Mutation profile of neurodegenerative mitochondriopathy - LHON in Southern India

Leber's Hereditary Optic Neuropathy (LHON) is a rare mitochondriopathy causing retinal ganglion cell degeneration resulting in central vision loss. It is caused by mitochondrial DNA (mtDNA) mutations and thus follows maternal inheritance pattern.

We analysed the whole mitochondrial genome in 100 South Indian LHON patients by utilizing Sanger and Next Generation Sequencing approaches. Haplogroup analysis was performed using HaploGrep2 to predict the risk group. Methylation changes in the mtDNA D-loop region were investigated by performing methylation-specific polymerase chain reaction (MSP).

LHON associated mutations were detected in 55% of the patients of which 42% harboured the primary mutations and 13% harboured potentially pathogenic variants that were previously reported to cause LHON. The candidate mutations identified with confirmed pathogenicity are: m.11778G > A (38%), m.14484 T > C (3%), m.4171C > A (1%) and m.11696G > A (1%). MSP results demonstrated that the D-loop region was unmethylated in all the study subjects including mutation-positive patients, mutation-negative patients, asymptomatic carriers, and controls. Haplogroup-M was prevalent (69%) in the study cohort followed by R (14%), U (9%), N (3%), HV (2%), G (2%), and W (1%). The frequency of the predominant mutation m.11778G > A was found lower (̴ 11%) in haplogroup-U.

South Indian LHON cohort shows a unique profile of mtDNA mutations and haplogroup association presumably with no role of D-loop methylation. MT-ND4, MT-ND5, and MT-ND1 serve as the hotspot genes in this cohort. The presence of LHON associated mutations in patients lacking the common primary mutations insists on the necessity of mitochondrial genome sequencing in individuals suspected with LHON.