Isolated and Combined Remethylation Disorders: Biochemical and Genetic Diagnosis and Pathophysiology

Abstract Genetic defects affecting the remethylation pathway cause hyperhomocysteinemia. Isolated remethylation defects are caused by mutations of the 5, 10-methylenetetrahydrofolate reductase (MTHFR), methionine synthase reductase(MTRR), methionine synthase(MTR), and MMADHC genes, and combined remethylation defects are the result of mutations in genes involved in the synthesis of either methylcobalamin or adenosylcobalamin, that is, the active cofactors of MTRR and methylmalonyl-CoA mutase. Diagnosis is based on the biochemical analysis of amino acids, homocysteine, propionylcarnitine, methylmalonic acid, S-adenosylmethionine, and 5-methylentetrahydrofolate in physiological fluids. Gene-by-gene Sanger sequencing has long been the gold standard genetic analysis for confirming the disorder and identifying the gene involved, but massive parallel sequencing is now being used to examine all those potentially involved in one go. Early treatment to rescue metabolic homeostasis is based on the following of an appropriate diet, betaine administration, and, in some cases, oral or intramuscular administration of vitamin B12 or folate. Elevated ROS levels, apoptosis, endoplasmic reticulum (ER) stress, the activation of autophagy, and alterations in Ca2+ homeostasis may all contribute toward the pathogenesis of the disease. Pharmacological agents to restore the function of the ER and mitochondria and/or to reduce oxidative stress-induced apoptosis might provide novel ways of treating patients with remethylation disorders.

Entire Mitochondrial DNA Sequencing on Massively Parallel Sequencing for the Korean Population

Mitochondrial DNA (mtDNA) genome analysis has been a potent tool in forensic practice as well as in the understanding of human phylogeny in the maternal lineage. The traditional mtDNA analysis is focused on the control region, but the introduction of massive parallel sequencing (MPS) has made the typing of the entire mtDNA genome (mtGenome) more accessible for routine analysis. The complete mtDNA information can provide large amounts of novel genetic data for diverse populations as well as improved discrimination power for identification. The genetic diversity of the mtDNA sequence in different ethnic populations has been revealed through MPS analysis, but the Korean population not only has limited MPS data for the entire mtGenome, the existing data is mainly focused on the control region. In this study, the complete mtGenome data for 186 Koreans, obtained using Ion Torrent Personal Genome Machine (PGM) technology and retrieved from rather common mtDNA haplogroups based on the control region sequence, are described. The results showed that 24 haplogroups, determined with hypervariable regions only, branched into 47 subhaplogroups, and point heteroplasmy was more frequent in the coding regions. In addition, sequence variations in the coding regions observed in this study were compared with those presented in other reports on different populations, and there were similar features observed in the sequence variants for the predominant haplogroups among East Asian populations, such as Haplogroup D and macrohaplogroups M9, G, and D. This study is expected to be the trigger for the development of Korean specific mtGenome data followed by numerous future studies.