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Up to 95% of tumours are not hereditary

Madrid, 13 January 2020. According to estimates by the Spanish Cancer Registers Network (REDECAN), at the end of 2019 more than 275,000 cases of cancer were diagnosed in our country, of which “only 5% will involve a gene that is inherited from parents to children, so 95% of tumours are not inherited,” stated  Dr Ricardo Cubedo, head of the MD Anderson Cancer Center Madrid Hereditary Cancer and Sarcomas Unit.

Contrary to popular belief, this specialist emphasises that “only one in ten or twenty cases of cancer is hereditary.” This occurs because we have two copies of each gene and transmit only one of these two copies to our offspring. The other copy is transmitted by the other parent. Thus, even if your father or mother has a genetic mutation in a certain gene, it is possible that you inherit the "good" copy and not the "bad" one, so, as Dr Cubedo points out, “the probability of inheriting a mutated gene from your parents is 50%.”

In addition, the penetrance phenomenon must also be taken into account; that is, the percentage of risk of developing a cancer if the mutation is inherited. “There is a huge variation. We do not know why there are families with very high penetrance and up to 80% chance of developing a cancer if the mutation is carried, compared to families with low penetrance, where this percentage is reduced to 30%,” notes the doctor.

Among the reasons, variables such as the phenotype are shuffled (the expression of one or another gene as a result of interaction with the internal and external environment) or the lifestyle, which is often similar between members of the same family but is a field currently under research about which very little is known yet. “If we really knew why, we could design much more effective prevention strategies,” acknowledges this specialist in hereditary cancer. 

Mutations that cause breast, ovarian and colon cancer, the most inherited

Any type of cancer can be inherited but the most frequently inherited tumours are breast cancer, ovarian cancer and colon cancer. In a somewhat less frequent second line of inheritance and prostate cancer and melanoma. However, not whenever a family has a history of one of these tumours does it mean that there is a genetic mutation in that family.

To start with, for example, “it is essential that repeated cases of the same tumour come from only the father or only the mother,” explains Dr Cubedo. Many people come in worried about the presence of cancer on both sides of the family but the reality is that this is precisely a reason to rule out inheritance of a mutated gene since both parents are not genetically related in any way.

In addition, to suspect a possible genetic inheritance of this type, it is important that the tumours always appear in the same organ and in first degree relatives (parents or siblings and their offspring), in addition to which there is one or no generation in which there is no case of this tumour. Another warning sign is that these tumours appear earlier than expected, such as, for example, breast cancer at the age of 20 or prostate cancer before the age of 50.

Likewise, other types of diagnoses considered "rare" would be breast cancer in men or both breasts in a woman, more than one diagnosis of colon cancer in a lifetime or the diagnosis of two types of cancer at the same time. “All of them could make us suspect a possible inherited genetic mutation that could increase our risk of developing cancer,” warns Cubedo.

The Psycho-Oncology Service, key in hereditary cancer consultations

Once it is confirmed that a person does indeed carry a genetic mutation that increases his or her risk of developing a certain type of cancer in the future, the experts in hereditary cancer who must analyse the degree of risk and, based on this, recommend and offer surgical and/or pharmacological options where possible.

In these scenarios, the Psycho-Oncology Service is very important and sometimes even requires more psychological attention for a negative diagnosis than for a positive one. “Feelings of guilt related to why my brother has been affected and I haven't, for example,” says this specialist.

Another topic widely discussed in consultation is anxiety about possible genetic inheritance to children. “We cannot carry out a genetic test at 5/8 years of age and, furthermore, if possible, it is more effective to carry out this test on older relatives who have developed the tumour to detect whether or not this mutation exists before trying to detect its presence in the offspring,” explains Dr Cubedo.

Dr Cubedo, who is currently writing the book 101 answers about cancer, aims to answer all those doubts that the general population has about hereditary cancer. Cubedo points to the key role of screening as a fundamental tool in preventing the appearance of cancer. “Often people who are concerned about the possible inheritance of a mutated gene come to the clinic but then they don't have mammograms or colonoscopies, for example,” he notes.