rules for X-inactivation: 1) normal females have just one active X chromosome; 2) X-inactivation occurs early in development; 3) the inactive X can be maternal or paternal, and the choice is random, and d) X-inactivation is irreversible in somatic cells and through all its descendants. Because females have two X chromosomes and males only one, the X-inactivation provides a mechanism called "compensation dose". As a result, females are mosaic .
The X-inactivation process is dependent on many factors and is tissue specific . It is initiated on the X-inactivation center (Xic) located at Xq13.2 . The X-inactivation specific transcript (Xist; MIM 314670) gene is responsible for the process of cis inactivation , and it is transcribed only in the inactive X chromosome. In Xic, it is also transcribed, in the antisense orientation, the X-inactive specific transcript-antisense (Tsix; MIM 300181) that has the function of regulating the early expression and the choice of which X chromosome will be inactivated . As the X-inactivation pattern is established, all descendents cells will have the same pattern. X-inactivation in somatic cells is very stable , permanent and irreversible, except in oogenesis and in spermatogenesis . The main steps of inactivation are: counting, choice and initiation . Actually, the first step is to maintain one of the active X chromosomes , and this is attributed to the existence of an autosomal limited factor . Many epigenetics modifications are necessary to guarantee the maintenance and the herdability of the X-inactivation . However, not all genes present in the X chromosome are inactive, and it is estimated that about 25% of the genes  escape from the inactivation, most of them located in the X chromosome short arm.
Lyonization follows the Gaussian distribution in the female population, and it is expected that females would be 50:50 for genes that underwent X-inactivation, but ratios 60:40 or 70:30 are the most common , and even 80:20 or 90:10 can be considered normal [51, 67]. However, it was shown that at least 5-10% of normal females present extreme skewing of X-inactivation , while some studies showed a range between 4% and 33% . Extreme skewed X- inactivation is defined as affecting more than 90% of one allele  and may be a consequence of the following: 1) biases in the choice of which X chromosome to inactivate; 2) X chromosome mutations or rearrangements which affect the viability of cells with one or the other active X, or 3) stochastic factors [73, 78]. Highly skewed X-inactivation, defined as more than 80:20, could be pathogenic .
Studies in peripheral blood cells show that skewing of inactivation increases with age, but the practical significance of this finding is uncertain. However, it is possible that skewing in an advanced age is correlated with the presence of a pathogenic allele in the X chromosome . The age-skewing process can be caused by stochastic clonal loss of hematopoietic cells  or the competitive advantage for hematopoietic stem cells with a specific genotype of X-linked genes .
Skewed X-inactivation and diseases
Traditionally, the main mechanisms that are said to influence the expression of the X-linked disorders are: skewed inactivation [43, 69, 85, 86], clonal expansion, and somatic mosaicism . Therefore, heterozygotes for X-linked diseases could be symptomatic if: 1) the mutation confers a proliferative advantage to the mutant cells, OR 2) there is skewed X-inactivation due to any reason (X/autosomal translocation, mutation in the XIST gene, etc), and the mutant is in the active X chromosome [36, 43, 87]. However, there are other explanations for the heterozygotes for an X-linked disease presenting with signs and symptoms, such as the occurrence of hemizygozity for the mutant allele (e.g., the patient is a carrier of the mutation but full X monosomy is present); the occurrence of uniparental disomy for the mutant X chromosome, and the absence (or deficiency) of cross-correction . Structural rearrangements involving the X chromosome, as X/autosomal translocations and deletions, could disrupt the normal allele and may cause loss of heterozygozity or contiguous gene syndromes. Several diseases, such as DMD, had their genes mapped to the X chromosome after patients with rearrangements involving the same X chromosome region were identified. In the case of MPS II, patients presenting deletions involving not only the IDS gene but also the FMR1 (involved in the X-fragile syndrome) have been described .
As previously mentioned, skewed X-inactivation has been shown to be responsible for clinical manifestations in female carriers of X-linked diseases, such as DMD . Skewed X-inactivation also seems to be involved in other situations such as recurrent pregnancy loss [89, 90] and breast cancer . Deviation from the random X-inactivation can be primary or secondary. Primary as a result of the deviation of X-inactivation itself, e.g., a mutation in XIST gene, or secondary if the non-random X-inactivation has a consequence like cell selection [22, 92].
It is possible to identify four X-inactivation patterns in heterozygotes for X-linked diseases as follows: 1) random inactivation is usually associated with a normal phenotype; 2) random inactivation leads to manifestations in a continuum spectrum, and a normal phenotype requires skewing, favoring the normal allele in the expressing tissue; 3) always extreme skewing, as mutant cells die or fail to develop, or migrate to destination; 4) gradual skewing because of cell selection due to a proliferative advantage of wild type (or mutant) cells expressing tissue .
Unbalanced X-inactivation can occur as a stochastic variation or due to genetic factors influencing the X-inactivation itself or the postinactivation selection mechanism . The first two are rare and occur independent of the kind of mutation in the gene of the disease [22, 67].
