Our exome sequencing of two siblings with a previously undiagnosed neurodegenerative disorder has detected compound heterozygous mutations c.101C>T (p.Ala34Val) and c.1547T>C (p.Ile516Thr) in HSD17B4 affecting the dehydrogenase and hydratase domains, respectively. Both missense mutations have been previously reported, but in both cases with a second missense mutation affecting the identical DBP domain on the other allele
. The first case was compound heterozygous for p.Ala34Val and p.Phe237Ser; both mutations affecting the dehydrogenase domain resulting in DBP type III and an isolated dehydrogenase deficiency. The second case was compound heterozygous for p.Ile516Thr and p.Asn457Tyr; in this instance both mutations occur within the hydratase domain causing DBP type II and an isolated hydratase deficiency. Although this patient with DBP type II survived >13.5 years, cognitive and language deficits were significant.
An obvious explanation for the relatively milder clinical phenotype observed in our patients is the fact that only one domain on each allele is affected with a less severe mutation. The attenuated clinical phenotype and biochemical testing indicates normal transcription, translation, and the normal import of the DBP enzyme into an intact and functional peroxisome. The latter is supported by normal catalase immunoflorescence observed in our patients indicating normal peroxisome biogenesis and morphology; a finding not seen in patients with the more severe form of DBP deficiency
. Little information is available on the impact of the p.Ala34Val mutation and analysis demonstrated low residual enzyme activity of the dehydrogenase domain. The p.Ile516Thr is located at the dimerization interface of the hydratase subunits but does not abolish dimerization completely, implying residual activity
. The apparent discrepancy between the residual hydratase activity (~40% of lower limit of normal) and absence of the hydratase domain on immunoblot may in part be accounted for by the fact that L-Bifunctional protein (L-BP) is responsible for part of the measured hydratase activity because it can metabolize the substrate THC:1-CoA to the (24S,25S)-isomer of 24OH-THC-CoA which cannot be metabolized further by the dehydrogenase domain of L-BP. The result could also, in part, represent a combination of altered structure and stability of the mutant DBP enzyme. Finally, both sets of homodimers (hydratase and dehydrogenase domains) likely have some physical and functional relationship to each other. A mutation in one domain has the potential to alter function and stability of the other unit even if a mutated domain (e.g. dehydrogenase) forms a dimer with an adjacent wild-type domain (e.g. hydratase). As such, the in vivo function of this enzyme appears more complicated than that predicted by in vitro studies.
Although some other peroxisome diseases resulting from single enzyme defects can present in adulthood (i.e. acyl-CoA oxidase (ACOX) deficiency
 and sterol carrier protein X (SCPx) deficiency
), this has not been reported for DBP deficiency
. Since both hydratase and dehydrogenase activities are affected, our patients would be deemed to be type I under the current DBP deficiency classification. However, the significant majority of type I-deficient patients have mutations in HSD17B4 encoding truncated or unstable proteins resulting in a severe phenotype and poor survival
. In our estimation, the mild clinical and biochemical phenotype in our patients warrant a new classification. We therefore propose a novel variant of DBP deficiency, designated DBP type IV, due to compound heterozygous mutations affecting two different domains of DBP but associated with a relatively milder clinical and biochemical phenotype.
Our newly proposed subtype of DBP deficiency (type IV) would also apply to two sisters recently diagnosed with Perrault syndrome caused by compound heterozygous mutations within HSD17B4, one affecting the dehydrogenase domain and one the hydratase domain, similar to our patients. In this instance, the sisters’ relatively milder phenotype was characterized by sensorineural deafness, mild intellectual disability, sensorimotor polyneuropathy, short stature and ovarian dysgenesis
. Exome sequencing was also essential in obtaining this diagnosis but complete biochemical testing including DBP enzyme activity measurement was not reported. Our patients differ from these sisters by their normal intelligence and pubertal development (in the older brother).
The overall incidence of peroxisomal disorders is approximately 1 in 5,000 newborns
, most of these cases are severe and are thus readily ascertained. In striking contrast to previously reported patients
[4, 5], the two brothers described here did not demonstrate any neonatal or infantile symptoms, moreover they continue to demonstrate normal cognition. Their slow clinical course of DBP deficiency has allowed, for the first time, serial electrodiagnostic testing in DBP patients; the clinical exams and nerve conduction studies spanning several years (Table
1) document a gradual decline of coordination reflecting increasing cerebellar and sensory nerve dysfunction. The progressive sensorimotor polyneuropathy demonstrated uniform conduction velocity slowing, reminiscent of that seen with many hereditary demyelinating polyneuropathies (e.g. Charcot-Marie-Tooth, type 1). The older sibling (Patient 1) not only demonstrated progressive demyelination (increasing latencies and conduction velocity slowing) but also evidence of progressive, length-dependent axonal loss. The introduction of readily available exome sequencing into rare disease clinics will lead to the recognition of additional patients with milder variants of DBP deficiency and will improve our understanding of the phenotypic spectrum and natural history of this and other diseases.