The Role Of Mitochondrial Dysfunction In The Pathogenesis Of Inherited Optic Neuropathies

Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (ADOA) are the two most common inherited optic neuropathies and they result in significant visual morbidity among young adults. Both disorders are the result of mitochondrial dysfunction. LHON from primary mitochondrial DNA (mtDNA) mutations affecting the respiratory chain complexes, and the majority of ADOA families have mutations in the OPA1 gene, which codes for an inner mitochondrial membrane protein critical for mitochondrilal morphology and mtDNA maintenance. Furthermore, additional modifier factors (such as mitochondrial haplogroup, mtDNA secondary mutations and nuclear genes) modulate the penetrance and expressivity of inherited optic neuropathy.Part I:m.l4484T>C (MT-ND6) mutation has been associated with LHON. Previous investigations revealed that the m.14484T>C mutation is a primary factor underlying the development of optic neuropathy but is not sufficient to produce a clinical phenotype. However, mitochondrial haplogroups have been proposed to modulate the phenotypic manifestation of the m.14484T>C mutation. Here, we performed the clinical, genetic evaluation and complete mitochondrial genome sequence analysis of 41 Chinese Han pedigrees carrying the m.14484T>C mutation. We showed that mitochondrial haplogroups M9, M10 and N9 increased the penetrance of optic neuropathy in these Chinese families. In particular, these mitochondrial haplogroup specific variants, m.3394T>C (MT-ND1), m.14502T>C (MT-ND4) and m.14693A>G (MT-TE) enhanced the penetrance of visual loss in these Chinese families. These data provided the direct evidence that mitochondrial modifiers modulate the variable penetrance and expressivity of optic neuropathy among Chinese pedigrees carrying the m.14484T>C mutation.Part II:In this report, we investigated the molecular mechanism underlying LHON-associated MT-ND1 m.3635G>A mutation. A mutational screening of MT-ND1 gene in a cohort of 1,070 Han Chinese LHON subjects identified the m.3635G>A mutation in nine Chinese families with suggestively maternally transmitted LHON. Using cybrids constructed by transferring mitochondria from lymphoblast cell lines derived from a Chinese family into mtDNA-less (ρ0) cells, we showed～27% decrease in the activity of complex I in mutant cybrids carrying the m.3635G>A mutation. The respiratory deficiency caused by the m.3635G>A mutation resulted in decreased efficiency of mitochondrial ATP synthesis and membrane potential (ΔΨm). These mitochondrial dysfunctions caused an increase in the production of ROS in the mutant cybrids. The data provide the direct evidence for the m.3635G>A mutation leading to LHON. Our findings may provide new insights into the pathophysiology of LHON.Part III:This study investigated the role of c.1198C>G (p.P400A) heterozygous mutation which is located in G3 motif of GTPase domain in OPA1gene. It may lead to mitochondrial structure and function disordered. In order to understanding the molecular mechanism of this mutation, we examined mitochondrial function in lymphoblost cell lines. We showed that～25% decrease in the expression of OPA1 gene. Transmission electron microscope revealed mitochondrial fragmentation, swollen and cristae structure disordered in mutant cells. The respiratory deficiency was responsible for the variable reductions in seven mtDNA-encoded polypeptides in mutant cells, ranging from 18.6% to 61.1%, with the average of～36.1% reduction, as compared with those of control cells. In addition, oxidative phosphorylation (OXPHOS) enzymology in ADOA patients revealed lower complex Ⅳ activity. Furthermore, marked decreases in the levels of mitochondrial ATP and ΔΨm were observed in mutant cells. These mitochondrial dysfunctions caused an increase in the production of ROS compared to controls. Mutant cells showed a higher rate of apoptosis and autophagy than controls. All those changes resulted in the selective loss of RGCs and vision loss.These research will provide the theory basis for diagnosis, treatment, intervention, visual prognosis and genetic counseling in inherited optic neuropathies.