Waardenburg syndrome type 4: report of two new cases caused by SOX10 mutations in Spain.

Shah-Waardenburg syndrome or Waardenburg syndrome type 4 (WS4) is a neurocristopathy characterized by the association of deafness, depigmentation and Hirschsprung disease. Three disease-causing genes have been identified so far for WS4: EDNRB, EDN3, and SOX10. SOX10 mutations, found in 45-55% of WS4 patients, are inherited in autosomal dominant way. In addition, mutations in SOX10 are also responsible for an extended syndrome involving peripheral and central neurological phenotypes, referred to as PCWH (peripheral demyelinating neuropathy, central dysmyelinating leucodystrophy, Waardenburg syndrome, Hirschsprung disease). Such mutations are mostly private, and a high intra- and inter-familial variability exists. In this report, we present a patient with WS4 and a second with PCWH due to SOX10 mutations supporting again the genetic and phenotypic heterogeneity of these syndromes. Interestingly, the WS4 family carries an insertion of 19 nucleotides in exon 5 of SOX10, which results in distinct phenotypes along three different generations: hypopigmentation in the maternal grandmother, hearing loss in the mother, and WS4 in the proband. Since mosaicism cannot explain the three different related-WS features observed in this family, we propose as the most plausible explanation the existence of additional molecular events, acting in an additive or multiplicative fashion, in genes or regulatory regions unidentified so far. On the other hand, the PCWH case was due to a de novo deletion in exon 5 of the gene. Efforts should be devoted to unravel the mechanisms underlying the intrafamilial phenotypic variability observed in the families affected, and to identify new genes responsible for the still unsolved WS4 cases.