A severe neurodevelopmental syndrome linked to a South Asian founder variant in the UFMylation adaptor CDK5RAP3.

We investigated the pathogenicity of a homozygous intronic variant in CDK5RAP3, a key UFMylation adapter, in three individuals from two unrelated families with a lethal neurodevelopmental disorder. CDK5RAP3 variants have not been linked to human disorders to date; however, murine Cdk5rap3 knockout is embryonic lethal and variants in five other UFMylation components cause severe neurodevelopmental conditions. A segregating homozygous variant, chr17(GRCh38):g.47974691G > A, CDK5RAP3 NM_176096.3:c.334 + 243G > A, was identified by trio whole-genome and proband RNA sequencing in Family A and by trio whole-exome sequencing data reanalysis in Family B. Variant pathogenicity investigations included RT-PCR, Western blot, co-immunoprecipitation and (phospho)proteomics to assess transcript, protein and UFMylation complex effects. Antisense oligonucleotide-mediated rescue of CDK5RAP3 expression combined with proteomics and phosphoproteomics defined the mechanistic impact of CDK5RAP3 deficiency and rescue in amniocytes from an affected individual. All three affected individuals showed foetal growth restriction, foetal akinesia, pontocerebellar hypoplasia, arthrogryposis and hepatic pathology. CDK5RAP3 c.334 + 243G > A activates a cryptic donor splice-site causing pseudoexon/intron inclusion triggering nonsense-mediated decay and deficiency of full-length CDK5RAP3 (NP_788276.1), while potentially allowing retained expression of C-terminal alternative isoforms. Co-immunoprecipitation revealed only full-length CDK5RAP3 binds UFL1, whereas C-terminal isoforms cannot. Primary amniocytes showed CDK5RAP3 deficiency was associated with impaired UFMylation of known substrates, RPL26 and UFBP1. Proteomic and phosphoproteomic analyses revealed dysregulation of extracellular matrix organisation, cell adhesion, mitotic/genome stability pathways, cytoskeletal networks and neuronal guidance, which were reversed by restoration of canonical CDK5RAP3 expression via splice-correcting antisense oligonucleotides. Phosphoproteomic data implicate CDK5RAP3 as an upstream regulator of UFL1 S462 phosphorylation, known to be regulated by Ataxia-telangiectasia mutated (ATM) signalling. Our findings provide strong evidence linking deficiency of full-length CDK5RAP3 to severe neurodevelopmental, liver and muscle dysfunction. This study further highlights the therapeutic potential of ASO-based deep-intronic splicing defect correction.