In-frame deletion of SMC5 related with the phenotype of primordial dwarfism, chromosomal instability and insulin resistance

SMC5/6 complex plays a vital role in maintaining genome stability, yet the relationship with human diseases has not been described.

SMC5 variation was identified through whole-exome sequencing (WES) and verified by Sanger sequencing. Immunoprecipitation, cytogenetic analysis, fluorescence activated cell sorting (FACS) and electron microscopy were used to elucidate the cellular consequences of patient's cells. smc5 knockout (KO) zebrafish and Smc5K371del knock-in mouse models were generated by CRISPR-Cas9. RNA-seq, quantitative real-time PCR (qPCR), western blot, microquantitative computed tomography (microCT) and histology were used to explore phenotypic characteristics and potential mechanisms of the animal models. The effects of Smc5 knockdown on mitotic clonal expansion (MCE) during adipogenesis were investigated through Oil Red O staining, proliferation and apoptosis assays in vitro.

We identified a homozygous in-frame deletion of Arg372 in SMC5, one of the core subunits of the SMC5/6 complex, from an adult patient with microcephalic primordial dwarfism, chromosomal instability and insulin resistance. SMC5 mutation disrupted its interaction with its interacting protein NSMCE2, leading to defects in DNA repair and chromosomal instability in patient fibroblasts. Smc5 KO zebrafish showed microcephaly, short length and disturbed glucose metabolism. Smc5 depletion triggers a p53-related apoptosis, as concomitant deletion of the p53 rescued growth defects phenotype in zebrafish. An smc5K371del knock-in mouse model exhibited high mortality, severe growth restriction and fat loss. In 3T3-L1 cells, the knockdown of smc5 results in impaired MCE, a crucial step in adipogenesis. This finding implies that defective cell survival and differentiation is an important mechanism linking growth disorders and metabolic homeostasis imbalance.