Fabry Disease: Explanations for the Female Phenotype
Symptomatic heterozygous females for Fabry disease can be as frequent as 60-70% [22, 93]. Sometimes, symptoms are as severe as in males [7, 93]. It has been suggested that X inactivation studies could be helpful in predicting the female phenotype in Fabry disease [11, 22]. For some authors [8, 15, 23] skewed X-inactivation plays an essential role in the phenotype expression in heterozygotes of Fabry disease. Dobrovolny et al.  showed that some females with preferential X-inactivation have more rapid disease progression and suggested that X-inactivation is the major factor for determining the severity of clinical involvement and morbidity in Fabry heterozygotes.
However, for other authors , the phenotype was not due to skewed X-inactivation, and severity was not correlated with deviation either. They concluded that the X-inactivation in leukocytes in females with Fabry disease is not useful to predict prognostic and should not be used to define therapeutic options. And disease progression was related to a cross-correction mechanism which would have a decreasing efficiency throughout the years, which would explain the fact that heterozygote females for Fabry disease get more symptomatic in time .
The cross-correction mechanism is also called metabolic cooperation. Lysosomal enzymes freely enter and leave lysosomes and are transferred from one cell to another by manose-6-phosphate-mediated endocytosis. When the enzyme is deficient, the non-digested products accumulate in the lysosome . In Fabry disease, either the amount of active enzyme secreted may be insufficient, or the secreted enzyme is not adequately taken up by normal cells, and this is not enough to cross-correct the deficient synthesis of the abnormal cells [16, 21, 87]. This last conclusion was suggested by the fact that the activity of the enzyme in plasma or leukocytes may not reflect the situation within the lysosome of relevant cell types of Fabry disease, which suggests that the uptake by normal cells may be defective [12, 16]. It seems that a distinct pattern of recaptation due to the mannose-6-phosphate receptors and the localization in the enzyme can be responsible for a lower-uptake enzyme, and, as a consequence, some females will manifest the disease . In the case of Pompe disease (α-glucosidase deficiency) a LD of autosomal recessive inheritance, abnormalities in the trafficking of intracellular mannose-6-phosphate receptors have been described. This would be the result of abnormal substrate storage, with impact on the internalization of the endogenous enzymes . According to Dobyns et al. cross-correction ability in Fabry disease would be largely restricted, as normal heterozygotes could present skewed X-inactivation in favour of the normal allele, and symptomatic heterozygotes may show random X-inactivation (Table 3). Another hypothesis is related to a new concept ("cross-induction"), for increased levels of plasma lyso-Gb3 (a deacylated form of Gb3) were found in symptomatic heterozygotes for Fabry disease and appear to be positively correlated with the severity of the clinical picture . If this is true, perhaps lyso-Gb3 is widely diffusible and has the capacity to inhibit α-GAL activity produced by cells which have an active non-mutated X-chromosome leading, therefore, to clinical symptomatology in most heterozygotes.
The following other factors affect expression in Fabry disease: 1) the nature of the mutation - and correlation with enzyme activity , and 2) the blood group (patients of blood groups B and AB are more severely affected than individuals of other groups , probably because these patients accumulate, in addition to globotriaosylceramie and galabiosylceramide, two further glycosphingolipids ).
MPS II Disease: explanations for female phenotype
Different from Fabry carriers, heterozygote MPS II females are spared from the disease. But why? It may be attributed to the cross-correction of cells (in which the active X is the one with the mutation) by the functional enzyme secreted by cells in which the non-mutant gene is active . Schwartz et al.  suggested that in some cells of MPS II carriers, like chondrocytes or hepatocytes, the X chromosome carrying the mutant allele would be preferentially inactivated or reduce cell viability. Consequently, MPS II heterozygous females are rarely affected unless there is a simultaneous presence of two mutant alleles or if a coincidental genetic defect happens, leading to skewed X-inactivation or hemizygozity in heterozygotes (Table 3). According to Dobyns et al.  as the cross-correction ability would be preserved in MPS II, the clinically normal heterozygotes would present randomic X-inactivation or skewed X-inactivation with predominant expression of the normal X chromossome, while the symptomatic heterozygotes would present skewed X-inactivation with predominant expression of the mutated allele (Table 3). These findings are in accordance with the literature, as shown in Table S2, Additional file 2.
Concluding remarks and the future
Most females are mosaics and have a mixture of cells expressing either their mother's or father's X-linked genes. Often, cell mosaicism is advantageous, for it ameliorates the deleterious effects of X-linked mutations and contributes to physiological diversity. Although sometimes mosaicism brings a significant biological advantage, the outcome is never certain .
It is interesting that, although Fabry disease and MPS II are both X-linked LD, they show many differences in clinical expression of heterozygous females. The main differences, at the moment, could be explained by: 1) cross-correction or cross-inducing mechanisms; 2) the skewed X-inactivation .
