Facts about genetic testing

Genetic conditions in the Ashkenazi Jewish population
There are a number of serious genetic conditions that are more common in the Ashkenazi Jewish population including Tay Sachs disease, cystic fibrosis, Bloom syndrome, Fanconi anaemia, familial dysautonomia, Canavan disease, Niemann Pick disease and mucolipidosis IV. These are all autosomal recessive conditions. Carriers of these conditions have one working copy of the gene, and one non-working copy of the gene. Carriers are healthy and won’t develop the condition, as the working copy of the gene compensates for the nonworking copy. However, if both members of a couple are carriers of the same condition, they have a one-in-four (25 per cent) chance of both passing on the non-working copy of the gene, and having a child with the condition.

What if no one in my family has a genetic condition?
Most people who find out that they are a carrier of one (or more) of these genetic diseases don’t have a family history of the condition. As carriers are healthy, they are generally unaware of their carrier status until they have a child affected with the condition or if they have carrier testing. As a result, these altered genes can be passed through many generations of a family without people realising. It is important to be aware that the absence of a family history doesn’t reduce the chance that an individual is a carrier of a genetic condition.

Conversely, if an individual has a relative who is a known carrier of a genetic condition, they have a higher chance of being a carrier. For example, if someone of Ashkenazi Jewish descent doesn’t have a family history of Tay Sachs disease, their chance of carrying the condition is 1 in 28. However, if their sibling or parent is a known carrier of Tay Sachs disease, their chance of being a carrier is 1 in 2, or 50 per cent. So while it may be a difficult topic to bring up, it can be useful to be aware if someone in the family carries a genetic condition.

When should I have carrier screening?
It is useful for couples to have a discussion around their attitudes towards genetic carrier screening to determine if they share the same beliefs and values towards this type of testing, and to consider what they might do if they are found to be at risk of having a child with a serious genetic condition. It is important to note that genetic screening is optional and voluntary, and couples should be aware of the type of information these types of tests can provide, and the implications of the results. The best time for couples to consider carrier screening is before starting a family, although screening can also be performed in early pregnancy. Couples who both carry the same genetic condition have a number of options if they wish to avoid having a child with the condition. These include:

1. Using IVF with preimplantation genetic testing (PGT), a technique where embryos are tested at an early stage of development, and only embryos without the genetic condition are placed in the uterus to achieve a pregnancy.
2. Conceiving naturally and testing the fetus early in pregnancy.
3. Using donor eggs or sperm from an individual who is not a carrier of the condition.
4. Adoption.

How can I be screened for these genetic conditions?
Genetic screening is available to individuals and couples who would like to know if they are carriers of these serious genetic conditions. In 1998, genetic carrier screening for Tay Sachs disease was made available to year 11 students attending Jewish schools in Melbourne, and more conditions were added to the test over the next 15 years. If you were tested at school, you are welcome to contact VCGS to check which conditions you were screened for. For example, if you were only tested for Tay Sachs disease at school, you could now consider screening for more genetic conditions before starting a family.

It is also recommended that all women, regardless of their ancestry, are offered carrier screening for cystic fibrosis, spinal muscular atrophy (SMA) and fragile X syndrome, the three most common genetic conditions in the general population. A more recent test option, called expanded carrier screening, tests couples for several hundred genetic conditions, including all of the ones already mentioned. This is a more comprehensive option for couples, particularly for those who don’t have full Ashkenazi Jewish ancestry. VCGS runs annual community genetic carrier screening evenings where testing for these conditions is available. Alternatively, you can contact associate genetic counsellor Kate Scarff (contact details below) to arrange a free consultation to discuss your genetic carrier screening options.

Kate Scarff is the associate genetic counsellor for the Victorian Clinical Genetics Services (a not-for-profit subsidiary of the Murdoch Children’s Research Institute). For more information, call 0427 798 860, email katrina.scarff@vcgs.org.au or visit vcgs.org.au/tsd.