According to statistics from the Singapore Cancer Registry Annual Report 2018, breast cancer was the most common cancer (29.3%) among Singaporean women followed by colon-rectal (13.3%) and lung cancers (7.5%) between 2014 and 2018. Besides, breast cancer was also reported to be the leading cause of death in cancers affecting women in Singapore during the same period. The age standardized mortality rate for breast cancer has increased from 5.7 per 100,000 in 1968-1972 to 12.6 per 100,000 population in 2014-2018 in tandem with increasing incidence rate.
Early detection of cancers plays an important role in reducing the mortality rate of cancers. However, breast cancer is usually detected at later stages of diseases. (1) The reliability of existing breast cancer biomarkers as an efficient means of detection needs to be reviewed. There is continuous effort to search for efficient breast cancer biomarkers as an early measure to detect breast cancer. At the same time, the biomarkers could serve as an important indicator for prognosis in breast cancer treatment.
CYP2B6 enzyme and CYP2C19 enzyme belong to cytochrome P450 (CYP450) metabolic enzymes which are categorized under phase I superfamily metabolic enzymes. Phase I and II enzymes are of particular interest with respect to breast cancer due to their involvement in the metabolism of steroid hormones, chemical carcinogens, and other environment toxicants. (2, 3) During phase I metabolism reaction, substrates usually undergo reduction, oxidation or hydroxylation to become more polar metabolites. CYP450 enzymes are the predominant mediators. (4) Usually, phase I metabolism is followed by phase II conjugation reactions whereby phase I metabolites, phase II exogenous or endogenous compounds are conjugated to a more polar molecule that produces inactive and water soluble compounds to be excreted by urine or bile. (5, 6) The combined phase I and phase II metabolism is mainly a detoxification and elimination process, however, both phases bear the risk of formation of toxic and highly reactive compounds which can induce or promote serious health problems such as cancer. (5, 7) Therefore, altered activity of metabolic enzyme holds the potential to increase the exposure to carcinogenic compounds and consequently the risk of tumor formation. (8)
In a recent study done by Germany investigators, CYP2B6*6 variant had an increased breast cancer risk with OR of 1.1 (p=0.027) (9). In 2016, Joanne Siok et al. discovered that CYP2C19*2, loss of function polymorphism, as well as the CYP2C19 H2 haplotype were found to be significantly associated with lower plasma concentrations of NorEND and lower formation rates of NorEND (10). NorEND is an active metabolite of tamoxifen that inhibits both aromatase and estrogen receptors, variability in its plasma concentration can potentially influence the therapeutic outcomes of tamoxifen therapy. These data suggest that CYP2C19 may potentially serve as a complementary biomarker for the identification of patients who may or may not benefit from tamoxifen treatment.
Another group of Chinese investigators in China has discovered a possible association of gene polymorphism of CYP2C19*3 with breast cancer in Chinese Han population. The OR for carriers of AG+AA genotype for breast cancer was 2.31 (95% confidence interval= 1.27-4.43) (11).
It has been known that elevated sex hormone levels (2fold increase) were found in women with breast cancer (12). This effect was attributed to estrogen-induced gene expression of factors involved in cell growth and division (13) as well as genotoxic action of metabolic compounds such as 4-hydroxy-catechol estrogens and estrogen-3, 4-quinones (14). Besides, progesterone also plays a part to hormone induced carcinogenesis by promotion of estrogen synthesis, expression of estrogen receptor and cell proliferation (15, 16). Aside from hormonal factors, environmental carcinogenic factors like tobacco smoke, and genetic factors such as mutation and polymorphisms all contribute to breast cancer susceptibility. A family study on genetic basis of breast cancer indicated 2 fold increased risk in the first degree relatives of women with the disease (17). In 1990, BRCA1 and BRCA 2 were identified as two major breast cancer susceptibility genes (18). Harmful mutations in these two genes confer to a cumulative disease risk by age 70 years of 65 and 45%, respectively (19). Recent genome wide association studies revealed strong evidence for more than 18 common breast cancer susceptibility alleles including FGFR2, CCND1, TNRC9, MAP3K1 and LSP1 (20). Most of these genes are related to DNA repair, cell cycle control, apoptosis, cell growth and division representing the most important pathways for the protection of cells against carcinogenic processes. Low coverage of genes coding for phase I and phase II enzymes in commercial genotyping arrays and lack of well-designed studies have downplayed the roles of phase I and phase II enzymes play in conferring breast cancer risk (21).
Although, CYP2B6 contributes about 2-5% of the total liver cytochrome content, it is also expressed in extra-hepatic tissues such as the breast; notably with higher expression level in breast tumor than normal breast. (8, 22)
A study also indicated that, estrogen receptor (ER) positive breast tumours show a higher CYP2B6 levels than ER-negative tumors (23, 24). Impaired CYP2B6 activity had increased the level of both estradiol and testosterone whereby testosterone conferred a stronger influence on breast cancer risk than estradiol in postmenopausal women (25).
In this pilot study, we investigated the percentage of CYP2B6*6, CYP2C19*2 and CYP2C19*3 in our breast cancer patients against the healthy population in Singapore. At the same time, we examined the prevalence of these SNPs in different demographic groups for instance ethnicity, marital status, family history, use of hormone therapy, amongst others. Furthermore, characteristics of breast cancer were studied to unravel the distribution of these SNPs in different stages of breast cancer. Besides, the percentage of these SNPs in different treatments for breast cancer were also presented.