As proposed by Dobyns et al. , the X-linked diseases show a continuum in penetrance, and new rules must be established to explain the various phenotypes observed. Despite the fact that over 30 years have passed since the discovery of the existence of the X-inactivation mechanism, we still have a lot to learn, and new information about cell biology is still needed to allow us to provide a more precise genetic counseling to affected families.
Written informed consent was obtained from the patient for publication and images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
The authors of this work are grateful for the fellowships provided by CAPES through the Postgraduate Program in Child and Adolescent Medicine, UFRGS (LLCP), CNPq (RG and IVDS), and FIPE.
- Wraith JE: Lysosomal disorders. Semin Neonatol. 2002, 7: 75-83. 10.1053/siny.2001.0088.View ArticlePubMedGoogle Scholar
- Platt FM, Walkley SU: Lysosomal Disorders of the Brain. Great Britain: Oxford Univerity Press; 2004.View ArticleGoogle Scholar
- Meikle PJ, Hopwood JJ: Lysosomal storage disorders: emerging therapeutic options require early diagnosis. Eur J Pediatr. 2003, 162 (Suppl 1): S34-37. 10.1007/s00431-003-1348-y.View ArticlePubMedGoogle Scholar
- Meikle PJ, Hopwood JJ, Clague AE, Carey WF: Prevalence of lysosomal disorders. JAMA. 1999, 281: 249-254. 10.1001/jama.281.3.249.View ArticlePubMedGoogle Scholar
- Wraith JE: Advances in the treatment of lysosomal storage disease. Dev Med Child Neurol. 2001, 43: 639-646. 10.1017/S0012162201001165.View ArticlePubMedGoogle Scholar
- Ballabio A, Gieselmann V: Lysosomal disorders: from storage to cellular damage. Biochim Biophys Acta. 2009, 1793: 684-696. 10.1016/j.bbamcr.2008.12.001.View ArticlePubMedGoogle Scholar
- Gibas AL, Klatt R, Johson J, Clarke JTR, Katz J: Disease rarity carrier status and gender: a triple disadvantage for womem with Fabry disease. J Genet Couns. 2008, 17: 528-537. 10.1007/s10897-008-9179-7.View ArticlePubMedGoogle Scholar
- Desnick RJ, Ioannou YA, Eng CM: α galactosidase A deficiency: Fabry disease. The metabolic and molecular basis of inherited disease New York: Edited by: MacGraw-HillScriver CR, Beaudet AL, Sly WS, Valle D, 8 2001, III: 3773-3774.Google Scholar
- Spada M, Pagliardini S, Yasuda M, Tukel T, Thiagarajan G, Sakuraba H, Ponzone A, Desnick RJ: High incidence of later-onset Fabry disease revealed by newborn screening. Am J Hum Genet. 2006, 79: 31-40. 10.1086/504601.PubMed CentralView ArticlePubMedGoogle Scholar
- The Human Gene Mutation Database at the Institute of Medical Genetics in Cardiff. [http://www.hgmd.cf.ac.uk/ac/index.php]
- Dobrovolny R, Dvorakova L, Ledvinoa J, Magagge S, Bultas J, Lubanda JC, Elleder M, Karetova D, Pavlikova M, Hrebicek M: Relationship between X inactivation and clinical involvement in Fabry heterozygotes. Eleven novel mutations in the α Galactosidase A gene in the Czech and Slovach population. J Mol Med. 2005, 83: 647-654. 10.1007/s00109-005-0656-2.View ArticlePubMedGoogle Scholar
- Mehta Atul, Beck Michael, Sunder-Plasmann Gere: Fabry disease perspectives from 5 years of FOS. United Kingdon: OXFORD Pharmagenesis 2006.Google Scholar
- MacDermot KD, Holmes A, Miners AH: Natural history of Fabry disease in affected males and obligate carrier females. J Inherited Metab Dis. 2001, 24 (Suppl 2): 13-14. 10.1023/A:1012447102358.View ArticlePubMedGoogle Scholar
- MacDermot KD, Holmes A, Miners AH: Anderson-Fabry disease: clinical manifestations and impact of disease in a cohort of 60 obligate Carrier females. J Med Genet. 2001, 38: 769-775. 10.1136/jmg.38.11.769.PubMed CentralView ArticlePubMedGoogle Scholar
- Whybra C, Kapmann C, Willers I, Davies J, Winchester B, Kriegsmann J, Bruhl K, Gal A, Bunge S, Beck M: Anderson-Fabry disease: clinical manifestations of the disease in female heterozygotes. J Inherit Metab Dis. 2001, 24: 715-724. 10.1023/A:1012993305223.View ArticlePubMedGoogle Scholar
