Genetics
Welcome to the genetics section of our website.
Background Information
Some cancers do run in families. People sometimes think that several cases of
cancer in their family might be connected, and that they have inherited an
increased risk of developing cancer. However, only a minority of cancers (5–10%,
or less than 10 out of 100 cases) are clearly linked to an inherited gene
change. Unfortunately, most cancers happen for reasons we don't fully understand
and it may well turn out that more cancers have a genetic origin.
Genes are the biological information passed on (inherited) from your parents.
They affect the way your body grows, works, and looks. Every single cell of your
body is a tiny building block and contains all the information you inherited.
The information is contained in 46 chromosomes (23 pairs) within the nucleus of
each cell. Chromosomes are like containers or filing cabinets filled with all
the genetic information your body needs to work properly. Chromosomes can
actually be seen through a strong microscope. It is estimated that each human
cell contains around 30,000 genes.
It seems that cancer, even if there is a strong family history, is almost always
the outcome of an interaction between genes and the environment.
A genetic or DNA test looks at the order in which the chemical "letters" of your
genetic code are found within a gene. For example BRCA1, the first gene
identified as playing a role in hereditary breast cancer, consists of around
100,000 "letters".
Each cell carries out a specific function for the body and it uses the
information contained in its DNA to know what to do. If there is a mistake
contained in any of the 'files' stored in your body cells, it can make things go
wrong with the cell. A cell could die or could grow into tissue that is not
quite right (like a cyst) or it could grow and multiply out of control and
develop into a cancer.
A genetic test tries to find these mistakes or genetic changes (mutations). A
genetic mutation is not in itself a cancer, but it might mean that you are more
likely to develop cancerous cells than other people in the population.
Genetic Mutations
Researchers are working hard to understand more about how genes work inside
the body and why things sometimes go wrong. Recently, cancer research has
focused
on understanding how a normal cell, through a series of genetic changes, turns
into a cancerous cell. A number of genes have been identified that play a part
in the development of some cancers. If a person is born with a gene change
(mutation) that makes them more likely to develop cancer, we say that they have
inherited a cancer gene. This mutation may then also be passed on to their
children.
There are two types of genetic changes or mutations:
- those that are passed down from generation to generation
- those that happen during the lifetime of a person and are not passed on to the next generation
The most common cancers that may, in some cases, be due to an inherited mutation
are breast, ovarian, bowel and womb (endometrial) cancer. Genetic tests can
identify some of the genes responsible for these cancers. In some cases, other
cancers such as prostate, pancreatic and testicular cancer may be caused by an
inherited mutation. However, although genes thought to be linked with these
cancers have been identified, genetic tests for these cancers have not yet been
developed.
In most cases, people who inherit a known cancer gene will not definitely get
cancer. Inheriting a cancer gene usually means that a person has a
significantly increased risk of developing cancer compared to other people.
If you have only one elderly relative with breast, bowel or any other cancer, it
is very unlikely that the cancer is hereditary. If there were an inherited
cancer gene in your family, it is very likely that more relatives would have
been diagnosed with the same type of cancer.
If two or more blood relatives on the same side of your family have developed
the same type of cancer at a fairly young age, (or cancers which run together,
like breast and ovarian or bowel and womb cancer) you might want to tell your
GP.
You should also let your GP know if you have a relative who developed breast,
ovarian, bowel or womb cancer when they were very young. Your GP will ask you
questions about your family history. If your GP thinks there is a chance your
family has an inherited increased risk of getting cancer (genetic
susceptibility), they will refer you to a genetic counsellor or a cancer
specialist.
For a list of all genetic centres in the NHS -
click here.
The increase in risk varies according to the particular genetic mutation and the
type of cancer it relates to. For example, 8 in 10 (80%) of women who have a
mutation in one of the known breast cancer genes, BRCA1, and a strong family
history of the disease, will develop breast cancer. This is a much higher risk
than women in the general population have. Only one in ten (10%) of women who
don't have a strong family history will develop breast cancer.
If you are found to have a cancer gene mutation, other members of your family might have inherited the same genetic change that you have. Each of your children will have a 1 in 2 chance (50%) of inheriting the change from you.
Testing
Screening becomes very important for some people who know that they are at
increased risk. Bowel screening can pick up pre-cancerous cell changes, but
other
types of cancer screening only pick up cancers at an earlier stage than they
might otherwise have been detected. Cancer screening often does not in itself
prevent cancer. But it is important to remember that cancers which are detected
earlier can be treated more effectively.
Many people say that having gone through genetic counselling and testing, and
having regular screening, means that they are doing everything they can to
increase their and their children's chances of good health. For that reason some
people also start keeping a simple record of their family's health and major illnesses. They want to have the information available for their children or
other relatives if they ever need it.
Cancers Linked To Genetics
1. Breast cancer
Some risk factors have been found to play a role in the development of breast cancer generally – not only for people who have a strong family history of the disease:
- An early first period (menarche) and late menopause seem to slightly increase a woman's breast cancer risk.
- The contraceptive pill and HRT may both slightly increase a woman's risk of breast cancer particularly if taken for more than five years. However, women's individual situations vary and it is important to talk to your doctor about your own particular risk.
- Other risk factors are being very overweight, particularly after the menopause, and drinking large amounts of alcohol.
The following factors seem to lower women's breast cancer risk:
- physical exercise
- a diet high in vegetable and fruit content
- having children early in life and breastfeeding them. However, this factor has been questioned for women who have (carry) a BRCA1 or 2 mutation
2. Ovarian cancer
All of the above risk factors also apply to ovarian cancer, except for the
contraceptive pill - the pill seems to reduce ovarian cancer risk. If you have a
significantly increased breast and ovarian cancer risk in your family (for
example, because a BRCA1 or 2 mutation has been found in your family), this can
make deciding whether to take the pill difficult. The pill might reduce your
ovarian cancer risk but increase your breast cancer risk.
3.
Bowel cancer
Here are some risk factors for bowel cancer:
- a history of polyps in the bowel or other chronic bowel problems
- a diet containing a lot of fat and meat and not enough fruit and vegetables
- alcohol and smoking.
The following factors can lower your bowel cancer risk:
- a diet high in vegetable and fruit content and low in fat and salt
- regular, moderate physical exercise.
Not all of the factors listed here are within a person's control. It is also not clear how all these different factors might
interact. There is also no good evidence about how much these factors affect the small group of people who have a strong family history of cancer. However, some people say that if they avoid some of the above risk factors it makes them feel they are doing whatever they can to control their risk of cancer. If you
are thinking about ways of reducing your own cancer risk, it is worth discussing your ideas with your specialist so that you can take into account
your personal circumstances.
It is clear that giving up smoking, eating a balanced diet, exercising
moderately and maintaining a healthy weight is good for you regardless of your
cancer risk and can make you feel better in many ways.
Other Cancers
A rare cancer of the eye called retinoblastoma, which occurs in childhood, can
be hereditary.
Scientists and doctors also believe that there is a genetic cause for
testicular,
pancreatic,
prostate,
stomach and
kidney cancer in a small number of people.
Currently there is no effective screening available for most of these cancers.
If you are worried about a lot of cases of these (or other) cancers in your family, you should talk to your GP. If there is the possibility of a genetic
factor playing a role in your family, you might be able to take part in a research study. You will also be told what symptoms to look out for to identify
any possible cancer as early as possible.