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Show ORIGINAL CONTRIBUTION Proportion of 11 778 Mutant Mitochondrial DNA and Clinical Expression in a Thai Population With Leber Hereditary Optic Neuropathy Wanicha L. Chuenkongkaew, MD, Rungnapa Suphavilai, MSc, Lookjan Vaeusorn, MD, Nopasak Phasukkijwatana, BSc, Patcharee Lertrit, MD, PhD, and Bhoom Suktitipat, MD Background: The proportion of mutant mtDNA in blood has been found to correlate with the frequency of visual loss in cases with mtDNA mutations associated with Leber hereditary optic neuropathy ( LHON), especially in men. We sought to determine this correlation in a Thai population of LHON. Methods: Densitometric quantification of blood mtDNA with the 11778 LHON mutation in 137 symptomatic cases and their asymptomatic maternal relatives in 30 Asian pedigree families was performed. Asymptomatic maternal relatives under the age of 16 years were excluded. The visual outcome in symptomatic cases with homoplasmy and heteroplasmy was compared. Results: Heteroplasmy was detected in eight ( 12.9%) symptomatic and 30 ( 40%) asymptomatic individuals. The quantification of blood mutant mtDNA in the eight symptomatic cases ranged from 44% to 93% ( mean = 75%). The visual outcome of the cases with heteroplasmy was not different from that of cases with homoplasmy. There was a correlation between the proportion of mutant mtDNA and the likelihood of visual loss. Conclusions: The prevalence of heteroplasmy among pedigrees of the 11778 LHON mutation in Thailand was similar to that of other Asian populations and may be greater than in 11778 LHON pedigrees from white backgrounds. The proportion of mutated mtDNA correlated with visual loss, but the effect of heteroplasmy on clinical expression seemed not to relate to gender. ( J Neuro- Ophthalmol 2005; 25: 173- 175) Departments of Ophthalmology ( WLC, LV) and Biochemistry ( RS, NP, PL, BS), Faculty of Medicine, Siriraj Hospital, Mahidol University, Bangkok, Thailand. Address correspondence to Wanicha L. Chuenkongkaew, MD, Department of Ophthalmology, Siriraj Hospital, 2 Prannok Rd., Bangkoknoi, Bangkok 10700, Thailand; E- mail: siwck@ mahidol. ac. th This research work is supported by a grant from the Faculty of Medicine, Mahidol University, and Thailand Research Fund ( TRF) grant number BRG 4580018 to PL, Bangkok, Thailand. Leber hereditary optic neuropathy ( LHON) is a maternally inherited disease characterized by the sudden onset of loss of central vision, usually in early adulthood. More than 50% of all cases with LHON carry a mitochondrial DNA ( mtDNA) mutation at nucleotide position 11778. This nucleotide change converts a highly conserved argi-nine residue to histidine at codon 340 in the NADH-ubiquinone oxidoreductase subunit 4 ( ND4) gene of mtDNA. This mutation is present in all mtDNA molecules of the individuals within the maternal lineage ( homoplasmy). However, some pedigrees harbor both mutant and wild- type mtDNA ( heteroplasmy). The proportion of the 11778 mutant mtDNA is an important risk factor, particularly in affected males, for the development of blindness in LHON ( 1). We studied the correlation between the proportion of blood mutant mtDNA in pedigrees of Asian 11778 LHON cases and its clinical expression. METHODS The cases enrolled in this study included individuals from Thai ( 27), Chinese ( 1), Chinese- Thai ( 1), and Indian ( 1) pedigree families who developed the typical clinical neuro- ophthalmic features of LHON and showed the 11778 LHON mutation in peripheral blood ( symptomatic group) as well as their visually intact maternal relatives aged 16 or older ( asymptomatic group). Younger asymptomatic maternal relatives were excluded as a result of the possibility of developing visual loss later in life. Whole blood samples obtained with informed consent were quantified by the restriction fragment length polymorphism ( RFLP) technique to estimate the degree of heteroplasmy, the proportion of mutant to total ( both wild-type and mutant) mtDNA in each individual. Homoplasmy was defined as mutant mtDNA of more than 95%. Data collected included age, age at onset, gender, maternal relation, and best- corrected visual acuity ( BCVA). For statistical analysis, BCVA was converted to logMAR ( logMAR = log 1/ Snellen visual acuity). The unmeasured visual acuities were at the following logMAR values: counting J Neuro- Ophthalmol, Vol. 25, No. 3, 2005 173 J Neuro- Ophthalmol, Vol. 25, No. 3, 2005 Chuenkongkaew et al fingers corresponding to Snellen visual