- Deegan PB, Bahner F, Barba Romero MA, Hughes DA, Kapmann C, Beck M, on behalf of European FOS Investigators: Natural history of Fabry disease in females in the Fabry Outcome Survey. J Med Genet. 2005, 43: 347-352. 10.1136/jmg.2005.036327.PubMed CentralView ArticlePubMedGoogle Scholar
- Nagasaki A, Nishie W, Sato K, Oba I, Noguchi E, Akitsu H, Sawamura D, Shimizu H: Clinical and genetic analysis of Fabry disease: report of six cases including three heterozygoutes females. J Dermatol Sci. 2008, 52: 61-64. 10.1016/j.jdermsci.2008.04.009.View ArticlePubMedGoogle Scholar
- Wilcox WR, Oliveira JP, Hopkin RJ, Ortiz A, Banikazemi M, Feldt-Rasmussen U, Sims K, Waldek S, Pastores GM, Lee P, Eng CM, Marodi L, Standford KE, Breunig F, Wanner C, Warnock DG, Lemay RM, Germain DP: Females with Fabry disease frequently have a major organ involvement: lessons from the Fabry Registry. Mol Genet Metab. 2008, 93: 112-12. 10.1016/j.ymgme.2007.09.013.View ArticlePubMedGoogle Scholar
- Kobayashi M, Ohashi T, Sakuma M, Ida H, Eto Y: Clinical manifestations and natural history of Japanese heterozygous females with Fabry disease. J Inherited Metab Dis. 2008Google Scholar
- Baehner F, Kapmann C, Whybra C, Miebach E, Wiethoff CM, Beck M: Enzyme replacement therapy in heterozygous females with Fabry disease: results of a phase IIB study. J Inherited Metab Dis. 2003, 26: 617-27. 10.1023/B:BOLI.0000005658.14563.77.View ArticlePubMedGoogle Scholar
- Migeon BR: Females are Mosaics: X inactivation and sex differences in disease. New York: Oxford University Press; 2007.Google Scholar
- Maier EM, Osterrieder S, Whybra C, Ries M, Gal A, Beck M, Roscher AA, Muntau AC: Disease manifestations and X inactivation in heterozygous females with Fabry disease. Acta Paediatr. 2006, 95 (Suppl): 30-38. 10.1080/08035320600618809.View ArticleGoogle Scholar
- Mehta A, Ricci R, Widmer U, Dehout F, Garcia de Lorenzo A, Kapmann C, Linhart A, Sunder-Plasmann G, Ries M, Beck M: Fabry disease defined: baseline clinical manifestations of 366 patients in the Fabry Outcome Survey. Eur J Clin Invest. 2004, 34: 236-342. 10.1111/j.1365-2362.2004.01309.x.View ArticlePubMedGoogle Scholar
- Kitagawa T, Suzuki K, Ishige N, Ohsahi T, Kobayashi M, Eto Y, Tanaka A, Odaka H, Owada M: Non-invasive high-risk screening for Fabry disease hemizygotes and heterozygotes. Pediatr Nephrol. 2008, 23: 1461-1471. 10.1007/s00467-008-0846-6.View ArticlePubMedGoogle Scholar
- Linthorst GE, Poorthius BJ, Hollak CE: Enzyme activity for determination of presence of Fabry disease in women results in 40% false-negative results. J Am Coll Cardiol. 2008, 51: 2082-2083. 10.1016/j.jacc.2008.02.050.View ArticlePubMedGoogle Scholar
- Kitagawa T, Ishige N, Suzuki K, Owada M, Ohsahi T, Kobayashi M, Eto Y, Tanaka A, Mills K, Winchester B, Keutzer J: Non-invasive screening method for Fabry disease by measuring globotriaylceramide in whole urine samples using tandem mass spectrometry. Mol Genet Metab. 2005, 85: 196-202. 10.1016/j.ymgme.2005.01.007.View ArticlePubMedGoogle Scholar
- Machili G, Barbujani G, Danieli GA, Hermann FH: Segregation and sporadic cases in families with Hunter's syndrome. J Med Genet. 1991, 28: 398-401. 10.1136/jmg.28.6.398.View ArticleGoogle Scholar
- Nelson J, Crowhust J, Carey B, Greed L: Incidence of the mucopolysaccharidosis in Western Australia. Am J Med Genet. 2003, 123A: 310-313. 10.1002/ajmg.a.20314.View ArticlePubMedGoogle Scholar
- Wilson PJ, Meaney CA, Hopwood JJ, Morris CP: Sequence of the human iduronate 2-sulfatase (IDS) gene. Genomics. 1993, 17: 773-775. 10.1006/geno.1993.1406.View ArticlePubMedGoogle Scholar
- Flomen RH, Green EP, Green PM, Bentley DR, Giannelli F: Determination of the organization of coding sequences within the iduronate sulphate (IDS) gene. Hum Mol Genet. 1993, 2: 5-10. 10.1093/hmg/2.1.5.View ArticlePubMedGoogle Scholar
