Transition to next generation analysis of the whole mitochondrial genome: a summary of molecular defects

The diagnosis of mitochondrial disorders is challenging because of the clinical variability and genetic heterogeneity. Conventional analysis of the mitochondrial genome often starts with a screening panel for common mitochondrial DNA (mtDNA) point mutations and large deletions (mtScreen). If negative, it has been traditionally followed by Sanger sequencing of the entire mitochondrial genome (mtWGS). The recently developed "Next-Generation Sequencing" (NGS) technology offers a robust high-throughput platform for comprehensive mtDNA analysis. Here, we summarize the results of the past 6 years of clinical practice using the mtScreen and mtWGS tests on 9,261 and 2,851 unrelated patients, respectively. A total of 344 patients (3.7%) had mutations identified by mtScreen and 99 (3.5%) had mtDNA mutations identified by mtWGS. The combinatorial analyses of mtDNA and POLG revealed a diagnostic yield of 6.7% in patients with suspected mitochondrial disorders but no recognizable syndromes. From the initial mtWGS-NGS cohort of 391 patients, 21 mutation-positive cases (5.4%) have been identified. The mtWGS-NGS provides a one-step approach to detect common and uncommon point mutations, as well as deletions. Additionally, NGS provides accurate, sensitive heteroplasmy measurement, and the ability to map deletion breakpoints. A new era of more efficient molecular diagnosis of mtDNA mutations has arrived.