acuity 6/ 600 ( logMAR = 2.0) and hand motion corresponding to Snellen visual acuity 6/ 6000 ( logMAR = 3.0). RESULTS Of the 30 pedigrees, 11 ( 37%) were heteroplasmic. In 14 pedigrees, at least 80% of the maternal relatives at risk were examined. Of 137 individuals, 62 ( 45.3%) were symptomatic and 75 ( 54.7%) were asymptomatic; 73 ( 53.3%) were male and 64 ( 46.7%) were female. In the symptomatic group, the male to female ratio was 3.1: 1; in the asymptomatic group, it was 0.5: 1. The age of onset of LHON in symptomatic individuals ranged from 8 to 68 years ( mean = 37.3 years); the age of asymptomatic individuals ranged from 18 to 90 years ( mean = 41.8 years). There was no statistical significant between the mean age of symptomatic and asymptomatic groups ( P = 0.10, T test). The age at onset of visual loss ranged from 6 to 44 years ( mean = 20.6 years) in men and from 10 to 53 years ( mean = 27.2 years) in women. Of 62 symptomatic individuals, eight ( 12.9%) were heteroplasmic and 54 ( 87.1%) were homoplasmic. Of 75 asymptomatic individuals, 30 ( 40%) were heteroplasmic and 45 ( 60%) were homoplasmic. The age at onset ranged from 10 to 42 years ( mean = 21.1 years) in symptomatic heteroplasmic individuals and from 6 to 53 years ( mean = 22.2 years) in homoplasmic individuals. There was no statistical difference between the age at onset in symptomatic heteroplasmic and homoplasmic groups ( P = 0.68, Spearman's rho). The age of asymptomatic heteroplasmic individuals ranged from 18 to 90 years ( mean = 46.9 years) and from 18 to 70 years ( mean = 38.5 years) in homoplasmic individuals. There was no statistical significance between the age of asymptomatic heteroplasmic and homoplasmic groups ( P = 0.95, Pearson correlation). The prevalence of heteroplasmy in the 137 individuals is shown in Table 1. The proportion of mutant DNA in symptomatic heteroplasmic individuals ranged from 44% to 93% ( mean = 75%) and from 18% to 94% ( mean = 62%) in asymptomatic heteroplasmic individuals. The BCVA ranged from 0.2 to 3.0 logMAR ( mean = 1.7 logMAR) ( approximate Snellen visual acuity = 20/ 1000) in heteroplasmic individuals and from 0.3 to 3.0 logMAR ( mean = 1.8 logMAR) ( approximate Snellen visual acuity = 20/ 1250) in homoplasmic individuals. The visual outcome of the cases with heteroplasmy was not different from that of cases with homoplasmy ( P = 0.74, Mann- Whitney tests). There was statistical significance between the proportion of mutant mtDNA and the expression of visual loss ( P = 0.001; chi- square tests) ( male group, P = 0.011; female group, P = 0.05; chi- square tests). TABLE 1. Prevalence of heteroplasmy in 137 cases with the 11778 mutation of Leber hereditary optic neuropathy Symptomatic Asymptomatic Total (%) (%) (%) Males Heteroplasmy Homoplasmy Total Heteroplasmy Homoplasmy Total ) tal 6 ( 12.8) 41 ( 87.2) 47 ( 100) 2 ( 13.3) 13 ( 86.7) 15 ( 100) 62 ( 45.3) 10 ( 38.5) 16 ( 61.5) 26 ( 100) 20 ( 40.8) 29 ( 59.2) 49 ( 100) 75 ( 54.7) 16 ( 21.9) 57 ( 78.1) 73 ( 100) 22 ( 34.4) 42 ( 65.6) 64 ( 100) 137 ( 100) Of 75 asymptomatic maternal relatives, 24 with homoplasmy and 11 with heteroplasmy were in the same generation as the proband of the family; 19 with homoplasmy and 19 with heteroplasmy were from the preceding generation; one with homoplasmy and one with heteroplasmy were from subsequent generations. DISCUSSION In our maternal pedigrees harboring the 11778 LHON mutation, heteroplasmy for the mutation in peripheral blood leukocytes was detected in 37% of pedigrees, whereas in previous studies of maternal pedigrees harboring this mutation, heteroplasmy was documented in at least one family member in approximately 15% of pedigrees ( 1- 3). Most of these pedigrees likely represented white individuals. In the study of Smith et al ( 2), heteroplasmy was found in 7% of symptomatic individuals and in 19% of asymptomatic individuals. In our study, heteroplasmy was detected in approximately 13% and 40% of symptomatic and asymptomatic individuals, respectively, which is similar to the 14% and 44% of symptomatic and asymptomatic Japanese 11778 LHON cases ( 4). Although one cannot draw definitive conclusions without a systematic screening of all maternal relatives within these pedigrees, the suggested higher prevalence of heteroplasmy reported in these two studies of Asian individuals might be the result of the different mtDNA haplotype backgrounds found in Asians and whites. To clearly classify affected and unaffected groups of our cases, asymptomatic individuals under