- Timms Km, Lu F, Shen Y, Pierson CA, Muzny DM, Nelson DL, Gibbs RA: 130 kb of DNA sequence reveals two new genes and a regional duplication distal to the human iduronate-2-sulfaatse locus. Genome Res. 1995, 5: 71-78. 10.1101/gr.5.1.71.View ArticlePubMedGoogle Scholar
- Neufeld EF, Muenzer J: The Mucopolysaccharidoses. The Metabolic and Molecular Basis of Inherited Disease New York: Edited by: MacGraw-HillScriver CR, Beaudet AL, Sly WS, Valle D, 8 2001, III: 3241-3452.Google Scholar
- Young ID, Harper PS, Archer IM, Newcombe RG: A clinical and genetic study of Hunter's syndrome 1. Heterogeneity. J Med Genet. 1982, 19: 401-407. 10.1136/jmg.19.6.401.PubMed CentralView ArticlePubMedGoogle Scholar
- Young ID, Harper PS, Archer IM, Newcombe RG: A clinical and genetic study of Hunter's syndrome 2. Differences between the mild and severe forms. J Med Genet. 1982, 19: 408-411. 10.1136/jmg.19.6.408.PubMed CentralView ArticlePubMedGoogle Scholar
- Neufeld EF, Liebaers I, Epstein CJ, Yatziv S, Milunsky A, Migeon BR: The Hunter syndrome in females: is there na autosomal recessive form of iduronate sulfatase deficiency?. Am J Hum Genet. 1977, 29: 455-456.PubMed CentralPubMedGoogle Scholar
- Mosmann J, Blunt S, Stephens R, Jones EE, Pembrey M: Hunter's disease in a girl: association with X:5 chromosomal translocation disrupting the Hunter gene. Arch Dis Child. 1983, 58: 911-915. 10.1136/adc.58.11.911.View ArticleGoogle Scholar
- Broadhead DM, Kirk JM, Burt AJ, Gupta V, Ellis PM, Besley GT: Full expression of Hunter's disease in a female with an X-chromosome deletion leading to non-random inactivation. Clin Genet. 1986, 20: 392-398.Google Scholar
- Clarke JT, Greer WL, Strasberg PM, Pearce RD, Skomorowski MA, Ray PN: Hunter disease (mucopolysaccharidosis type II) associated with unbalanced inactivation of X chromosomes in a karyotypically normal girl. Am J Hum Genet. 1991, 49: 289-297.PubMed CentralPubMedGoogle Scholar
- Winchester B, Young E, Geddes S, Genet S, Hurst J, Middleton-Price H, Williams N, Webb M, Habel A, Malcom S: Female twin Hunter disease due to nonrandom inactivation of the X-chromosome: a consequence of twinning. Am J Med Genet. 1992, 1: 44:834-838.Google Scholar
- Sukegawa K, Song XQ, Masuno M, Fukao T, Shimozawa N, Fukuda S, Isogai K, Nishio H, Matsuo M, Tomatsu S, Kondo N, Orii T: Hunter disease in a girl caused by R468Q mutation in the iduronate-2-sulfatase gene and skewed inactivation of the X chromosome carrying the normal allele. Hum Mut. 1997, 10: 316-317.View ArticleGoogle Scholar
- Sukegawa K, Matsuzaki T, Fukuda S, Masuno M, Fukao T, Kokutyu M, Iwata S, Tomatsu S, Orri T, Kondo N: Brother/sister sibilings affected with Hunter disease: evidence for skewed X chromosome inactivation. Clin Genet. 1998, 53: 96-101.View ArticlePubMedGoogle Scholar
- Cudry S, Tigaud I, Frossairt R, Bonnet V, Maire I, Bozon D: MPS II in females: molecular basis of two different cases. J Med Genet. 2000, 37: E29-10.1136/jmg.37.10.e29.PubMed CentralView ArticlePubMedGoogle Scholar
- Tuschl K, Gal A, Paschke E, Kircher S, Bodamer OA: Mucopolysaccharidosis type II in females case report and review of literature. Pediatr Neurol. 2005, 32: 270-272. 10.1016/j.pediatrneurol.2004.10.009.View ArticlePubMedGoogle Scholar
- Zlogotora J, Bach G: Heterozygote detection in Hunter syndrome. Am J Med Genet. 1986, 17: 661-665. 10.1002/ajmg.1320170317.View ArticleGoogle Scholar
- Adinolfi M: Hunter syndrome: cloning of the gene, mutations and carriers detection. Dev Med Neurol. 1993, 35: 79-85.View ArticleGoogle Scholar
- Schwartz IVD: Estudo clinico, bioquímico e genético de pacientes com mucopolissacaridoses II e de possíveis heterozigotas. PhD tesis. Universidade Federal do Rio Grande do Sul, Departamento de Genética e Biologia Molecular; 2004.Google Scholar
- Wraith JE, Scarpa M, Beck M, Bodamer AO, De Meileir L, Guffon N, Meldgaard Lund A, Malm G, Ploeg Van der AT, Zeman J: Mucopolysaccharidosis type II (Hunter syndrome): a clinical review and recommendations for treatment in the era of enzyme replacement theraphy. Eur J Pediatr. 2008, 167: 267-277. 10.1007/s00431-007-0635-4.PubMed CentralView ArticlePubMedGoogle Scholar
