People with bipolar disorder as well as their unaffected siblings appear to be at increased risk for cancer, particularly of the breast, according to new research from Taiwan.

“To our knowledge, our study is the first to report an increased overall cancer risk as well as increased risks of breast and ectodermal cancer among the unaffected siblings aged < 50 years of patients with bipolar disorder,” Ya-Mei Bai, MD, PhD, of National Yang-Ming University, Taipei, Taiwan, and colleagues write in an article published online December 22, 2021 in the International Journal of Cancer.

Most, but not all, previous studies have shown a link between bipolar disorder and cancer. Whether the elevated risk of malignancy extends to family members without the mental-health condition has not been elucidated.

To investigate, the researchers turned to the National Health Insurance Research Database of Taiwan. They identified 25,356 individuals diagnosed with bipolar disorder by a psychiatrist between 1996 and 2010 and the same number of unaffected siblings, as well as more than 100,000 age-, sex-, income-, and residence-matched controls without severe mental illness.

Compared with the control group, people with bipolar disorder (odds ratio, 1.22) and their unaffected siblings (OR, 1.17) both had a higher risk of developing malignant cancer of any kind. The researchers also found that both groups were at higher risk for breast cancer, with odds ratios of 1.98 in individuals with bipolar disorder and 1.73 in their unaffected siblings.

However, the risk of skin cancer was only high in people with bipolar disorder (OR, 2.70) and not in their siblings (OR, 0.62). And conversely, the risk of kidney cancer was significantly increased in unaffected siblings (OR, 2.45) but not in people with bipolar disorder (OR, 0.47).

When stratified by the embryonic developmental layer from which tumors had originated — ectodermal, mesodermal, or endodermal — the authors observed a significantly increased risk only for ectodermal cancers. In addition, only people under aged 50 in both groups (OR, 1.90 for those with bipolar disorder; OR, 1.65 for siblings) were more likely to develop an ectodermal cancer, especially of the breast, compared with the control group. The risks remained elevated after excluding breast cancer but were no longer significant.

When stratified by age, the risk of developing any cancer in both groups also only appeared to be greater for those under aged 50 (OR, 1.34 in people with bipolar disorder; OR, 1.32 in siblings) compared with those aged 50 and over (OR, 0.97 and 0.99, respectively). The authors highlighted these figures in the supplemental data set but did not discuss it further in the study beyond a brief mention that “younger patients with bipolar disorder and younger unaffected siblings (< 50 years), but not older ones (≥ 50 years), were more likely to develop any malignancy during the follow-up than matched controls.”

“This paper essentially finds what we have found in our previous work — that people with bipolar disorder have a greater risk of cancer,” said Michael Berk, MBBCh, PhD, a professor of psychiatry at the Deakin University School of Medicine in Geelong, Australia, who published a systematic review and meta-analysis last spring on cancer risk and the role of lithium treatment in bipolar disorder.

“The interesting finding in our work,” Berk told Medscape Medical News, “is that this risk is attenuated by use of lithium but not other agents.”

The Taiwanese researchers propose a “biopsychosocial explanation” for their results, noting that both the nervous system and the breast and skin develop from the ectoderm, and that cancer risk factors such as smoking and obesity are more common in people with bipolar disorder and their unaffected siblings.

“The findings,” they write “imply a genetic overlap in neurodevelopment and malignancy pathogenesis and may encourage clinicians to closely monitor patients with bipolar disorder and their unaffected siblings for cancer warning signs.”

The authors, however, caution that their study needs validation and had several limitations, including lack of adjustment for drug treatment and lifestyle and environmental factors.

“Our findings may persuade clinicians and researchers to reevaluate the cancer risk among the unaffected siblings of patients with schizophrenia and bipolar disorder because these two severe mental disorders may have a common biopsychosocial pathophysiology,” the team writes.

The study was supported by a grant from Taipei Veterans General Hospital, Yen Tjing Ling Medical Foundation, and the Ministry of Science and Technology, Taiwan.

Int J Cancer. Published online December 22, 2021. Abstract

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