Mitochondrial DNA variant 16189T>C is associated with susceptibility to endometrial cancer

Mutations in mitochondrial DNA (mtDNA) have been implicated in a wide range of human disorders as different as neuromuscular disease, deafness, and diabetes mellitus [Wallace, 1999]. Surprisingly, in some cases the same mtDNA mutation has been related to different phenotypes of disease. Thus the infrequently occurring base substitution A4G at nucleotide position (np) 3243 in the tRNA-leu gene (MTTL1; MIM# 590050) is associated with MELAS disease as well as Type 2 diabetes [Wallace, 1999]. Another recent example is the more common T4C base substitution at np 16189 (16189T4C) within the non-coding displacement (D) loop region, which has been found to be associated with susceptibility to Type 2 diabetes [Poulton et al., 1998] and dilated cardiomyopathy [Khogali et al., 2001]. Recently, many human tumors were found to contain somatic mtDNA mutations [Fliss et al., 2000; Liu et al., 2001] but relatively little attention has been paid to the possible role of germline sequence variants (mtDNA polymorphisms) in the occurrence of cancer. We have tested the proposition that the latter type of mtDNA variant is associated with particular human cancers. The 16189T4C base substitution lies within the highly polymorphic D-loop region of mtDNA (spanning np 16024 to 576). Numerous variants in this region, including single-nucleotide and repetitive-sequence polymorphisms, are present in the human population. In the present study, in order to test for association with cancer susceptibility, the frequency of the 16189T4C variant in patients with endometrial, cervical, and ovarian cancers was analyzed and compared with normal controls. In addition, two other polymorphic loci, namely a T4C base substitution at np 16519 and a CA-repeated sequence starting at np 514, were also analyzed (Fig. 1). All three polymorphic loci are usually homoplasmic and occur commonly in the local Chinese population under study. Tumor DNA was randomly selected from 51, 67, and 53 samples of primary endometrial, cervical, and ovarian carcinomas, respectively. The 107 normal DNA controls were extracted from serum taken from female noncancer patients. All cancer patients and control subjects recruited in the present study are of Chinese origin. The genotypes were examined using standard PCR amplification followed by automated DNA sequencing [Liu et al., 2001]. The frequencies of polymorphic genotypes in the four groups of samples are shown in Table 1. By two-sided w analysis, the frequencies of occurrence of the CA-repeat sequence variants (classified into short and long groups) and variant 16519T4C in the four groups of samples were not different from one another. In contrast, the frequency of occurrence of 16189T4C between endometrial cancer and the other three groups are different. The frequency was statistically significantly increased (p=0.009) in patients with endometrial cancer (43.1%) when compared with controls (21.5%) (Table 2). However, the frequencies of this variant found in cervical (26.9%) and ovarian (24.5%) cancers were similar to the controls. Furthermore, the frequency of 16189T4C in endometrial cancer was higher than cervical cancer (p=0.064) and statistically significantly higher than ovarian cancer (p=0.045). The increased incidence of 16189T4C found in endometrial cancer is not due to the preferential occurrence of somatic mutation at np 16189 because mtDNA of the matched normal tissue has the same genotype as that of the endometrial cancer in each case (data not shown). We conclude, therefore, that there is an association between the 16189T4C variant in mtDNA and endometrial cancer. The 16189T4C variant is located 30 to the so-called termination-associated sequence (TAS) (Fig. 1). This particular base substitution generates an uninterrupted homopolymorphic C-tract with heteroplasmic lengths