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Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas

Daniel Covarrubias

Department of Statistics, Rice University, Houston, Texas

Aiyi Liu

Biometry and Mathematical Statistics Branch, National Institute of Child Health and Human Development, NIH, Rockville, Maryland

In Response: We recently reported that European-American females with haplogroup K mitochondrial DNA (mtDNA) variants have a significant increase in the risk of developing breast cancer, whereas women bearing haplogroup U have a decreased breast cancer risk (1). Mosquera-Miguel et al. studied sporadic breast cancer patients from continental Spain and the Canary Islands and found no evidence of an association for any mtDNA variants with breast cancer (CAN-07-5706). First, all cases in our study (1) have a family history of breast cancer; therefore, the study populations are not equivalent. Our own unpublished data also suggests that mtDNA variants associated with familial breast cancer risk may not be related to sporadic breast cancer.

The mtDNA haplogroup classification used in our study is based on previously published articles (refs. 2–5; Table 1 ). Haplogroup U is distinguished from haplogroup K by 9055G>A; both haplogroups U and K contain 12308G. To date, there is no widely accepted standard for the classification of haplogroups U and K. Although haplogroup K is a subclade of haplogroup U in Europeans, most, if not all, published articles on haplogroups and disease association do not include haplogroup K within haplogroup U.

References

1. Bai RK, Leal SM, Covarrubias D, Liu A, Wong LJ. Mitochondrial genetic background modifies breast cancer risk. Cancer Res 2007;67:4687–94.[Abstract/Free Full Text]
2. Herrnstadt C, Elson JL, Fahy E, et al. Reduced-median-network analysis of complete mitochondrial DNA coding-region sequences for the major African, Asian, and European haplogroups. Am J Hum Genet 2002;70:1152–71.[CrossRef][Medline]
3. Macaulay V, Richards M, Hickey E, et al. The emerging tree of West Eurasian mtDNAs: a synthesis of control-region sequences and RFLPs. Am J Hum Genet 1999;64:232–49.[CrossRef][Medline]
4. Torroni A, Huoponen K, Francalacci P, et al. Classification of European mtDNAs from an analysis of three European populations. Genetics 1996;144:1835–50.[Abstract]
5. van der Walt JM, Nicodemus KK, Martin ER, et al. Mitochondrial polymorphisms significantly reduce the risk of Parkinson disease. Am J Hum Genet 2003;72:804–11.[CrossRef][Medline]

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