‘Growth is a mirror of health’, James Tanner wrote these words, which have become fundamental to understanding the concept of auxology. For a rare disease, we could paraphrase them and say that ‘growth is a mirror of the severity of the disease’. In our previous studies, we evaluated and described growth in patients with MPS I, MPS II, MPS IVA and MPS VI [4,5,6, 9]. In this study, we attempt to answer the questions of the causes of differences in the growing process of MPS groups. In our cohort, mean values for birth length and weight were greater than in the general population, with statistically significant differences in birth length. This observation was congruent with further studies [4, 10, 11]. In our study, the birth length of MPS IVA patients was the largest in comparison to the healthy population and other types of MPS. However, the tendency for increased birth body length in patients with MPS IV was also observed in another study . Patients with MPS IVA are longer at birth than children in the general population maybe because excessive laxity of the connective tissue allows the newborn to stretch out better when measured.
There is a scarcity of literature concerning the process of growth in children with different types of MPS diseases. Growing categorises clinical severity of the MPS disease. MPS IVA shows the most significant growth impairment. After birth, children with MSP IV grew slowly, and their final height corresponded to that of a 7-year-old child. The same observation was corroborated in previous publications [13,14,15]. A very similar trend of growth was observed in MPS VI patients. They were larger at birth and grew slower than their healthy peers. MPS VI patients fell down on percentile charts until about 8, and then the growth rate z-scores ranged between − 3 and − 4. The growth of patients with MPS I, II and III up to the 24th month of life was between 0 and 2 in body height z-scores. Afterwards, growth slowed down after the 2nd year of life for MPS I patients, around the 4th year of life for MPS II patients and around the 6th year of life for MPS III patients. After this period of intensive growth, in subsequent years, body height showed significantly lower values when compared with the reference charts. This trend was corroborated by earlier publications [10, 11, 16, 17]. The reasons for differences in growth dynamics between patients with different types of MPS are yet to be fully explained. One explanation is the accumulation dependency of DS. There is preclinical and clinical evidence of prenatal accumulation of GAGs in chondrocytes in MPS [2, 18,19,20,21]. The overgrowth in foetal and early postnatal life could be connected to HS acting as a coreceptor that binds to several proteins, including growth factors. Therefore, an increased level of HS might overstimulate axial bone growth in children with MPS I, II and III [22, 23]. This is contradicted by the fact that HS is not accumulated in MPS IVA, but they still are longer at birth than in the general population. At the same time, the accumulation of DS over time would cause inhibition in the analysis of MPS growth plates showing clusters of enlarged and GAG-containing cells that disrupt a columnar architecture of growth plate cartilage, presumably leading, in part, to abnormal bone growth . Simonaro et al. suggested that the main tissue of these disorders is the cartilage rather than the bone itself. However, Hinek and Wilson  reported that elastogenesis takes place in the shaft of long bones during foetal life, and accumulations of DS by fibroblasts induces the functional deficiency in the elastin-binding protein and, consequently, leads to disruption of normal elastogenesis. Presence of elastic fibers in the limb buds and their primitive perichondrial tissue has been suggested as a crucial factor in maintaining the proper shape of the normal embryonal skeleton. DS damage to tropoelastin could be relevant to skeletal deformations found in MPS disease.. Melbouci hypothesise that MPS IVA is the most severely affected because C6S and KS are GAGs that are accumulated, both of which directly impact bone growth. KS is localised mainly in connective tissue, and C6S is primarily in articular cartilage. Cumulations of these GAGs have a degenerative impact on bones and lead to skeletal deformities . In MPS III, the only GAG that is accumulated is HS. HS is localised in the plasma membrane, the extracellular matrix in visceral organs and the central nervous system . In this study, patients with MPS III showed better growth compared to other MPS types, perhaps because they do not accumulate dermatan sulphate. DS is accumulated in MPS I, MPS II and MPS VI. The differences in growth dynamics between patients with MPS I, MPS II and MPS VI could be explain by various levels of accumulation of toxic DS . Although DS is the GAG accumulated in MPS VI, the upward trend is not intensive, and after the 5th year of life, the growth rate is fairly constant. Growth in MPS VI is related to the severity of the disease. Classification systems describe MPS VI as severe (with early onset of symptoms and fast progression), attenuated (with later onset, slower disease progression and variable clinical presentation) and intermediate; however, MPS VI spans a continuum of disease . The relatively attenuated disease form is characterised by a later onset of symptoms due to lower levels of dermatan sulphate . A study indicates a mild form of MPS VI due to a local East European mutation—p.R152W—in the homozygous state that seems to significantly differ from other MPS VI types .
The GAGs that accumulate in MPS I and MPS II are DS and HS. Therefore, it appears that the explanation of growth pattern differences between MPS I and MPS II should be sought in the levels of DS accumulation. MPS I patients show a higher accumulation of DS than MPS II. At the same time, MPS II patients show a higher accumulation of HS than MPS I [28, 29].
We also found a tendency for relationships between growth and the severity of the disease with more severe phenotypes resulting in cognitive impairement. For boys with Hurler syndrome, the body height below the 3rd percentile was reached after the 24th month of life, for patients with severe MPS II between the 6th and 7th year of life, and for patients with attenuated MPS II between the 8th and 9th year. Range of motion limitations (ROM) in patients with MPS II correlate with patients’ height and are more pronounced in a severe form of MPS II. In patients with cognitive impairment, ROM limitations are greater and impact body height more . With age, the number of GAGs excreted decreases, so their highest quantities are accumulated at a young age when they can affect the growth cartilage and cause the most significant damage; hence, we observe a sharp decline in the growth curve.
There are some limitations of this study. Mucopolysaccharidoses are rare diseases, and we only had a limited data sample. Our study has the mix-longitudinal character this method can be used when there is an insufficient number of subjects [31, 32], but non-uniform longitudinal research resulted in a different amount of data for calendar age groups. Therefore, greater reliability of results was obtained for the period, which is better represented.
Although we had a relatively large sample size further studies that would substantiate the observed trends and relationships reported for this group. This could be important for future research that would document changes in growth patterns for children who have access to enzyme replacement therapy. Therefore, every major medical research center should have an anthropologic laboratory with personnel qualified not only to conduct anthropometric measurements but also to accurately interpret the results. Better understanding of the natural history of the disease can be achieved with long-term observations, which help detect and establish patterns of growth and physical development in the evaluated group.