the age of 16 years of age were excluded. Only one case with homoplasmy and one case with heteroplasmy were from subsequent generations. Therefore, the unaffected maternal relatives who were recruited for this study were less likely to develop subsequent visual loss. 174 © 2005 Lippincott Williams & Wilkins Proportion of 11778 mtDNA in Thai J Neuro- Ophthalmol, Vol. 25, No. 3, 2005 TABLE 2. Comparison of the quantity of 11778 mutant mtDNA in the study of Smith et al ( 2) and the current study No. of cases with 11778 mutant mt DNA (%) Mutant mtDNA 0- 25% 26- 50% 51- 75% 76- 90% > 90% Total Smith et al ( 2) Symptomatic 0( 0) 0( 0) 0( 0) 5( 7) 70 ( 93) 75 Asymptomatic 2( 2) 2( 2) 4( 4) 11( 11) 82 ( 81) 101 Current Symptomatic 0( 0) 0( 0) 3( 5) 3( 5) 56 ( 90) 62 study Asymptomatic 2( 3) 16 ( 21) 7( 9) 5( 7) 45 ( 60) 75 Our symptomatic cases in both heteroplasmic and homoplasmic groups developed loss of vision at the same age and had the same visual outcome. We found a correlation between the proportion of mutant mtDNA and the likelihood of having visual loss; this finding agrees with that of a previous study ( 1). In contrast to cases with mutant mtDNA of less than 75% in the study of Smith et al ( 2), 5% of our cases who had blood mutant mtDNA of less than 75% developed visual loss ( Table 2). The percentage of cases with mutant mtDNA less than 75% who had visual loss was not statistically significantly different from that of cases who had blood mtDNA of more than 75% and had visual loss ( P < 0.01, chi- square tests). However, when analyzing results using an odds ratio calculation, the cases who had blood mutant mtDNA of greater than 75% were more likely to develop blindness than those who had mutant mtDNA of less than 75% ( odds ratio = 6.41 [ 2.52, 16.78]). Interestingly, the frequency of visual loss has been documented to have a relationship to the mutation load of mtDNA in the peripheral blood of male cases with LHON but not to that of female cases ( 1). By contrast, our study did not show any gender difference. Also interesting is that in whites, the mtDNA haplogroup J has preferential correlation to the LHON mutation, whereas in Asians, the haplo-groups B and BM have a close relationship to the 11778 LHON mutation ( 5). The mtDNA haplotype background may influence disease expression. We conclude that the prevalence of heteroplasmy among pedigrees of 11778 LHON in Thailand is similar to that of other Asian populations ( 4) and may be more frequent than in 11778 LHON pedigrees from white backgrounds ( 2). Similar to previous studies ( 1,2), the proportion of mutated mtDNA in Thai individuals with 11778 LHON correlated with the clinical expression of visual loss. However, in contrast to one previous study of English LHON pedigrees ( 1), the effect of heteroplasmy on clinical expression in our cases seemed not to relate to gender. Acknowledgments The authors thank Nancy J. Newman, MD, for her critical review of the manuscript; Associate Professors Anuchit Poonyathalang, Sukhuma Warrasak, and Thanyachai Sura from the Department of Ophthalmology and the Department of Medicine, Ramathibodi Hospital, Mahidol University, and Dr. Parima Hiranwiwatkul from the Department of Ophthalmology, Chulalongkorn University for their kind contribution of blood samples for DNA analysis; and Miss Sasima Tongsae, Department of Ophthalmology, Siriraj Hospital, Mahidol University for her assistance in statistical analysis. REFERENCES Chinnery PF, Andrews RM, Turnbull DM, et al. Leber hereditary optic neuropathy: Does heteroplasmy influence the inheritance and expression of the G11778A mitochondrial DNA mutation? Am J Med Genet 2001; 98: 235^ 3. Smith KH, John DR, Heher KL, et al. Heteroplasmy in Leber's hereditary optic neuropathy. Arch Ophthalmol 1993; 111: 1486- 90. Newman NJ, Lott MT, Wallace DC, et al. The clinical characteristics of pedigrees of Leber's hereditary optic neuropathy with the 11778 mutation. Am J Ophthalmol 1991; 111: 750- 62. Hotta Y, Fujiki K, Hayawaka M, et al. Clinical features of Japanese Leber's hereditary optic neuropathy with 11778 mutation of mitochondrial DNA. Jpn J Ophthalmol 1995; 39: 96- 108. Sudoyo H, Suryadi H, Lertrit P, et al. Asian- specific mtDNA backgrounds associated with the primary G11778A mutation of Leber's hereditary optic neuropathy. J Hum Genet 2002; 47: 594- 60. Harding AE, Sweeney MG, Govan GG, et al. Pedigree analysis in Leber hereditary optic neuropathy families with a pathogenic mtDNA mutation. Am J Hum Genet 1995; 57: 77- 86. 175 |
