Unbalanced bidirectional causal association between thyroid cancer and ER-positive breast cancer: should we recommend screening for thyroid cancer in breast cancer patients?

Background The association between breast cancer (BC) and thyroid cancer (TC) has been studied in several epidemiological studies.

However, the underlying causal relationship between them is not yet clear.

Methods The data from the latest large-sample genome-wide association studies (GWAS) of BC and TC were searched in the public GWAS database.

The BC GWAS data included estrogen receptor (ER)-positive and negative subgroups.

Two-way two-sample Mendelian Randomization (MR) was used to explore the potential causal relationship between BC and TC.

Inverse variance weighting (IVW) and the MR-Egger method were used to combine the estimation of each single nucleotide variation (previous single nucleotide polymorphism).

BC was taken as the result, and the effect of TC exposure was analyzed.

Then, the effect of BC exposure on the result of TC was analyzed.

Results Both IVW and MR-Egger results indicated that gene-driven thyroid cancer does not cause estrogen receptor-positive breast cancer and is a protective factor (β = -1.203, SE = 4.663*10^–4, P  = 0.010).

However, gene-driven estrogen receptor-positive breast cancer can lead to the development of thyroid cancer (β = 0.516, SE = 0.220, P  = 0.019).

Conclusion From the perspective of gene drive, people with TC are less likely to have ER-positive BC.

In contrast, people with ER-positive BC are more likely to have TC.

Therefore, it is recommended that patients with BC be screened regularly for TC.