Whole-exome sequencing diagnosis of two autosomal recessive disorders in one family.

Autosomal recessive congenital ichthyosis (ARCI) is a genetically heterogeneous disorder for which subtyping through molecular analysis can help determine the eventual phenotype and prognosis. We used whole-exome sequencing to identify a new homozygous splice-site mutation in ST14 (IVS5+1G>A), encoding matriptase, in a 4-year-old girl with ARCI from a consanguineous Kuwaiti family. Clinically, she also had hypotrichosis, which supported a diagnosis of ARCI type 11. Only four previous examples of pathogenic mutations in ST14 have been reported, and our findings expand the genotype-phenotype correlation for this subtype of ARCI. Our patient was the second child born to these parents; the first (deceased) and third children had congenital brain and eye abnormalities, of uncertain aetiology and with no precise diagnosis. Further analysis of our patient's exome dataset revealed heterozygosity for a splice-site mutation in POMT1 (IVS4+1G>T), encoding the protein O-mannosyltransferase, a gene implicated in Walker-Warburg syndrome. DNA sequencing in the third child showed homozygosity for this mutation in POMT1. The first-cousin parents were both heterozygous for the splice-site mutations in ST14 and POMT1. In this family, whole-exome sequencing provided accurate subtyping of a form of ARCI in one child and provide an explanation for an undiagnosed developmental disorder in two other children, findings that improve the prospects for diagnostic accuracy and genetic counselling, and demonstrate the impact of next-generation sequencing technologies on clinical genetics.

Founder mutation in dystonin-e underlying autosomal recessive epidermolysis bullosa simplex in Kuwait.

Only two homozygous nonsense mutations in the epidermal isoform of the dystonin gene, DST-e, have been reported previously in autosomal recessive epidermolysis bullosa simplex (EBS); the affected pedigrees were Kuwaiti and Iranian. This subtype of EBS is therefore considered to be a rare clinicopathological entity. In this study, we identified four seemingly unrelated Kuwaiti families in which a total of seven individuals had predominantly acral trauma-induced blistering since infancy. All affected individuals were homozygous for the mutation p.Gln1124* in DST-e, the same mutation that was identified in the originally reported family from Kuwait. Haplotype analysis in the five pedigrees (including the previous case) revealed a shared block of ~60 kb of genomic DNA across the site of the mutation, consistent with a founder effect. Most heterozygotes had no clinical abnormalities although one subject had mild transient skin fragility during childhood, an observation noted in the previously reported Iranian pedigree, suggesting that the condition may also be semidominant in some pedigrees rather than purely autosomal recessive. Our study reveals propagation of a mutant ancestral allele in DST-e throughout Kuwait, indicating that this subtype of EBS may be more common in Kuwait, and perhaps other Middle Eastern countries, than is currently appreciated.

Mutations in EXPH5 result in autosomal recessive inherited skin fragility.

Several different genes have been implicated in the pathophysiology of inherited blistering skin diseases. Recently, autosomal recessive loss-of-function mutations in EXPH5 (encoding exophilin-5, also known as Slac2-b, a protein involved in intracellular vesicle transport) were identified in a new mechanobullous disease resembling a form of epidermolysis bullosa simplex (EBS). Here, we searched for mutations in EXPH5 in a 4-year-old white boy with EBS in whom initial Sanger sequencing of known genes implicated in intraepidermal skin fragility failed to identify pathogenic mutations. Transmission electron microscopy of rubbed nonlesional patient skin revealed disruption of keratinocytes in the lower epidermis with cytolysis and acantholysis, keratin filament clumping and prominent perinuclear cytoplasmic vesicles, and provided the clue to the candidate gene pathology. Sanger sequencing of genomic DNA showed compound heterozygosity for two new mutations in EXPH5, c.1947dupC (p.Pro649fsPro*11) and c.2249C>A (p.Ser750*). Immunofluorescence microscopy of patient skin showed a complete absence of exophilin-5 labelling. This case represents the third pedigree with EXPH5 mutations resulting in inherited skin fragility. The clinical and molecular data expand genotype-phenotype correlation in this new form of EBS and demonstrate the important role of exophilin-5 in keratinocyte cell biology.