PostScript .............................................................................................. LETTERS Correction: no evidence of an association between the T16189C mtDNA variant and late onset dementia (Gibson et al ) We believe that the title of Chinnery et al’s paper should be corrected because the data the authors present do not include an analysis of the 16189 variant of mtDNA (Table 1). We defined the 16189 variant as the DNA sequence associated with a polydC tract, 2 resulting from a T16189C transition that may generate heteroplasmic length variation, table 1. Heteroplasmic length variation does not occur when the polymeric tract is inter- rupted by a cRt transition, which occurs at several different sites but commonly at nucleo- tice 16186 or 16192. Individuals with these additional polymorphisms are excluded from our definition of the 16189 variant because they no longer have a long homopolymeric c tract. The variant does not alter any coding sequences yet lies near to mtDNA control sequences, which can explain its effects on mitochondrial function. In studies of disease associations with variants in this region we chose to investigate the 16189 variant rather than any other sequence change, because of the likely functional effects of the homopolymeric C tract and heteroplasmic length variation. Gibson et al 1 have shown that the overall prevalence of the T16189C allele in their population is 12.6%, which is substantially higher than the 6.4–8.8 % prevalence of the 16189 variant reported in other studies. 23 This is because they have quantfied the prevalence of the T16189C transition per se rather than the variant. Including these additional polymorph- isms may dilute out a real association with the 16189 variant. The authors have shown that the T16189C transition per se is not a risk factor for late onset dementia, 1 but to our knowledge this has not, in any case, been implicated with any disease phenotypes. However, they found that the heteroplasmic length variation, which implies the presence of the 16189 variant, was associated with a 2.2 fold increased risk. They did not, however, quantify the relative risk for the 16189 variant per se, which could well be significant, in direct contradiction of their title. From their data, it is possible that the variant might in fact predispose to late onset dementia. The 16189 variant is a risk factor for type 2 diabetes, 4 thinness at birth, 5 and aged 20 years 6 iron loading in haemochromato- sis, 7 dilated cardiomyopathy, 8 endometrial cancer, 9 and other multifactorial disorders. 10 This variant may be mildly detrimental. 11 Unlike many other mtDNA polymorphisms implicated in type 2 diabetes, this variant probably has bona fide functional conse- quences because it has arisen many times independently in the various populations studied, 2 4 12 excluding a founder effect. Because the authors did not perform mito- chondrial haplotyping 1 to exclude a founder effect, their results may reflect the conse- quences of other co-segregating genes. J Poulton, S Das Nuffield Department of Obstetrics and Gynaecology, University of Oxford, Women’s Centre, John Radcliffe Hospital, Oxford, UK Correspondence to: Professor J Poulton, Nuffield Department of Obstetrics and Gynaecology, University of Oxford, Women’s Centre, John Radcliffe Hospital, Oxford, OX3 9DU, UK; joanna.poulton@ obs-gyn.ox.ac.uk doi: 10.1136/jmg.2004.019208 Received 13 February 2004 Accepted for publication 29 May 2004 Conflict of interest: none declared References 1 Gibson AM, Edwardson JA, Turnbull DM, McKeith IG, Morris CM, Chinnery PF. No evidence of an association between the T16189C mtDNA variant and late onset dementia. J Med Genet 2004;41:e7. 2 Poulton J, Marchington D, Brown MS, Phillips D, Hagelberg E. Does a common mitochondrial DNA polymorphism underlie susceptibility to diabetes and the thrifty genotype? Trends Genet 1998;14:385–7. 3 Miller K, Dawson J, Hagelberg E. A concordance of nucleotide substitutions in the first and second hypervariable segments of the human mtDNA control region. Int J Legal Med 1996;109:107–13. 4 Poulton J, Luan J, Macaulay V, Hennings S, Mitchell J, Wareham NJ. Type 2 diabetes is associated with a common mitochondrial variant: evidence from a population-based case-control study. Hum Mol Genet 2002;11:1581–3. 5 Casteels K, Ong K, Phillips D, Bendall H, Pembrey M. Mitochondrial 16189 variant, thinness at birth, and type-2 diabetes. ALSPAC study team. Avon Longitudinal Study of Pregnancy and Childhood Lancet, 1999;353:1499–500. 6 Poulton J, Macaulay V, Livesey K, Wareham N, Parker E, Phillips D, Simmons D, Mayosi B, Knogali S, Robson KA. A common mtDNA variant may be a susceptibility factor in 4 important multifactorial conditions. Am J Hum Genet 2001;69(Suppl):579. 