Biallelic variants in DAP3 result in reduced assembly of the mitoribosomal small subunit with altered intrinsic and extrinsic apoptosis and a Perrault syndrome-spectrum phenotype

The mitoribosome synthesizes 13 protein subunits of the oxidative phosphorylation system encoded by the mitochondrial genome. The mitoribosome is composed of 12S rRNA, 16S rRNA and 82 mitoribosomal proteins encoded by nuclear genes. To date, variants in 12 genes encoding mitoribosomal proteins are associated with rare monogenic disorders, and frequently show combined oxidative phosphorylation deficiency. Here, we describe five unrelated individuals with biallelic variants in the nuclear gene encoding mitoribosomal small subunit 29 (MRPS29), with variable clinical presentations ranging from Perrault syndrome (sensorineural hearing loss and ovarian insufficiency) to an early childhood neurometabolic phenotype. Assessment of respiratory chain function and proteomic profiling of fibroblasts from affected individuals demonstrated reduced MRPS29 protein levels, and consequently decreased levels of additional protein components of the mitoribosomal small subunit, associated with a combined complex I and IV deficiency. Lentiviral transduction of fibroblasts from affected individuals with wild-type cDNA increased DAP3 mRNA expression, and partially rescued protein levels of MRPS7, MRPS9 and complex I and IV subunits, demonstrating the pathogenicity of the variants. Protein modelling suggested that disease-associated missense variants can impact ADP binding, and assays demonstrated variants can consequently reduce both intrinsic and extrinsic apoptotic sensitivity, DAP3 thermal stability and DAP3 GTPase activity. Our study presents genetic and functional evidence that biallelic variants in result in a multisystem disorder of combined oxidative phosphorylation deficiency with pleiotropic presentations, consistent with mitochondrial dysfunction.