ABSTRACT
Defective mitochondrial proteins are emerging as major contributors to human disease. Nicotinamide nucleotide transhydrogenase (NNT), a widely expressed mitochondrial protein, has a crucial role in the defence against oxidative stress. NNT variations have recently been reported in patients with familial glucocorticoid deficiency (FGD) and in patients with heart failure. Moreover, knockout animal models suggest that NNT has a major role in diabetes mellitus and obesity. In this study, we used experimental structures of bacterial transhydrogenases to generate a structural model of human NNT (H-NNT). Structure-based analysis allowed the identification of H-NNT residues forming the NAD binding site, the proton canal and the large interaction site on the H-NNT dimer. In addition, we were able to identify key motifs that allow conformational changes adopted by domain III in relation to its functional status, such as the flexible linker between domains II and III and the salt bridge formed by H-NNT Arg882 and Asp830. Moreover, integration of sequence and structure data allowed us to study the structural and functional effect of deleterious amino acid substitutions causing FGD and left ventricular non-compaction cardiomyopathy. In conclusion, interpretation of the function-structure relationship of H-NNT contributes to our understanding of mitochondrial disorders.
Subject(s)
Mitochondrial Diseases/genetics , Mutation , NADP Transhydrogenase, AB-Specific/chemistry , NADP Transhydrogenase, AB-Specific/genetics , Amino Acid Sequence , Binding Sites , Genetic Predisposition to Disease , Humans , Mitochondrial Proteins/chemistry , Mitochondrial Proteins/genetics , Mitochondrial Proteins/metabolism , Models, Molecular , NAD/metabolism , NADP Transhydrogenase, AB-Specific/metabolism , Protein Binding , Protein Conformation , Protein DomainsABSTRACT
Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder characterized by low levels of cortisol despite high adrenocorticotropin (ACTH) levels, due to the reduced ability of the adrenal cortex to produce cortisol in response to stimulation by ACTH. FGD is a heterogeneous disorder for which causal mutations have been identified in MC2R, MRAP, MCM4 and TXNRD2. Also mutations in STAR and CYP11A1 can sometimes present with a phenotype resembling FGD. Recently, it has been indicated that FGD can also be caused by mutations in NNT (nicotinamide nucleotide transhydrogenase). We identified a 6.67 Mb homozygous region harboring the NNT gene by SNP haplotyping in a 1-year old Dutch boy presenting with FGD, but without mutations in MC2R and MRAP. Exome-sequencing revealed a novel homozygous mutation (NM_012343.3: c.1259dupG) in NNT that was predicted to be disease-causing. The mutation is located in exon 9 and creates a frameshift leading to a premature stop-codon (p.His421Serfs*4) that is known to result in FGD. Both parents were shown to be heterozygous carriers. We reviewed the literature for all the reported NNT mutations and their clinical presentation. The median age of disease onset in 23 reported patients, including the present patient, was 12 months (range 3 days-39 months). There was no difference in age of disease onset between truncating and non-truncating NNT mutations. Based on recent literature, we advise to monitor patients with FGD due to NNT mutations for possible combined mineralocorticoid insufficiency and extra-adrenal manifestations.