Identification of a nonsense mutation in the very low-density lipoprotein receptor gene (VLDLR) in an Iranian family with dysequilibrium syndrome.

We have investigated a consanguineous Iranian family with eight patients who suffer from mental retardation, disturbed equilibrium, walking disability, strabismus and short stature. By autozygosity mapping we identified one region with a significant LOD score on chromosome 9(p24.2-24.3). The interval contains the VLDLR gene, which codes for the very low-density lipoprotein receptor. This protein is part of the reelin signalling pathway, which is involved in neuroblast migration in the cerebral cortex and cerebellum. A homozygous deletion encompassing VLDLR has previously been found to cause a syndrome of cerebellar ataxia and mental retardation associated with cerebellar hypoplasia in the Hutterite population known as dysequilibrium syndrome (DES). The reported deletion however, contains an additional brain expressed gene of unknown function, whose involvement in the aetiology of the phenotype could so far not be excluded. We screened the coding region of VLDLR for mutations in our patients and found a homozygous c.1342C>T nucleotide substitution, which leads to a premature stop codon in exon 10. This is the first report of a mutation in patients with DES that affects VLDLR exclusively, confirming the central role of the very low-density lipoprotein receptor in the aetiology of this condition.