Heterozygous loss-of-function variant in <i>METTL5</i> is associated with intellectual disability

Biallelic pathogenic variants in METTL5, which encodes an RNA methyltransferase involved in the m6A modification of 18S rRNA, have been reported as a rare etiology of autosomal recessive intellectual disability (ID). However, However, the clinical relevance of heterozygous variants in this gene remains unclear. In this study, we report a heterozygous frameshift variant located in exon 1 of the METTL5 gene identified in a father and son presenting with ID. Functional analyses demonstrated that this variant abolishes conserved domains, markedly reduces protein expression, and impairs 18S rRNA m6A modification, accompanied by decreased global protein synthesis. Proteomic profiling further revealed a downregulation of neuronal proteins involved in neuron projection, providing biological evidence supporting a potential role of METTL5 haploinsufficiency in neurodevelopmental processes. These observations suggest that heterozygous loss-of-function variants in METTL5, particularly those occurring early in the coding sequence, may be associated with an increased risk of neurodevelopmental disorders. Taken together, our study expands the mutational and phenotypic spectrum of METTL5-related disorders.