Segregation patterns and heteroplasmy prevalence in Leber's hereditary optic neuropathy

To investigate the segregation pattern of the mitochondrial DNA mutation at nucleotide position 3460 responsible for Leber's hereditary optic neuropathy (LHON) and to determine the prevalence of heteroplasmy for the three primary LHON mutations at positions 11778, 3460, and 14484.

Segregation analysis was performed in a cross-sectional study by determining the level of heteroplasmy in blood leukocytes of 23 LHON patients and unaffected carriers from four unrelated families. One family comprising two affected and three unaffected carriers was followed over 5.5 years for a longitudinal segregation analysis of heteroplasmy. The percentage of mutant mtDNA was determined using a novel procedure of fluorescence-based primer extension and restriction fragment length polymorphism analysis. The prevalence of heteroplasmy was assessed by determining the number of genealogically unrelated LHON pedigrees with heteroplasmic maternal family members from the LHON patient records of the Department of Ophthalmology, University of Tübingen, Germany.

The authors observed a marked variability in the degree of heteroplasmy levels within each pedigree and a tendency toward a higher mutant allele frequency in offspring generations. Disease expression was correlated with higher levels of mutant mtDNA molecules. Longitudinal analysis revealed no statistically significant decrease in the heteroplasmy level in the family studied but a reduction of 11% and 12% in one affected and one unaffected individual, respectively. In 167 genealogically unrelated LHON families the prevalence of heteroplasmy was 5.6%, 40%, and 36.4% for the 11778, 3460, and 14484 LHON mutations, respectively.

Cross-sectional studies of heteroplasmy for the 3460 LHON mutation suggest that the genotype shifts toward a higher mutational load in offspring generations. Long-term decrease in the blood mutant load in single cases indicates negative selection of the mutant allele in the hematopoietic cell system. The prevalence of heteroplasmy varies significantly between the different primary LHON mutations, suggesting genotypical differences in disease expression.