Mitochondrial disorders: challenges in diagnosis & treatment.

Mitochondrial dysfunctions are known to be responsible for a number of heterogenous clinical presentations with multi-systemic involvement. Impaired oxidative phosphorylation leading to a decrease in cellular energy (ATP) production is the most important cause underlying these disorders. Despite significant progress made in the field of mitochondrial medicine during the last two decades, the molecular mechanisms underlying these disorders are not fully understood. Since the identification of first mitochondrial DNA (mtDNA) mutation in 1988, there has been an exponential rise in the identification of mtDNA and nuclear DNA mutations that are responsible for mitochondrial dysfunction and disease. Genetic complexity together with ever widening clinical spectrum associated with mitochondrial dysfunction poses a major challenge in diagnosis and treatment. Effective therapy has remained elusive till date and is mostly efficient in relieving symptoms. In this review, we discuss the important clinical and genetic features of mitochondrials disorders with special emphasis on diagnosis and treatment.

Co-occurrence of m.1555A>G and m.11778G>A mitochondrial DNA mutations in two Indian families with strikingly different clinical penetrance of Leber hereditary optic neuropathy.

BACKGROUND: Mitochondrial DNA (mtDNA) mutations are known to cause Leber hereditary optic neuropathy (LHON). However, the co-occurrence of double pathogenic mutations with different pathological significance in pedigrees is a rare event. METHODS: Detailed clinical investigation and complete mtDNA sequencing analysis was performed for two Indian families with LHON. The haplogroup was constructed based on evolutionarily important mtDNA variants. RESULTS: We observed the existence of double pathogenic mutations (m.11778G>A and m.1555A>G) in two Indian LHON families, who are from different haplogroup backgrounds (M5a and U2e1), with different clinical penetrance of the disease (visual impairment). The m.11778G>A mutation in the MT-ND4 gene is associated primarily with LHON; whereas, m.1555A>G in the 12S rRNA gene has been reported with aminoglycoside-induced non-syndromic hearing loss. CONCLUSIONS: The absence of hearing abnormality and widely varying clinical expression of LHON suggest additional nuclear modifier genes, environmental factors, and population heterogeneity might play an important role in the expression of visual impairment in these families.

Outcome of epilepsy in patients with mitochondrial disorders: Phenotype genotype and magnetic resonance imaging correlations.

OBJECTIVES: Studies exploring the outcome of epilepsy in patients with mitochondrial disorders are limited. This study examined the outcome of epilepsy in patients with mitochondrial disorders and its relation with the clinical phenotype, genotype and magnetic resonance imaging findings. PATIENTS AND METHODS: The cohort was derived from the database of 67 patients with definite genetic diagnosis of mitochondrial disorders evaluated over a period of 11years (2006-2016). Among this, 27 had epilepsy and were included in final analysis. Data were analyzed with special reference to clinical phenotypes, genotypes, epilepsy characteristics, EEG findings, anti epileptic drugs used, therapeutic response, and magnetic resonance imaging findings. Patients were divided into three groups according to the seizure frequency at the time of last follow up: Group I- Seizure free; Group II- Infrequent seizures; Group III- uncontrolled seizures. For each group the clinical phenotype, genotype, magnetic resonance imaging and duration of epilepsy were compared. RESULTS: The phenotypes & genotypes included Mitochondrial Encephalopathy Lactic Acidosis and Stroke like episodes (MELAS) & m.3243A>G mutation (n = 10), Myoclonic Epilepsy Ragged Red Fiber syndrome (MERRF) & m.8344A>G mutation (n = 4), Chronic Progressive External Ophthalmoplegia plus &POLG1 mutation (CPEO, n = 6), episodic neuroregression due to nuclear mutations (n = 6; NDUFV1 (n = 3), NDUFA1, NDUFS2, MPV17-1 one each), and one patient with infantile basal ganglia stroke syndrome, mineralizing angiopathy &MT-ND5 mutations. Seven patients (25.9%) were seizure free; seven had infrequent seizures (25.9%), while thirteen (48.1%) had frequent uncontrolled seizures. Majority of the subjects in seizure free group had episodic neuroregression & leukoencephalopathy due to nuclear mutations (85.7%). Patients in group II with infrequent seizures had CPEO, POLG1 mutation and a normal MRI (71%) while 62% of the subjects in group III had MELAS, m.3243A>G mutation and stroke like lesions on MRI. CONCLUSIONS: A fair correlation exists between the outcome of epilepsy, clinical phenotypes, genotypes and magnetic resonance imaging findings in patients with mitochondrial disorders. The recognition of these patterns is important clinically because of the therapeutic and prognostic implications.

