Functional characterization of CDK5 and CDK5R1 mutations identified in patients with non-syndromic intellectual disability

Cyclin-dependent kinase 5 ( CDK5 ) and cyclin-dependent kinase 5, regulatory subunit 1 ( CDK5R1 ), encoding CDK5 activator p35, have a fundamental role in central nervous system (CNS) development and function, and are involved in the pathogenesis of several neurodegenerative disorders, thus constituting strong candidate genes for the onset of intellectual disability (ID). We carried out a mutation screening of CDK5 and CDK5R1 coding regions and CDK5R1 3′-UTR on a cohort of 360 patients with non-syndromic ID (NS-ID) using denaturing high performance liquid chromatography (DHPLC) and direct sequencing. We found one novel silent mutation in CDK5 and one novel silent mutation in CDK5R1 coding regions, three novel intronic variations in CDK5 , not causing any splicing defect, and four novel heterozygous variations in CDK5R1 3′-UTR. None of these variations was present in 450 healthy controls and single-nucleotide polymorphism (SNP) databases. The functional study of CDK5R1 p.A108V mutation evidenced an impaired p35 cleavage by the calcium-dependent protease calpain. Moreover, luciferase constructs containing the CDK5R1 3′-UTR mutations showed altered gene expression levels. Eight known polymorphisms were also identified displaying different frequencies in NS-ID patients compared with the controls. In particular, the minor allele of CDK5R1 3′-UTR rs735555 polymorphism was associated with increased risk for NS-ID. In conclusion, our data suggest that mutations and polymorphisms in CDK5 and CDK5R1 genes may contribute to the onset of the NS-ID phenotype.