Can leaky splicing and evasion of premature termination codon surveillance contribute to the phenotypic variability in Alkuraya-Kucinskas syndrome?

Disease-associated variants in KIAA1109 associate with autosomal recessive Alkuraya-Kucinskas syndrome, which is typified by cerebral parenchymal underdevelopment, clubfeet, and arthrogryposis. Biallelic truncating variants occur with severe disease resulting in miscarriage or early neonatal death, whereas biallelic missense variants can occur with a milder phenotype of global developmental delay and intracranial malformation. This suggests that hypomorphic alleles in KIAA1109 give rise to a milder phenotype than do amorphic alleles. We describe a consanguineous family with pseudodominant segregation of a homozygous noncanonical splice donor variant (NM_015312.2:c.[13438+3A>G];[13438+3A>G]) in mother and daughter. In peripheral blood, sequencing of cDNA detected skipping of exon 76 (NM_015312.3:c.13281_13438del) and, by qRT-PCR quantification, occurred in 82-95% of peripheral blood KIAA1109 mRNA. Although the deletion of exon 76 is predicted to encode p.(Trp4428Serfs*4), 46-83% of KIAA1109 mRNA in peripheral blood evaded nonsense mediated mRNA decay as measured by qRT-PCR. These observations expand understanding of the genotype-phenotype association in KIAA1109-related disease and suggest hypotheses for milder presentations of Alkuraya-Kucinskas syndrome.