CONFIGURACIÓN DE UNA ESTRATEGIA PARA LA IDENTIFICACIÓN GENÉTICO-MOLECULAR DE PACIENTES CON ENFERMEDAD MITOCONDRIAL

Abstract

The mitochondrial disorders (MD) are a group of inherited metabolic diseases characterized by an alteration in the correct function of the mitochondria, mainly due to deficiencies of the oxidative phosphorylation system. These diseases are genetically and clinical heterogenic, due to their dual genetic origin (they can be caused due to mutations in the nuclear and in the mitochondrial genome) and their great phenotypic variability (the disease can affect only one organ or it can be multisystemic). Many times this group of disorders lacks of a genotype-phenotype correlation, since the same mutation can generate different phenotypes, and one specific phenotype can be caused by mutations in different genes, and this fact hampers the genetic diagnose of these disorders._x000D_ The hypothesis of this thesis proposes that the use of the next generation sequencing (NGS) technologies in the genetic diagnostic of the MD will improve its performance. The use of the NGS in the diagnostic of these diseases will allow the detection of new point mutations in genes already described. Furthermore, the diagnostic through NGS also will allow to associate genes related with a specific MD to new phenotypes, or even to discover new genes causing MD, thus expanding the genetic and phenotypic spectrum of MD and improving the performance of the genetic diagnostic._x000D_ In the present work we have integrated massive sequencing analysis of mitochondrial and nuclear genes to the diagnostic of patients with MD. We have set up and validated the complete mitochondrial DNA (mtDNA) sequencing that has enabled us to detect low levels of mtDNA heteroplasmy. Furthermore, the covering homogeneity of the complete mtDNA molecule has been optimized. This methodology has allowed us to precisely determine the concrete deleted sequences in single mtDNA deletions. Additionally, we have designed à la carte and validated a panel of genes directly related to mtDNA maintenance for diagnostic purposes. In patients with MD the clinic (n=8) and complete (n=9) exomes have been studied and analyzed. In this regard, new potentially mutagenic variants in both nuclear and mitochondrial DNA genes have been completely characterized, as well as potential phenotypic modulator variants in Non-Syndromic Sensoryneural Hearing Loss patients carrying the homoplasmic m.1555A>G mutation in the MTRNR1 gene. _x000D_ The application of the massive sequencing analysis technique in the diagnosis of these MD has represented an increase in the diagnosis efficiency, specially in those patients that had been well characterized clinically. MtDNA sequencing has allowed the detection in peripheric blood samples of pathogenic variants with a very low degree of heteroplasmy that were not previously detected by the classic Sanger sequencing, reducing the necessity of performing a muscle biopsy. In the complete mtDNA sequencing study, we have detected pathogenic variants that had not been previously associated to the phenotype of the patients. Additionally, the application of the genes panel related with mtDNA maintenance has proven to be very efficient, specially in those patients with multiple mtDNA deletions, being the POLG and TK2 the most representative in those patients. The clinic exome study in patients with Leigh syndrome has allowed us to genetically diagnose 2 out of 6 patients studied. Finally, in two patients with multi-enzymatic deficit of the oxidative phosphorylation system, we have found new potentially pathogenic variants, one in a gene previously associated with the YARS2 clinical phenotype, while the other in a mitochondrial gene previously not associated to any MD.

Date of Award	16 Nov 2017
Original language	Spanish
Supervisor	Elena Garcia Arumí (Director)