Although recessively inherited ABCA2 variants have been reported in two patients who had intellectual disability with global developmental delays, our study demonstrates the role of an ABCA2 variant in the pathogenesis of ataxia with dysarthria.
In the last family of three affected siblings, a novel syndrome of intractable itching, hypercholanemia, short stature, and intellectual disability was mapped to a single locus that contains a homozygous truncating variant in WDR83OS (C19orf56), known to interact with ATP13A2 and BSEP.
Therefore, it is likely that one or more of these MRX genes, subject to X-inactivation, are lost from the ring X chromosome, and that reduced expression of the MRX gene(s) caused by random X-inactivation has resulted in mental retardation in the mother and daughter.
Mutations in most of more than 20 known genes causing nonspecific form of X-linked MR (MRX) are very rare and may account for less than 0.5-1% of MR. Linkage studies in extended pedigrees followed by mutational analysis of known MRX genes in the linked interval are often the only way to identify a genetic cause of the disorder.
The results imply that an MRX gene subject to X inactivation is present in a roughly 4 Mb region between DXS7182 and DAX-1, and that reduced expression of the normal MRX gene caused by random X inactivation results in MR in carrier females.
Linkage studies followed by mutational analysis of known X-chromosomal genes related to mental retardation (MRX genes) localized within defined genetic intervals represent a rational strategy to identify a genetic cause of the disorder.
The aberrations are associated with the phenotype in five patients (4.6%), based on the following criteria: de novo aberration; involvement of a known or candidate X-linked nonsyndromic(syndromic) MR (MRX(S)) gene; segregation with the disease in the family; absence in control individuals; and skewed X-inactivation in carrier females.