Bi-allelic variants in TM2D3 cause a severe syndromic neurodevelopmental disorder associated with endoplasmic reticulum and mitochondrial abnormalities.

We identified via exome sequencing bi-allelic variants in TM2D3 in four affected individuals from four unrelated families with overlapping clinical presentations, including microcephaly, severe global developmental delay with absent speech, autistic features, heart malformation, and dysmorphic facial features. TM2D3 encodes a transmembrane protein present in many tissues, with a higher abundance in the central nervous system, but little is known about its function and cell localization. Here, by using chemical and genetically encoded probes in SNB75 cells, we show that TM2D3 is an endoplasmic reticulum (ER) protein. Further analysis on SNB75 TM2D3-knockout cells as well as skin fibroblasts from affected individuals harboring the recurrent c.503G>A (p.Gly168Asp) allele revealed an impact of TM2D3 on ER-stress response, with dysregulated expression of ATF4, HSPA5, and DDIT3. Transmission electron microscopy highlighted ER swelling as well as unexpected secondary mitochondrial alterations including increased length, cristae width, and ER-mitochondria distance. To gain further insights into the pathomechanisms at play, we performed RNA sequencing from the fibroblasts of the three individuals harboring the p.Gly168Asp variant and four available parents and disclosed 21 differentially expressed genes, including genes coding for extracellular matrix components involved in the migration of neuronal precursors. Altogether, these clinical and experimental data show that bi-allelic TM2D3 variants underlie a severe syndromic neurodevelopmental disorder linked to exacerbated ER-stress sensitivity, secondary mitochondrial alterations, and altered extracellular matrix gene expression.