- Dobyns WB, Filauro A, Tomson BN, Chan AS, Ho AW, Ting NT, Osterwijk JC, Ober C: Inheritance of most X-linked traits is not dominant or recessive, just X-linked. Am J Med Genet. 2004, 129A: 136-143. 10.1002/ajmg.a.30123.View ArticlePubMedGoogle Scholar
- Morgan TH, (Ed): Croonian Lecture- On the mecanism of heredity. Proceeding of the Royal Society, New York. 1992, 94: 162-197. 10.1098/rspb.1922.0053.Google Scholar
- Franco B, Ballabio A: X-inactivation and human disease: X-linked dominant male-lethal disorders. Curr Opin Genet Dev. 2006, 16: 254-259. 10.1016/j.gde.2006.04.012.View ArticlePubMedGoogle Scholar
- Willard 2000 HF: The sex chromosomes and X chromosome inactivation. The metabolic and molecular basis of inherited disease New York: Edited by: MacGraw-HillScriver CR, Beaudet AL, Sly WS, Valle D, 8 2000, , III: 1191-1211.Google Scholar
- Chabas A, Happle R: Understanding the biology of X linked diseases. Acta Paediatr. 2006, 451 (Suppl 95): 9-10. 10.1080/08035320600619104.View ArticleGoogle Scholar
- Dobyns WB: The pattern of inheritance traits is not dominant or recessive, just X-linked. Acta Paediatr Suppl. 2006, 95: 11-15. 10.1080/08035320600618759.View ArticlePubMedGoogle Scholar
- Redonnet-Vernhet I, Ploss van Amstel JK, Jansen RPM, Wevers RS, Salvayre R, Levade TJ: Uneven X inactivation in a female monozigotic twin par with Fabry disease and discordant expression of a novel mutation in the alpha-galatosidase A gene. J Med Genet. 1996, 33: 682-688. 10.1136/jmg.33.8.682.PubMed CentralView ArticlePubMedGoogle Scholar
- Levade T, Giordano F, Maret A, Margery MC, Bazex J, Salvayre R: Different phenotypic expression of Fabry disease in female monozigotic twins. J Inherited Metab Dis. 1991, 14: 105-16. 10.1007/BF01804397.View ArticlePubMedGoogle Scholar
- Marguery MC, Giordano F, Parant M, Samalens G, Levade T, Salvayre R, Maret A, Clavas P, Bourrouillou G, Cantala P, Bazex J: Fabry's Disease: Heterozygous form of different expression in two monozygous twin sisters. Dermatology. 2003, 187: 9-15. 10.1159/000247189.View ArticleGoogle Scholar
- Richards CS, Watkins SC, Hoffman EP, Schneider NR, Mildark IW, Katz KS, Cook JD, Kunkel LM, Cortada JM: Skewed X inactivation in a female MZ twin results in Duchene muscular dystrophy. Am J Hum Genet. 1990, 46: 672-681.PubMed CentralPubMedGoogle Scholar
- Lupski JR, Garcia CA, Zoghbi HY, Hoffamn EP, Fenwick RG: Discordance of muscular dystrophy in monozygotic females twins:evidence supporting asymmetric splitting of the inner cell mass in a manifesting carrier of Duchene Dystrophy. Am J Hum Genet. 1991, 40: 354-364.View ArticleGoogle Scholar
- Valleix S, Vinciguerra C, Lavergne JM, Leuer M, Delphec M, Negrier C: Skewed X-chromosome inactivation in monochorionic diamniotic twin sisters results in severe and mild hemophilia A. Blood. 2002, 100: 3034-3036. 10.1182/blood-2002-01-0277.View ArticlePubMedGoogle Scholar
- Tiberio G: MZ females twins discordant for X-linked diseases: a review. Acta Genet Med Gemellol. 1994, 43: 207-214.PubMedGoogle Scholar
- Burn J, Povey S, Boyd Y, Munro EA, West L, Harper K, Thomas D: Duchene muscular dystrophy in one of monozygotic twin girls. J Med Genet. 1986, 23: 494-500. 10.1136/jmg.23.6.494.PubMed CentralView ArticlePubMedGoogle Scholar
- Nance WE: Do twin Lyons have large spots?. Am J Hum Genet. 1990, 46: 646-648.PubMed CentralPubMedGoogle Scholar
- Côté GB, Gyftodimou J: Twinning and mitotic crossing-over: some possibilities and their implications. Am J Hum Genet. 1991, 47: 120-130.Google Scholar
- Gardner RL, Lyon MF, Evans EP, Burtenshaw MD: Clonal analysis of X-chromosome inactivation and the origin of the germ line in the mouse embryo. J Embryol Exp Morphol. 1985, 88: 349-363.PubMedGoogle Scholar
