Challenges in Genetic Diagnosis of Mitochondrial Diseases: What Can Functional Genomics' Studies Do?

INTRODUCTION: Mitochondrial oxidative phosphorylation (OXPHOS) diseases are challenging both from clinical and therapeutic perspectives. The advent of next-generation sequencing (NGS) boosted the discovery of new genetic defects affecting OXPHOS, with pathogenic variants identified in >350 genes to date [1]. However, in many patients, novel variants of unknown clinical significance are found. Subsequent functional studies may clarify its pathogenic consequences and modify the variant's classification, establishing a genetic diagnosis [2, 3].

METHODS: Analysis of data obtained from patients (P1-P5) with novel genetic causes and functional genomics' studies performed, namely OXPHOS respiratory/glycolytic rates (Seahorse XF), enzymatic activity and assembly (BN-page), protein levels (SDS-WB), single muscle fiber assay, NGS and bioinformatics.

RESULTS/CASE REPORT: P1-Leigh syndrome (40y, male); Complex IV activity deficiency (full assembly absent), homozygous deletion (c.-11_13del, SURF1), not detected by NGS[2]. P2- Epileptic encephalopathy (8y, male); homozygous c.882-1G>A, FASTKD2; OXPHOS decrease; reduced FASTKD2 expression and abnormal respiratory/glycolytic rates. P3-Cardiomyopathy/ nephropathy (39y, male); c.29G>C, FASTKD2; OXPHOS decrease; reduced FASTKD2 levels. P4-CPEO (62y, female); multiple OXPHOS deficiency; mtDNA alterations (m.7486G>A, MTTS1; 4,977bp del); higher levels of mutant mtDNA alterations in COX-deficient fibers [3]. P5- Polyneuropathy (15y, female); heterozygous c.1437C>A, POLG; combined def. or normal OXPHOS activity/respiratory capacity (tissue variable), raised CI assembly; normal POLG levels. Also, proteins' expression levels were reduced (P1-4), confirming pathogenicity. In P5, data do not support pathogenicity.

CONCLUSION: If specific functional results are similar to controls, one might inquire about the pathogenicity of the studied variant and more genetic or bioinformatics analyses and family investigations are needed. There are also limitations of NGS in mutation detection that Sanger sequencing can overcome (P1). When performed first, the OXPHOS activity may guide to genetic screening or interpretation, concordant to later assembly results. All cases were solved and data may be crucial for genetic counseling.