Ethnically diverse causes of Walker-Warburg syndrome (WWS): FCMD mutations are a more common cause of WWS outside of the Middle East

Abstract

Walker-Warburg syndrome (WWS) is a genetically heterogeneous autosomal recessive disease characterized by congenital muscular dystrophy, cobblestone lissencephaly, and ocular malformations. Mutations in six genes involved in the glycosylation of á-dystroglycan (POMT1, POMT2, POMGNT1, FCMD, FKRP and LARGE) have been identified in WWS patients, but account for only a portion of WWS cases. To better understand the genetics of WWS and establish the frequency and distribution of mutations across WWS genes, we genotyped all known loci in a cohort of 43 WWS patients of varying geographical and ethnic origin. Surprisingly, we reached a molecular diagnosis for 40% of our patients and found mutations in POMT1, POMT2, FCMD and FKRP, many of which were novel alleles, but no mutations in POMGNT1 or LARGE. Notably, the FCMD gene was a more common cause of WWS than previously expected in the European/American subset of our cohort, including all Ashkenazi Jewish cases, who carried the same founder mutation.

Figure 1. FKRP mutations in classic WWS patients

A. Patient 11 was born to first-cousin parents and carried a homozygous 9 basepair duplication starting at bp.962 in the FKRP gene. This mutation leads to a 3 amino acid duplicagtion. B. Two heterozygous missense mutations were identified in Patient 10. C. Schematic representation of the FKRP protein and mutation summary. The initiation codon is codon 1. Mutations listed above the protein cause WWS: previously reported mutations are in blue, mutations identified in this study are in red. Below the protein mutations causing MEB and CMD associated with variable brain malformations are shown. (h) means heterozygous mutations. Compound heterozygous mutations are connected by an arrow. FKRP mutations causing only CMD are not listed. TM: transmembrane domain.

Figure 2. FCMD mutations in classic WWS patients

A. Patient 15 is of Ashkenazi Jewish origin. She carried a homozygous insertion in exon 8 of the FCMD gene, which was inherited from both parents. SNP genotyping results are indicated below each individual. The same mutation and the same SNP haplotype were found in two additional Ashkenazim affected in our cohort suggesting a founder effect in this population. B. Patient 12 carried two heterozygous nonsense mutations, while Patient 13 carried a homozygous missense mutation. C. Schematic representation of the fukutin protein and mutation summary. Mutations listed above the protein cause WWS: previously reported mutations are in blue (G125S was identified in heterozygosity with a small 3’ UTR deletion, which is different from the classic FCMD mutations (Cotarelo, et al., 2008)), mutations identified in this study are in red. (h) means heterozygous mutations. Compound heterozygous mutations are connected by an arrow. Below the protein coding mutations causing FCMD are shown; (+3’) next to each mutation indicates that the mutation is in heterozygosity and associated with the 3’ retransposon insertion characteristic of Japanese FCMD patients. TM: transmembrane domain.

Figure 3. Brain imaging of children with WWS of varying genetic etiologies

Magnetic resonance imaging (MRI) using T2-weighted axial (top row) and T1-weighted sagittal (bottom row) sequences demonstrates that affected individuals with mutations in the POMT1 (left), FCMD (middle), and FKRP (right) genes have indistinguishable radiological phenotypes. All demonstrate classic WWS findings including cobblestone lissencephaly with an absence of sulcation and an irregular gray-white junction, marked ventriculomegaly, and severe dysplasia of posterior fossa structures, including cerebellar hypoplasia and a “kinked” appearance of the brainstem.