An island of bladder cancer susceptibility

From the Bhatti Group, Public Health Sciences Division

Although bladder cancer is less common in women, the prognosis is typically worse than it is for men. Symptoms of bladder cancer can be easily mistaken for less severe or benign conditions, and the lower survival rates in women may be in part explained by bladder cancer diagnoses occurring at a more advanced stage. Thus, the identification of biomarker(s) in readily accessible tissues that could predict risk for future development of this disease would be highly valuable. Researchers from Dr. Parveen Bhatti’s group in the Public Health Sciences Division conducted a study to investigate whether a specific type of DNA modification, measurable in blood, may provide insight into bladder cancer risk. These results were recently published in Cancer Epidemiology, Biomarkers & Prevention.

Epigenetic modifications are changes to DNA that are heritable but do not involve alterations to the sequence of the DNA building blocks themselves. Methylation of DNA, in which methyl groups are attached to DNA, is one such epigenetic modification that occurs at specific DNA sites referred to as CpG loci. Regions of DNA that contain a high density of CpG sites are referred to as islands and are often found near transcriptional regulatory elements such as promoter regions. Aberrant DNA methylation at CpG sites, previously associated with expression level changes in tumor suppressor genes, has received attention as a possible contributor to the development of different types of cancer. A previous study reported significant changes in DNA methylation levels at specific loci in the blood of bladder cancer patients as compared to controls without cancer. However, the blood samples used in the analyses were collected post-diagnosis which precludes the ability to relate differential methylation levels to risk for cancer development, as methylation status may have changed as a consequence of the cancer itself or treatment initiated after diagnosis.

In the new Fred Hutch case-control study, the authors utilized baseline blood samples collected from participants of the Women’s Health Initiative (WHI), a prospective study of over 150,000 postmenopausal women. The new study included 440 pairs of bladder cancer cases and controls matched for a variety of factors, including age and year of enrollment into WHI. All of the cases were transitional cell carcinoma, the most common subtype of bladder cancer. The median length of time from enrollment, when the baseline blood sample was collected, to diagnosis of bladder cancer was 7.2 years; cases then continued to be followed for other outcomes, and the follow-up time for the controls was equal to or greater than that of their matched case. 

Plot of over 300,000 CpG sites in association with bladder cancer risk in participants from the Women’s Health Initiative.
Plot of over 300,000 CpG sites in association with bladder cancer risk in participants from the Women’s Health Initiative. Methylation of a CpG site in CITED4 was strongly associated with bladder cancer risk (shown in purple). Image provided by Kristina Jordahl

Methylation of over 300,000 CpG sites was measured in DNA isolated from the baseline blood samples. Statistical modeling was then conducted to assess the association between methylation status of the individual CpG sites and risk of bladder cancer. In their analyses, the authors controlled for potential confounding factors that may influence DNA methylation status and bladder cancer risk, including participant race/ethnicity, smoking status, pack-years of smoking, and the relative proportions of the major types of circulating white blood cells contributing to the blood samples.

Of the thousands of CpG sites assessed, many of the top ten sites with the strongest association with bladder cancer largely resided in genes that function in regulation of gene expression and DNA repair, and many of these genes have previously been implicated in cancer. Increased methylation at the top CpG site was significantly associated with an 82% reduction in risk of bladder cancer (see Figure). This site is located in a CpG island of the CITED4 gene, the product of which is known to interact with several other proteins that control the expression of a diverse set of genes.

This study is novel in its ability to predict bladder cancer susceptibility, made possible by the use of pre-diagnostic blood samples from a prospective study with years of participant follow-up. As described by Kristina Jordahl, graduate student and lead author of the study, “This is the first study to demonstrate that a change to the methylation of DNA at a specific site is associated with a future risk of developing bladder cancer. This type of etiologic clue has the potential to inform strategies for bladder cancer screening and prevention.” This finding may be especially important for women, in whom bladder cancer outcomes tend to be poorer.

There are several known risk factors for bladder cancer, including modifiable environmental factors such as cigarette smoking, chronic exposure to certain toxic chemicals, and chronic exposure to diesel fuel, among others, as well as non-modifiable factors such age, gender, and race. “The functional and practical significance of the methylation differences we observed need further investigation. Our next step is to determine if there are certain exposures that lead to an increased risk of bladder cancer through methylation changes,” said Jordahl.

This research was supported by the American Cancer Society.

Research reported in the publication is a collaboration between Cancer Consortium members at the University of Washington (Drs. Amanda Phipps and Emily White) and Fred Hutch (Drs. Timothy Randolph, Amanda Phipps, Emily White, and Parveen Bhatti)

Jordahl KM, Randolph TW, Song X, Sather CL, Tinker LF, Phipps AI, Kelsey KT, White E, Bhatti P. 2018. Genome-wide DNA methylation in pre-diagnostic blood and bladder cancer risk in the Women’s Health Initiative. Cancer Epidemiology, Biomarkers & Prevention. doi: 10.1158/1055-9965.EPI-17-0951.