Original ArticleA pilot study on DNA hypermethylation status in promoter region of P16 gene in patients with sporadic breast cancer
Introduction
Carcinoma breast is one of the common cancers seen across the world and is the fifth leading cause of cancer deaths worldwide.1,2 Women from both developed and developing countries have the risk of developing breast cancer.2 Over the last decade, the incidence of breast cancer is the leading cause of cancer among the Indian population.3 Breast cancer is a multistep process resulting from the accumulation of genetic mutations leading to dysfunction of critical genes involved in biological process. Besides genetic alterations, it has become apparent over the years that epigenetic changes have significant effects on tumor initiation and progression. The aberrant methylation of the promoter region of a gene can lead to its silencing and contribute to the development of the tumor. Promoter methylation of various genes was explored previously as a biomarker of the disease.4 Hypermethylation of DNA occurs when a methyl group from S-adenosylmethionine is transferred to 5 methyl-Cytosine (5mC) as a new base on DNA. Promoter region CpG islands are usually differentially methylated in all normal tissues. The silencing of oncosuppressor genes may lead to the inactivation of the apoptotic pathway in different stages of cell cycles.5
The p16 gene encodes a cyclin-dependent kinase inhibitor, p16 INK4A, which regulates cell cycle at transition from G-1 to S-phase. It is located on chromosome 9p21.5 The transcripts of p16 INK4A gene translates into two functional proteins, p16 INK4A and p19 ARF. The promoter methylation of CpG island in p16 gene is associated with many tumors and has been seen in both human breast cancer cell lines and 20–30% of primary breast cancers. Thus, p16 silencing due to methylation is considered a possible contributor to breast tumor genesis. The studies have shown that use of DNA methylation inhibitors like 5-aza-2′-deoxycytidine can reactivate the repressed p16 gene, thus restoring the normal cell growth control.6,7
Most of the previous studies on promoter methylation of p16 gene were done using histopathological tissue. Evidence for an association between blood-based DNA methylation and breast cancer risk is inconsistent.8 Blood DNA methylation levels could be a replacement for breast tissue methylation;9, 10, 11, 12 moreover, there is the paucity on the availability on the Indian data. Hence, this study was undertaken to explore p16 gene hypermethylation on peripheral blood sample on 75 histo pathologically proven carcinoma of breast cases.
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Materials and method
The study was conducted in the department of biochemistry of a tertiary care teaching hospital between October 2016 and October 2019. Sample size was calculated with 95% confidence interval estimation, 25.4% anticipated prevalence of methylation in histopathologically proven carcinoma of breast and 10% absolute error of margin by using formula: N = Z2 p (1-p)/d2. Thus, minimum sample size was calculated to be 72.03. But keeping in mind loss/attrition, data were collected for 75 patients. A
Patient characteristics
The age of patients included in the present study ranged between 26 and 81 years with the mean age of patient as 54.03 ± 11.20 years. Lymph node involvement was seen in 33 (44%) patients, and the remaining 42 (56%) patients had no lymph node involvement. Out of 75 patients, Grade I cancer was seen in 8 (10%) patients, Grade II cancer was seen in 47 (63%) patients, and Grade III cancer was seen in 20 (27%) patients. Sixty-four (85%) patients had ductal carcinoma of breast, and eleven (15%)
Discussion
Breast cancer is the most frequent cancer among women and is also the most important cause of cancer-related deaths among women.13,14 The CpG islands are located in the promoter region of most genes and are normally unmethylated. However, in cancer cells, the aberrant hypermethylation of the promoter regions is associated with the decrease in expression of these genes. The promoter hypermethylation of tumor suppressor gene like p16 may lead to inactivation of tumor suppressor function in
Conclusion
Statistically significant methylation in blood of histopathologically proven sporadic cases of breast cancer was observed in 33% patients in promoter region of p16 gene suggesting that epigenetic modification may be a possible underlying mechanism accounting for sporadic carcinoma of breast. Ease of collection of blood sample in comparison with tissue may make it a non-invasive marker for screening of breast cancer. Loss of p16 gene function due to its promoter methylation in sporadic breast
Acknowledgment
This article is based on Armed Forces Medical Research Committee Project No 4818/2016 granted by the office of the Director General Armed Forces Medical Services and Defence Research Development Organization, Government of India.
Disclosure of competing interest
The authors have none to declare.
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