- Monteiro J, Derom C, Vlietnick R, Kohn N, Lesser M, Gregerson PK: Commitment to X inactivation precedes the twinning event in monochorionic MZ twins. Am J Hum Genet. 1998, 63: 339-346. 10.1086/301978.PubMed CentralView ArticlePubMedGoogle Scholar
- Chitnis S, Derm C, Vlietinck R, Deron R, Monterio J, Gregersn PK: X chromosome inactivation patterns confirm the late timing of monoamniotic MZ twining. Am J Hum Genet. 1999, 65: 570-571. 10.1086/302502.PubMed CentralView ArticlePubMedGoogle Scholar
- Puck JM, Willard HF: X inactivation in females with X linked disease. N England J Med. 1998, 330: 325-328. 10.1056/NEJM199801293380511.View ArticleGoogle Scholar
- Lyon MF: Gene Action in the X-chromosome of the Mouse (Mus musculus L.). Nature. 1961, 190: 372-373. 10.1038/190372a0.View ArticlePubMedGoogle Scholar
- Lyon MF: Sex chromatin and gene action in the mammalian X-chromosome. Am J Hum Genet. 1962, 14: 135-148.PubMed CentralPubMedGoogle Scholar
- Brown CJ, Lafreniere RG, Powers VE, Sebastio G, Ballabio A, Pettigrew AL, Ledbetter DH, Levy DH, Levy E, Craig IW, Willard HF: Localization of the X-inactivation centre on the human X chromosome in Xq13. Nature. 1991, 349: 82-84. 10.1038/349082a0.View ArticlePubMedGoogle Scholar
- Willard HF: X chromosome inactivation, XIST, and pursuit of the X inactivation center. Cell. 1996, 86: 5-7. 10.1016/S0092-8674(00)80071-9.View ArticlePubMedGoogle Scholar
- Lee JT, Lu N: Targeted mutagenesis of Tsix leads to nonrandom X inactivation. Cell. 1999, 99: 47-57. 10.1016/S0092-8674(00)80061-6.View ArticlePubMedGoogle Scholar
- Brown CJ, Robinson WP: The causes and consequences of random and non-random X chromosome inactivation in humans. Clin Genet. 2000, 58: 353-363. 10.1034/j.1399-0004.2000.580504.x.View ArticlePubMedGoogle Scholar
- Salido EC, Yen PH, Mohandas TK, Shapiro LJ: Expression of the X-inactivation-associated gene associated XIST during spermatogenesis. Nat Genet. 1992, 2: 196-199. 10.1038/ng1192-196.View ArticlePubMedGoogle Scholar
- Avner P, Heard E: X-chromosome inactivation: counting, choice and initiation. Nat Rev Genet. 2001, 2: 59-67. 10.1038/35047580.View ArticlePubMedGoogle Scholar
- Heard E, Rougeulle C, Arnaud D, Avner P, Allis CD, Spector DL: Methylation of histone H3 at Lys-9 is an early mark on the X chromosome during X inactivation. Cell. 2001, 107: 727-738. 10.1016/S0092-8674(01)00598-0.View ArticlePubMedGoogle Scholar
- Talebizadeh Z, Simon SD, Butler MG: X chromosome gene expression in human tissues: male and female comparisons. Genomics. 2006, 88: 675-681. 10.1016/j.ygeno.2006.07.016.View ArticlePubMedGoogle Scholar
- Busque L, Mio R, Brais E, Blais N, Lalonde Y, Maragh M, Gilliand DG: Non-random X inactivation patterns in normal females: lyonization ratios vary with age. Blood. 1996, 88: 59-65.PubMedGoogle Scholar
- Naumova AK, Plenge RM, Bird LM, Leppert M, Morgan K, Willard HF, Sapienza C: Heritability of X chromosome-inactivation phenotype in a large family. Am J Hum Genet. 1996, 58: 1111-1119.PubMed CentralPubMedGoogle Scholar
- Fey MF, Liechti-Gallati S, von Rohr A, Borisch B, Theirlkas L, Scnheider V, Oestreicher M, Nagel S, Ziemiecki A, Tobler A: Clonality and X-inactivation patterns in hematopoietic cell populations detected by the highly informative M27 beta probe. Blood. 1994, 83: 931-938.PubMedGoogle Scholar
- Sharp A, Robinson D, Jacobs P: Age-and tissue specific variation of X chromosome inactivation ratio in normal women. Hum Genet. 2000, 107: 343-347. 10.1007/s004390000382.View ArticlePubMedGoogle Scholar
- Lyon MF: X-chromosome inactivation and human genetic disease. Acta Paediatr Suppl. 2002, 91: 107-112. 10.1080/080352502762458030.View ArticlePubMedGoogle Scholar
- Gale RE, Fielding AK, Harrison CN, Linch DC: Acquired skewing of X-chromosome inactivation patterns in myeloid cells of the elderly suggests stochastic clonal loss with age. Br J Haematol. 1997, 98: 512-519. 10.1046/j.1365-2141.1997.2573078.x.View ArticlePubMedGoogle Scholar