Leber's Hereditary Optic Neuropathy-Specific Mutation m.11778G>A Exists on Diverse Mitochondrial Haplogroups in India.

Purpose: Leber's hereditary optic neuropathy (LHON; OMIM 535000) is one of the most common maternally inherited mitochondrial disorders. Three mitochondrial DNA point mutations-m.3460G>A (MT-ND1), m.11778G>A (MT-ND4), and m.14484T>C (MT-ND6)-account for the majority of reported LHON cases. Only approximately 50% of males and approximately 10% of females carrying these mutations develop optic neuropathy and blindness. Additional factors, such as mtDNA/nuclear genetic background and environmental modifiers, are likely to contribute toward the observed incomplete penetrance and gender bias. We aimed to investigate whether mtDNA haplogroup influences LHON clinical expression in Indian patients harboring the m.11778G>A mutation. Methods: Detailed clinical assessment and complete mitochondrial genome sequencing was undertaken in 64 LHON families harboring the m.11778G>A mutation. Mitochondrial haplogroup was assigned based on evolutionarily conserved mtDNA variations. Results: A total of 543 individuals (295 male, 248 female) from 64 unrelated families harboring the m.11778G>A mutation were recruited to the study. The overall disease penetrance was 27.07% (146 of 543) and higher in males (37.9%; 112 of 295) than females (13.7%; 34 of 248). The mtDNA haplogroup analysis revealed that all affected probands belonged to different mtDNA haplogroups. No association between the m.11778G>A mutation and the background mtDNA haplogroup was detected. Conclusions: The first detailed study of Indian LHON patients confirm that the m.11778G>A-related LHON in India coexists with multiple different mtDNA haplogroups, unlike the preferential association of west Eurasian haplogroup J and the reported increased clinical penetrance with the J2 subhaplogroup. However, we observed variable penetrance of LHON in different Indian mtDNA haplogroup backgrounds, indicating their possible influence on clinical expression. These data suggest that a similar heterogeneity, resulting from the mtDNA haplogroup, might also exist in other mitochondrial diseases among Indian populations.

Mitochondrial oxidative phosphorylation disorders in children: Phenotypic, genotypic and biochemical correlations in 85 patients from South India.

Mitochondrial oxidative phosphorylation (OXPHOS) disorders account for a variety of neuromuscular disorders in children. In this study mitochondrial respiratory chain enzymes were assayed in muscle tissue in a large cohort of children with varied neuromuscular presentations from June 2011 to December 2013. The biochemical enzyme deficiencies were correlated with the phenotypes, magnetic resonance imaging, histopathology and genetic findings to reach a final diagnosis. There were 85 children (mean age: 6.9±4.7years, M:F:2:1) with respiratory chain enzyme deficiency which included: isolated complex I (n=50, 60%), multiple complexes (n=24, 27%), complex IV (n=8, 9%) and complex III deficiencies (n=3, 4%). The most common neurological findings were ataxia (59%), hypotonia (59%) and involuntary movements (49%). A known mitochondrial syndrome was diagnosed in 27 (29%) and non-syndromic presentations in 57 (71%). Genetic analysis included complete sequencing of mitochondrial genome, SURF1, POLG1&2. It revealed variations in mitochondrial DNA (n=8), SURF1 (n=5), and POLG1 (n=3). This study, the first of its kind from India, highlights the wide range of clinical and imaging phenotypes and genetic heterogeneity in children with mitochondrial oxidative phosphorylation disorders.

The "double panda" sign in Leigh disease.

