Pyroxd1 is essential for murine viability with the homozygous N155S recurrent variant linked to myopathy, muscle hypotrophy and osteopenia

Biallelic variants in PYROXD1 are associated with a life limiting muscle and connective tissue disorder characterised by generalised muscle weakness, breathing and feeding difficulties, distal laxity, hypernasal speech, blue sclera and osteopenia. PYROXD1 encodes an oxidoreductase implicated in mitochondrial function and tRNA ligase activity. Herein we present the first comprehensive suite of mouse models designed to elucidate PYROXD1 redox functions and the underlying pathogenetic basis for PYROXD1 disorders. A LacZ reporter strain reveals ubiquitous expression of Pyroxd1, particularly in the developing head, eye, heart and skin. Complete knockout (KO) of Pyroxd1 resulted in embryonic lethality between ED4.5–E9.5, suggesting Pyroxd1 performs a unique function during embryogenesis that cannot be substituted by other redox enzymes. Skeletal muscle tissue-specific KO, or stage-specific tamoxifen-induced KO during gestation or post-weaning, were viable but not sustainable models. Tamoxifen-inducible KO cell lines derived from these models provide valuable tools for dissecting Pyroxd1 function. Homozygous mice harbouring the recurrent human variant NM_024854.5:c.464A > G;p.(N155S), termed Pyroxd1N155S, phenocopied a severe PYROXD1 disorder, presenting from ~ 10 weeks of age with a progressive myopathy, myofibrillar disorganisation, decreased contractile strength, muscle hypotrophy and osteopenia. Conversely, homozygous Pyroxd1N155G mice, created incidentally during CRISPR editing, were phenotypically normal and provide an important benign control. Proteomic analyses reveal distinct molecular signatures between acute Pyroxd1 KO and Pyroxd1N155S models, highlighting differential effects of complete loss-of-function (KO) compared to partial enzymatic activity (N155S). Pyroxd1 activity is critical for numerous cell essential processes, including protein biosynthesis and turnover, tRNA ligase complex activity and the unfolded protein response, mitochondrial respiratory chain function (especially complex 1), with an implicated broader role in the exon-ligation step of pre-mRNA splicing. Together, these murine Pyroxd1 models establish an enabling platform for gaining mechanistic insight and guiding the path of future therapeutic development for PYROXD1 disorders.