Reassessment of the Pathologic Significance of the 9438 Mitochondrial DNA mutation Associated with LHON

To the Editor:

In 1993, Johns and Neufeld described five unrelated patients with Leber hereditary optic neuropathy (LHON) who had the nt 9438 G > A mitochondrial DNA (mtDNA) nucleotide change in the gene coding for cytochrome c oxidase subunit III (COX III) of complex IV of the mitochondrial respiratory chain. They did not find this sequence change in 400 controls .and postulated that it was pathologically important.¹ The pathologic role of 9438 was questioned after it was detected in healthy African and Cuban individuals²,³ and in two Cubans with optic and peripheral neuropathy.⁴ In 1995, it was found together with the nt 11778 primary LHON mutation in two of 87 LHON patients (2.3%). The respiratory activity of cytochrome c oxidase was measured in cultured fibroblasts of a patient harbouring both the 11778 and 9438 nucleotide changes and of a patient with only the 11778 mutation, and presence of the 9438 nucleotide change had no effect on respiratory activity.⁵ It has now been reported in twelve of 2716 individuals (0.44%) in the Human Mitochondrial Genome Database (http://www.genpat.uu.se/mtDB), but the MitoMap website (http://www.mitomap.org) still lists 9438 as a secondary LHON mutation.

Four years ago we established a series of control populations for mitochondrial research and diagnostic testing at the King Faisal Specialist Hospital and Research Centre, in part by sequencing the full mtDNA coding region of 159 individuals (106 males and 53 females, mean age 46.3 ± 3.8 years) representing the spectrum of the Saudi Arabian population who reported no metabolic, genetic, mitochondrial, or ocular disorder on an extensive questionnaire regarding family history, past medical problems, and current health. Six of these 159 controls (3.8%) had the 9438 sequence variant. This variant was not present in 35 Saudi patients with LHON-like optic neuropathy,⁶ in 27 patients with primary open angle glaucoma⁷ or in 73 patients with other mitochondrial syndromes (3 with LHON; 10 with mitochondrial myopathy; 4 with primary lactic acidosis; 29 with mitochondrial cardiomyopathy; 16 with mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes; 5 with chronic progressive external ophthalmoplegia; 2 with Kearns-Sayre syndrome; and 4 with Leigh syndrome [⁸ and unpublished data]).

Pathogenicity of a mtDNA nucleotide change is difficult to establish unequivocally⁹; nevertheless, application of stringent criteria for potential pathogenicity⁶,⁷ to 9438 revealed that: 1) this sequence change resulted in the substitution of Serine for a highly conserved Glycine at codon 78 of the COX III gene; 2) the PolyPhen database (http://genetics.bwh.harvard.edu/pph/), which applies physical and interspecies conservation considerations to assessing the possible impact of an amino acid substitution on a human protein, predicted that this sequence change is benign with no substantial effect on the COX protein structure or function; and 3) the Protean program (part of the LASERGENE V.6 software; DNASTAR, Inc. Madison, WI, USA), which analyzes and displays secondary structural changes resulting from an amino acid change, predicted no alteration in the protein structure. A slight change in the hydropathy index from 0.84 to 0.89 was not considered significant.¹⁰

Based on the above data, we predict that nt 9438 G > A is a benign polymorphism with low prevalence in Saudi Arabian and other populations that appears to play no pathologic role in the occurrence of LHON or other mitochondrial disorders screened thus far.