- Kristiansen M, Knudesen GP, Bathum L, Naumova AK, Sorensen TI, Brix TH, Svendsen AJ, Christensen K, Kyvik KO, Orstavik KH: Twin study of genetic and aging effects on X chromosome inactivation. Eur J Hum Genet. 2005, 13: 599-606. 10.1038/sj.ejhg.5201398.View ArticlePubMedGoogle Scholar
- Maier EM, Kammer S, Muntau AC, Wichers M, Braun A, Roscher AA: Symptoms in carriers of adrenoleukodystrophy relate to skewed X inactivation. Ann Neurol. 2002, 52: 683-688. 10.1002/ana.10376.View ArticlePubMedGoogle Scholar
- Plenge RM, Stevenson RA, Lubs HA, Schwartz CE, Willard HF: Skewed X-chromosome inactivation is a common feature of X-linked mental retardation disorders. Am J Hum Genet. 2002, 71: 168-173. 10.1086/341123.PubMed CentralView ArticlePubMedGoogle Scholar
- Migeon BR: The role of X inactivation and cellular mosaicism in women's health and sex specific diseases. JAMA. 2006, 295: 22-29. 10.1001/jama.295.12.1428.View ArticleGoogle Scholar
- Birot AM, Delobel B, Gronnier P, Bonnet V, Maire I, Bozon D: A 5-megabase familial deletion removes the IDS and FMR1 genes in a male Hunter patient. Hum Mut. 1996, 7: 266-268. 10.1002/(SICI)1098-1004(1996)7:3<266::AID-HUMU12>3.0.CO;2-0.View ArticlePubMedGoogle Scholar
- Uehara S, Hashiyada M, Sato K, Sato Y, Fujimori K, Okamura K: Preferential X-chromosome inactivation in women with idiopathic recurrent pregnancy loss. Fertil Steril. 2001, 76: 908-914. 10.1016/S0015-0282(01)02845-X.View ArticlePubMedGoogle Scholar
- Lanasa MC, Hogge WA, Kibik C, Blancato J, Hoffman EP: Highly skewed X-chromosome inactivation is associated with idiopatic recurrent spontaneous abortion. Am J Hum Genet. 1999, 65: 252-254. 10.1086/302441.PubMed CentralView ArticlePubMedGoogle Scholar
- Kristiansen M, Knudsen GP, Maguire P, Margolin S, Pedersen L, Lindblom A, Orstavik KH: High incidence of skewed X chromosome inactivation in young patients with familial non-BRCA1/BRCA2 breast cancer. J Med Genet. 2005, 42: 877-880. 10.1136/jmg.2005.032433.PubMed CentralView ArticlePubMedGoogle Scholar
- Belmont JW: Genetic control of X inactivation and process leading to X inactivation skewing. Am J Hum Genet. 1996, 58: 1101-1108.PubMed CentralPubMedGoogle Scholar
- Wang RY, Lelis A, Mirocha J, Wilcox WR: Heterozygotes Fabry women are not just carriers, but have a significant burden of disease and impaired quality of life. Genet Med. 2007, 9: 34-45. 10.1097/GIM.0b013e31802d8321.View ArticlePubMedGoogle Scholar
- Cardone M, Porto C, Tarallo A, Vincinanza M, Rossi B, Polishchuck E, Donaudy F, Andria G, De Matteis MA, Parenti G: Abnomal mannose-6-phosphate receptor trafficking impairs recombinant alpha-glucosidase uptake in Pompe disease fibroblasts. Pathogenetics. 2008, 1: 1-6. 10.1186/1755-8417-1-6.View ArticleGoogle Scholar
- Aerts JM, Groener JE, Kuiper S, Donker-Koopman WE, Strijland A, Ottenhoff R, van Roomen C, Mirzaian M, Wiljburg FA, Linthorst GE, Vedder AC, Rombach SM, Cox-Brinkmann J, Somerharju P, Boot RG, Hollak CE, Brady RO, Poorthius BJ: Elevated globotriaosylsphingosine is a hallmark of Fabry disease. Proc Natl Acad Sci USA. 2008, 105: 2812-2817. 10.1073/pnas.0712309105.PubMed CentralView ArticlePubMedGoogle Scholar
- Braun SE, Arnovich EL, Anderson RA, Crotty PL, Mc Ivor RS, Whitley CB: Metabolic correction and cross-correction of mucopolysaccharidosis type II (Hunter syndrome) by retroviral-mediated gene transfer and expression of human iduronate-2-sulfatase. Proc Natl Acad Sci USA. 1993, 90: 11830-11834. 10.1073/pnas.90.24.11830.PubMed CentralView ArticlePubMedGoogle Scholar
- Schwartz IVD, Pinto LLC, Ribeiro MG, Mota JG, Acosta AX, Correia P, Horovitz D, Porciuncula CGG, Lipinski-Figueiredo E, Fett-Conte A, Oliveira Sobrinho RP, Norato DYJ, Paula AC, Kim CA, Duarte AR, Boy R, Leistner-Segal S, Burin MG, Giugliani R: Clinical and Biochemical Studies in Mucopolysaccharidosis type II Carriers. J Inherit Metab Dis. 2009, 1275-1279.Google Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.