HADH Knockout Cell Line (HEK293)

Short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), encoded by the HADH gene, is a ubiquitously expressed mitochondrial enzyme involved in fatty acid oxidation. This protein also plays a role in insulin secretion as recessive HADH mutations cause congenital hyperinsulinism of infancy (CHI) via loss of an inhibitory interaction with glutamate dehydrogenase (GDH). Here, we present a functional evaluation of 16 SCHAD missense variants identified either in CHI patients or by high-throughput sequencing projects in various populations. To avoid interactions with endogenously produced SCHAD protein, we assessed protein stability, subcellular localization, and GDH interaction in a SCHAD knockout HEK293 cell line constructed by CRISPR-Cas9 methodology. We also established methods for efficient SCHAD expression and purification in E. coli, and tested enzymatic activity of the variants. Our analyses showed that rare variants of unknown significance identified in populations generally had similar properties as normal SCHAD. However, the CHI-associated variants p.Gly34Arg, p.Ile184Phe, p.Pro258Leu, and p.Gly303Ser were unstable with low protein levels detectable when expressed in HEK293 cells. Moreover, CHI variants p.Lys136Glu, p.His170Arg, and p.Met188Val presented normal protein levels but displayed clearly impaired enzymatic activity in vitro, and their interaction with GDH appeared reduced. Our results suggest that pathogenic missense variants of SCHAD either make the protein target of a post-translational quality control system or can impair the function of SCHAD without influencing its steady-state protein level. We did not find any evidence that rare SCHAD missense variants observed only in the general population and not in CHI patients are functionally affected.

BACKGROUND:Short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) is a ubiquitously expressed mitochondrial enzyme with a role in the degradation of fatty acids. Because the protein also is a negative regulator of insulin secretion in pancreatic β-cells, inactivating mutations in the SCHAD gene (HADH) cause congenital hyperinsulinism of infancy (CHI) and severe hypoglycemia. Here we sought to identify novel interaction partners of SCHAD that might be particularly relevant for the endocrine pancreas. RESULTS:Employing the SCHAD protein as bait, we performed yeast 2-hybrid screening of a cDNA library made from human islets of Langerhans. Surprisingly, the screening revealed the intermediate filament protein keratin 8 (K8) as a putative interaction partner of SCHAD with very high confidence. Previous reports have linked K8 to glucose homeostasis, and we confirmed the SCHAD interaction by co-immunoprecipitation in HEK293 cells. SCHAD and K8 expression were then characterized in the human β-cell model EndoC-βH1. By using proximity ligation assay, we demonstrated that stimulating the cells with a high level of glucose triggered a transient increase in the interaction. However, when studying knockout mice, we found that the loss of either K8 or SCHAD did not change the expression level of the other interaction partner. Still, when K8 knockout mice were challenged with a ketogenic diet, upregulation of SCHAD expression was blunted compared to the upregulation observed in wildtype littermates. CONCLUSIONS:We propose that the SCHAD protein interacts with K8 in a way that might be relevant for proper functioning of the pancreatic β-cell. Whether the SCHAD-K8 interaction influences the phenotype of CHI remains to be demonstrated.