Introduction to Genetic Testing
Most genetic tests examine DNA, the chemical in our cells that gives our bodies instructions about how to grow, develop and function. DNA is a string of coded messages organised into specific instructions called genes. Humans have 30,000 different genes, arranged on a number of thread like structures, called chromosomes. We inherit our chromosomes from our parents, 23 from our mother and 23 from our father, so we have two sets of 23 chromosomes, or 23‘pairs’. A good description is If you think of genetics as the book of life, then the DNA are the letters, the genes are words, and the chromosomes are the chapters.
If you think of a mutation as a spelling mistake or a series of words changed in a sentence, then this causes a problem in the meaning and interpretation and it is just this that happens when there is a mistake or a mutations occurs in a gene – it can cause a condition like an IRD or RED.
Sometimes a mutation will have no effect at all. This depends on environmental factors, an element of chance, or mutations in other genes. Mutations can cause problems if they stop the gene or chromosome communicating the correct instructions needed for the body to function properly. Genetic tests therefore aim to find mutations in a particular gene or chromosome.
Below is further information about genetic testing.
A genetic test takes the form of a blood or tissue sample that is analysed in a laboratory by scientists to identify which gene or chromosome abnormality is causing a particular condition in an individual or family.
The results are very important to the understanding of diseases helping health care providers to better advise patients about how their condition is likely to progress. It provides them with more clarity on how genetic defects are inherited (passed from generation to generation). Genetic testing plays a big role in assisting the researchers to better understand diseases pathways in the hope that it will speed up the process of appropriate treatment for REDs and IRDs. But most importantly Genetic Testing allows patients and their families to have a greater understanding of their condition empowering them with the knowledge to make informed decisions regarding their healthcare and life choices.
Having a genetic test is important not just for the person who is affected but often for the whole family. It provides more clarity on the pattern of inheritance, how these genes with the mistakes in them are passed from generation to generation and how severe the retinal disease may be. It also provides clarity where there is no other evidence of the disease which can be the case with an only child or a recessive condition. Understanding inheritance is particularly important to increase the accuracy of genetic counselling for the family. It helps in identification of family members at risk, or in the risk analysis for a couple and for affected siblings.
Genetic counsellors are health care professionals with specialist expertise in medical genetics and counselling. They genetic counsellor as well as a clinical geneticist and genetic nurse are also referred to as genetics professionals.
Genetic professionals work as members of health care teams providing information and support to individuals or families who have genetic disorders or may be at risk of having an inherited condition. Their work includes the following:
· To assess the risk of a genetic disorder by researching a family's history, evaluating medical records, and conducting a physical examination of the patient and other family members when indicated.
· To examine and evaluate the impact of genetic testing from a medical, social and ethical perspective in each case.
· To provide the support and information needed to assist an individual and their family in making a decision about genetic testing.
· To interpret the results of genetic tests and medical data.
· To provide counselling or refer individuals and families to support services.
· To be an advocate for the patient and their family in the healthcare system.
· To explain the next steps for a patient or their family including possible treatments options.
· To discuss life choices including family planning.
Your health care provider may refer you to a genetic professional. Universities and medical centres also often have affiliated genetic professionals, or can provide referrals to a genetic professional or a genetics clinic.
As more has been learned about genetics, genetic professionals have become more specialised. For example, they may work only on a specific disease like an IRD. Some may work with specific groups only including age groups for example with children or with parents.
A genetic counsellor or genetic nurse can help with testing decisions and support issues, a medical geneticist will make the actual diagnosis of a disease or condition. In the case of IRDs and REDs only a geneticist can provide the most complete and current information about your condition.
Along with a medical geneticist, you may also be referred to a physician who is a specialist in the type of disorder you have. For example, if a genetic test is positive for an IRD then you are referred to a retinal specialist.
Knowing your gene provides invaluable assistance to the researchers who are seeking to better understand the pathways of disease in the hope that they can work more expediently towards effectively treating these conditions in the future. Some research initiatives are gene-specific and sometimes mutation-specific.
The importance of a genetic test, Prof Dra Juliana M. Ferraz Sallum
Ophthalmologist and Geneticist, São Paulo, Brazil
Patients with a retinal dystrophy and their families look for doctors to cure or at least to improve their vision. Currently there is no treatment that can cure or stop the progression of these diseases. Nevertheless it is still important to follow medical care and be regularly seen by your ophthalmologist because there are optical and low vision prescriptions, advice and treatment of the complications that can be associated with these conditions, such as cataracts and macular oedema
Genetic tests are also important as it is not enough to have a clinical diagnosis. In recent years a lot of the genetic causes of these diseases have been identified. Doing a genetic test can identify in most of the cases, but not yet in all, the genetic pathogenic variations. These pathogenic variations are mutations that disrupt the gene and consequently the protein coded by this gene. There are more than 260 genes that can be mutated in persons with retinal dystrophies. In each family, one of these genes is mutated. The identification of the variants in a genetic test is important for 3 reasons:
- First, the precise diagnosis of the dystrophy subtype. It is important for the doctors to better understand the disease and improve care for the specific subtype of disease. It is important for research to look for the missing disease genes and missing variations in these genes. Mutations in many of these genes can also affect non-ocular organs and this fact makes the identification important for proper medical care.
- Second, improve the confidence in the genetic counselling for the patients for example in family planning for other affected members in families. It is important to highlight that sometimes the risk of having a child with the disease is not only for the patient, the risk could be for a relative. It is important to consider the risks in the whole family.
- And the third reason is despite the fact that there is no ready treatment available, there are a lot of clinical trials performed for specific cases due to pathogenic variations in specific genes. So knowing the genotype can allow the patient to be included in a clinical trial.
Many healthcare providers, national health systems and insurers are of the belief that in the case of non-life limiting diseases such as IRDs if there is no treatment option available then there is no need for a genetic test. Retina International and its members disagree with this statement as to many healthcare providers in our community. For people living with these conditions understanding the causative gene can be life changing. The burden of not knowing is a heavy one for many affected and their families. The clarity of a diagnosis can bring to understanding inheritance patterns has been cited by patients as enormous. It allows those affected to plan their future life choices. This is not only reserved for family planning but impacts on decisions such as education, career and even where to live. It can also be a case that you or your family have a condition that can be affected by what you eat as is the case in Stargardt Disease.
Knowing your gene is a prerequisite for participation in a research study that may aid discovery leading, potentially to new treatment possibilities. Importantly knowing the gene causing the IRD or RED in your family can enable you to be on a register at national or international level so that you can participate in appropriate clinical trials and potentially have access to emerging treatments
Because genetic test have very important implications for an individual and their family, they are treated very carefully. Numerous steps are taken to ensure that the correct result is given. If a mutation is found it is always double checked to ensure that the result is correct Often, scientists will perform another test to ‘cross check’ the first result. Procedures are also in place to make sure that samples do not get mixed up. Additionally, many laboratories take part in Quality Assurance (QA) schemes which helps ensure that they perform good quality, reliable genetic tests.
Note: Unless a patient requests that their sample be discarded after testing, a laboratory will usually store the DNA, and may store chromosome samples. Laboratories will be happy to let you know about your sample, and individuals can request at any time that their DNA be destroyed or returned to them.