Although the "face of the giant panda" sign on magnetic resonance imaging (MRI) is traditionally considered to be characteristic of Wilson disease, it has also been reported in other metabolic disorders. This study describes the characteristic "giant panda" sign on MRI in a child with Leigh disease. The diagnosis was based on the history of neurological regression; examination findings of oculomotor abnormalities, hypotonia, and dystonia; raised serum lactate levels; and characteristic brain stem and basal ganglia signal changes on MRI. The midbrain and pontine tegmental signal changes were consistent with the "face of the giant panda and her cub" sign. In addition to Wilson disease, metabolic disorders such as Leigh disease should also be considered in the differential diagnosis of this rare imaging finding.

Mitochondrial DNA variations associated with hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy (HCM) is a primary disorder, characterized by unexplained hypertrophy of the left ventricle that frequently involved in the inter-ventricular septum. Mitochondrial DNA (mtDNA) mutations and haplogroups have been found to be associated with several diseases. Therefore, in the present study, we have sequenced the complete mtDNA of 114 clinically well-characterized HCM patients to look for the role of mtDNA variations and haplogroups in HCM phenotype among Indian patients. Complete mtDNA analysis revealed 28 novel variations, 25 disease-associated and 50 private mutations. We found 13 (11.40%) HCM patients having novel non-synonymous and/or MT-tRNA variations, of which two (m.4797C>M and m.8728T>Y) were in heteroplasmic condition. In silico prediction showed that a few mutations are pathogenic, which may affect the energy production in the heart. Unlike some of the other studies, we did not find association of mitochondrial haplogroup with HCM.

Mitochondrial DNA variations in Madras motor neuron disease.

Although the Madras motor neuron disease (MMND) was found three decades ago, its genetic basis has not been elucidated, so far. The symptom at onset was impaired hearing, upper limb weakness and atrophy. Since some clinical features of MMND overlap with mitochondrial disorders, we analyzed the complete mitochondrial genome of 45 MMND patients and found 396 variations, including 13 disease-associated, 2 mt-tRNA and 33 non-synonymous (16 MT-ND, 10 MT-CO, 3 MT-CYB and 4 MT-ATPase). A rare variant (m.8302A>G) in mt-tRNA(Leu) was found in three patients. We predict that these variation(s) may influence the disease pathogenesis along with some unknown factor(s).

Haplogroup heterogeneity of LHON patients carrying the m.14484T>C mutation in India.

PURPOSE: To investigate the clinical and mitochondrial DNA (mtDNA) haplogroup background of Indian Leber hereditary optic neuropathy (LHON) patients carrying the m.14484T>C mutation. METHODS: Detailed clinical investigation and complete mtDNA sequencing analysis was carried out for eight Indian LHON families with the m.14484T>C mutation. Haplogroup was constructed based on the evolutionarily important mtDNA variants. RESULTS: In the present study, we characterized eight unrelated probands selected from 187 LHON cases. The overall penetrance of the disease was estimated to be 19.75% (16/81) in eight pedigrees with the m.14484T>C mutation and showed substantially higher sex bias (male: female = 13:3). The mtDNA haplogrouping revealed that they belong to diverse haplogroups; i.e., F1c1, M31a, U2a, M*, I1, M6, M3a1, and R30a. Interestingly, we did not find an association of the m.14484T>C mutation with any specific haplogroup within the Indian population. We also did not find any secondary mutation(s) in these pedigrees, which might affect the clinical expression of LHON. CONCLUSIONS: Contrary to earlier reports showing preferential association of the m.14484T>C mutation with western Eurasian haplogroup J and increased clinical penetrance when present in J1 subhaplogroup background, the present study shows that m.14484T>C arose independently in a different mtDNA haplogroup and ethnic background in India, which may influence the clinical expression of the disease.

Mitochondrial dysfunction and genetic heterogeneity in chronic periodontitis.

We performed an extensive study on mitochondrial dysfunction in chronic periodontitis (CP). Electron microscopic analysis of gingival cells revealed abnormal mitochondria in 60% of the patients. Mitochondrial membrane potential and oxygen consumption of gingival cells were reduced by 4 fold and 5.8 fold, respectively; whereas ROS production was increased by 18%. The genetic analysis by complete mitochondrial DNA sequencing revealed the identification of 14 novel mutations only in periodontal tissues but not in the blood, suggesting a role of oxidative stress on periodontal tissues. Thus, our functional and genetic analysis provided an evidence for the mitochondrial dysfunction in CP.