PNC2 (SLC25A36) Deficiency Associated With the Hyperinsulinism/Hyperammonemia Syndrome

Abstract

Context The hyperinsulinism/hyperammonemia (HI/HA) syndrome, the second most common form of congenital hyperinsulinism, has been associated to dominant mutations in GLUD1, coding for the mitochondrial enzyme glutamate dehydrogenase, that increase enzyme activity by reducing its sensitivity to allosteric inhibition by GTP. Objective To identify the underlying genetic aetiology in two siblings who presented with the biochemical features of HI/HA syndrome but did not carry pathogenic variants in GLUD1, and to determine the functional impact of the newly identified mutation. Main Outcome Measures The patients were investigated by whole exome sequencing. Yeast complementation studies and biochemical assays on the recombinant mutated protein were performed. The consequences of stable slc25a36 silencing in HeLa cells were also investigated. Results A homozygous splice site variant was identified in solute carrier family 25, member 36 (SLC25A36), encoding the pyrimidine nucleotide carrier 2 (PNC2), a mitochondrial nucleotide carrier that transports pyrimidine as well as guanine nucleotides across the inner mitochondrial membrane. The mutation leads to a 26 aa in-frame deletion in the first repeat domain of the protein which abolished transport activity. Furthermore, knockdown of slc25a36 expression in HeLa cells caused a marked reduction in the mitochondrial GTP content which likely leads to an hyperactivation of glutamate dehydrogenase in our patients. Conclusions We report for the first time a mutation in PNC2/SLC25A36 leading to HI/HA and provide functional evidence of the molecular mechanism responsible for this phenotype. Our findings underscore the importance of mitochondrial nucleotide metabolism and expand the role of mitochondrial transporters in insulin secretion.

Hyperinsulinism/hyperammonemia syndrome caused by biallelic SLC25A36 mutation

“…The results indicated that the mutation changes the 5′ consensus sequence of the donor site of exon 3 and leads to exon 3 skipping (Figure 2A,B). These data confirmed the findings of Shahroor et al 9 …”

“…Through linkage analysis, whole‐exome sequencing (WES), and segregation analysis, a homozygous SLC25A36 mutation was identified at the splicing canonical donor site between exons 3–4, resulting in skipping of exon 3. Our results are in line with the two recent independent studies, demonstrating SLC25A36 mutations causing HI/HA syndrome 8,9 . We clearly delineate the disease phenotype and summarize the seven cases known to date.…”

“…While most cases of HI/HA syndrome described to date are due to dominant GLUD1 mutations, two recent reports demonstrated that this syndrome can be caused also by biallelic mutations in SLC25A36 8,9 . We now confirm and extend these findings: we present four children of consanguineous Bedouin kindred with an apparently autosomal recessive consistent HI/HA syndrome phenotype, yet no mutations in GLUD1 .…”

Ribonucleotide synthesis by NME6 fuels mitochondrial gene expression

“…While several mitochondrial solute carriers are known to exchange adenine nucleotides across the inner membrane, only two mitochondrial pyrimidine (deoxy)nucleotide carriers (SLC25A33 and SLC25A36) have been identified in mammalian cells 24,25 , both of which also transport guanine nucleotides in vitro 24 . Defective mitochondrial uptake of guanine nucleotides was recently associated with hyperinsulinism/hyperammonemia syndrome in patients with deleterious mutation of SLC25A36 26 . Loss of the single homologue of SLC25A33 and SLC25A36 in yeast, Rim2, leads to mtDNA depletion and blocks growth on non-fermentable carbon sources 27 .…”

Congenital hyperinsulinism in a Nigerian infant: A case report and review of literature

“…[10] e other common type of CHI is the activating mutation of glutamate dehydrogenase enzyme gene (GLUD1 on chromosome 10), which causes hyperinsulinism (HI)/hyperammonemia syndrome. [11,12] ese mutations impair the regulation of amino acid-stimulated insulin secretion leading to fasting hypoglycemia and leucine sensitive hypoglycemia. [10] Other rarer forms of HI exist and most of them are diazoxide responsive like the GLUD1 mutations.…”