Is Cancer Contagious Between Identical Twins?

No, cancer is not contagious. It is a genetic disease that arises from mutations and changes in cell behavior. Despite sharing the same genetic material, identical twins are not at an increased risk of developing cancer from each other. However, there are some factors that contribute to the possibility of one twin developing cancer if the other twin has already been diagnosed with the condition.

Cancer: A Genetic Disease

Despite the strong metabolic effects on the body, cancer is fundamentally a genetic disease. The vast majority of cancers are not hereditary. Some cancers, such as those associated with the BRCA1 or 2 genes, can be inherited. In cases where mutations in these genes are present, both identical twins are at heightened risk for certain types of cancers.

Another example is the p53 gene, which functions as a tumor suppressor. If both twins are born with mutations in the p53 gene, they are predisposed to a variety of cancers. The presence of two damaged p53 genes can lead to cancer development at a much younger age, sometimes as early as the 30s or even younger. This premutation puts individuals on a faster track to cancer development due to the lack of p53's DNA repair function.

Prevalence and Rarity of Hereditary Cancers

While hereditary cancers like BRCA1 and 2 mutations exist, they are relatively rare. Only about 5% of all breast cancers are linked to these inherited mutations. Similarly, mutations in the p53 gene, while significant, are also rare. Identical twins sharing these mutations might face a higher risk of developing certain cancers earlier in life, but this does not imply a higher overall cancer risk for identical twins compared to non-identical twins.

Environmental and Genetic Predispositions

Study results from nations such as Sweden, Denmark, and Finland show that if one twin has a specific type of cancer, the identical twin has a higher likelihood of developing the same cancer compared to a non-identical twin. For example, the risk of colorectal cancer due to smoking can increase by 30%, while the risk of lung cancer can rise by a staggering 2400%. This highlights the importance of lifestyle factors in the development of cancer.

The combination of genes that predispose one twin to cancer is identical in the other twin, but this does not mean that identical twins have a higher overall risk of developing cancer. The risk is higher if one twin has already been diagnosed with cancer, but it is essential to understand that shared environmental and genetic factors play a significant role in the development of cancer.

Rare Examples of Contagious Cancers

While the vast majority of cancers are not contagious, certain rare cases have been observed. For example, Tasmanian devils carrying the facial tumour disease (DFTD) and some dog breeds with transmissible cancer have demonstrated non-traditional routes of cancer transmission. These cases are so rare that they do not pose a significant risk of cancer spreading between individuals.

However, some viruses, such as human papillomavirus (HPV), can contribute to the development of cancer. While most people will be exposed to HPV during their lifetime, the majority will not develop cancer. In the UK, all girls receive HPV vaccinations to prevent the development of cancers linked to this virus, which has shown to be particularly relevant in recent years due to changes in sexual practices.

Conclusion

Most cancers in humans are not hereditary, and the majority of cancer cases are due to non-genetic factors such as lifestyle choices and environmental exposures. While identical twins may share certain genetic and environmental predispositions, this does not make cancer contagious between them. If one twin has cancer, it is worth consulting a genetic counselor to assess any potential inherited component or predisposition. However, the overall risk of one twin developing cancer from the other is low, and the focus should be on understanding and mitigating the risk factors present in each individual's life.

Keywords: cancer, contagious, identical twins