7 Livesey KJ, Wimhurst VL, Carter K, Worwood M, Cadet E, Rochette J, Roberts AG, Pointon JJ, Merryweather-Clarke AT, Bassett ML, Jouanolle AM, Mosser A, David V, Poulton J, Robson KJ. The 16189 variant of mitochondrial DNA occurs more frequently in C282Y homozygotes with haemochromatosis than those without iron loading. J Med Genet 2004;41:6–10. 8 Khogali S, Mayosi B, Beattie J, McKenna W, Watkins H, Poulton J. A common mitochondrial DNA D-loop variant is associated with idiopathic dilated cardiomyopathy in two different populations. Lancet 2001;357:1265–7. 9 Liu VW, Wang Y, Yang HJ, Tsang PC, Ng TY, Wong LC, Nagley P, Ngan HY. Mitochondrial DNA variant 16189TRC is associated with susceptibility to endometrial cancer. Hum Mutat 2003;22:173–4. 10 Momiyama Y, Furutani M, Suzuki Y, Ohmori R, Imamura S, Mokubo A, Asahina T, Murata C, Kato K, Anazawa S, Hosokawa K, Atsumi Y, Matsuoka K, Kimura M, Kasanuki H, Ohsuzu F, Matsuoka R. A mitochondrial DNA variant associated with left ventricular hypertrophy in diabetes. Biochem Biophys Res Commun 2003;312:858–64. 11 Morten K, Jen C, Poulton J. The 16189 variant of mtDNA in type 2 diabetes: towards a molecular mechanism. Diabet Med 2003;20(Suppl 2):13. 12 Poulton J, Bednarz AL, Scott-Brown M, Thompson C, Macaulay VA, Simmons D. The presence of a common mitochondrial DNA variant is associated with fasting insulin levels in Europeans in Auckland. Diabet Med 2002;19:969–71. No evidence of an association between the mtDNA 16184-93 polyC tract and late onset dementia We are grateful to Professor Poulton and Dr Das for clarifying their definition of the 16189 variant. In our study we determined the allele status at position 16189 of mitochondrial DNA (mtDNA), and found no evidence of an association between the 16189C polymorphic sequence variant and late onset dementia. 1 The title of our manuscript therefore reflects our observations and does not need to be corrected. Part of the confusion seems to have arisen because of different definitions of the ‘‘16189 variant’’ in the literature. The standard ‘‘Cambridge’’ reference mtDNA sequence 23 has a run of cytosine residues from nucleotide position (np) 16184 to 16193 inter- rupted by a thymidine residue at nucleotide position 16189. In approximately 12% of the UK population, there is a TRC substitution at np 16189. 1 In most individuals this results a tract of 10 C residues (polyC tract). This sequence is unstable and is associated with length variation of the polyC tract, probably because of slippage during genome replication, 4 generating larger and smaller polyC tracts within the same indivi- dual during life (heteroplasmy). 4 However, occasional individuals have other polymorph- isms between np 16184 and 16193, which appear to stabilise the tract and do not lead to the generation of length variants. 4 We suggest a more accurate definition should be used when referring to the homopolymeric C tract that is present in the majority of individuals with the T16189C substitution. The term ‘‘mtDNA 16184-93 polyC tract’’ will hopefully prevent confusion in the future. Poulton and Das comment on the results of our logistic regression analysis (table 1 in our paper 1 ), and suggest that individuals with homopolymeric tract length heteroplasmy have a 2.2 fold increased risk of developing Alzheimer’s disease (AD) compared with con- trols. This would imply an association between their definition of the 16189 variant and late onset dementia. However, the confidence intervals for the relative risk of 2.2 are 0.85 to 5.81, comfortably including 1. The relative risk is therefore not statistically significant and does not support a link between AD and homo- polymeric length tract heteroplasmy. In our original study, 1 we measured homo- polymeric tract length heteroplasmy using a trimmed PCR approach with a fluorescent forward primer. However, not all individuals with a 16184-93 polyC tract also have length heteroplasmy. Therefore, to address the con- cerns of Poulton and Das experimentally, we directly sequenced the relevant region of Table 1 Sequences of identified variants Variant Nucleotides Wild type sequence ccccctcccc Sequences included in Chinnery’s analysis ccccccctcc cctccccccc cccccccccc etc 16189 variant 2 cccccccccc Heteroplasmic length variants included within 16189 variant ccccccccc ccccccccccc etc J Med Genet 2004;41:957–960 957 www.jmedgenet.com on December 8, 2021 by guest. Protected by copyright. http://jmg.bmj.com/ J Med Genet: first published as 10.1136/jmg.2004.019133 on 9 December 2004. Downloaded from