Genetic Screening Tests

I recently attended a fascinating talk as part of the South Australian Obstetric Shared Care update by Associate Professor Chris Barnett from the Women’s and Children’s Hospital about the developments in carrier screening and what it means for families. Below are some of those insights. Warning – acronyms aplenty ahead!

The Royal Australian College of Obstetricians and Gynaecologists (RANZOCG) is now recommending that information on carrier screening should be offered to all women planning a pregnancy or in the first trimester of pregnancy. This is not the same as first trimester screening or NIPT (Non Invasive Prenatal Testing) which is offered to test a foetus for Trisomy 21 (Down Syndrome), Trisomy 18 and in some cases additional chromosomal anomalies

Carrier screening is a genetic test to determine if you are a carrier for 3 genetic conditions – spinal muscular atrophy (SMA), cystic fibrosis (CF) and Fragile X syndrome (FXS). They are some of the most common inherited conditions in our population and have serious health consequences. A person may be a carrier without knowing and may not have any family history of these conditions.

Cystic fibrosis is an inherited condition affecting the respiratory and gastrointestinal systems. CF results in thick mucus accumulating in the lungs, trapping bacteria and leading to recurrent chest infections and subsequent lung damage. In the gut, the digestion of food is difficult, leading to inadequate nutrition at times. People with CF often have to take a number of different medications and require multiple hospital admissions. Tragically, people with CF have a reduced life expectancy and there is currently no cure. Before carrier screening, testing for CF was done on the neonatal heel prick (Guthrie) test soon after birth. For more information on CF, click here. 

Fragile X Syndrome is the most common cause of inherited intellectual disability. Those affected may have developmental delay, epilepsy, autism and learning and fertility difficulties. Males are more commonly affected than females. There is currently no cure but multidisciplinary teams focus on optimising management. For more information on Fragile X Syndrome, click here.

Spinal Muscular Atrophy is a condition that affects nerves and muscles, resulting in weakness, which in turn affects the muscles used to breathe. There are 4 types of SMA, with SMA type 1 being the most common and most severe, with babies affected soon after birth and not usually living beyond 2 years of age. There is currently no cure. For more information on SMA, click here. 

The test can be performed in early pregnancy or in the first trimester by blood or saliva sample. Mum is usually tested first and Dad only needs to be tested if she is a carrier for CF or SMA. If both parents are carriers their children have a 1 in 4 chance of developing the condition and a 1 in 2 chance of being a carrier for the condition. In the case of Fragile X, male carriers are not considered to be at risk of having children affected by Fragile X, although their daughters will inherit a premutation and be at risk of having affected children themselves.

An individual has a 1 in 25 chance of being a CF carrier, a 1 in 40 chance of being a SMA carrier and a 1 in 250 chance of being a fragile X carrier. You only need to be tested once (i.e. not with each pregnancy) as your carrier status stays the same. All carriers would be referred to a genetic counselling service for further support and information. Couples may decide to undertake genetic testing of a conceived foetus using chorionic villus sampling (CVS – sample of placental tissue to assess genetic material of the foetus) for a definite diagnosis or to undergo in-vitro fertilization (IVF) with pre-implantation genetic diagnosis to determine if any embryos are affected. Other families may wish to continue a pregnancy regardless but want to know a potential diagnosis before the birth. Other options for couple are considering using donor egg/sperm/embryo, adoption or not having children. As with many things, it is a very personal decision.

It is important to remember that no test guarantees a healthy baby. The screening tests are 90-99% accurate depending on the condition due to the presence of rare genetic mutations in some individuals. There are more extensive panels also available which test for a number of different, less common conditions.

This test is not Medicare rebatable at present, although thanks to Mackenzie’s Mission (read more about this here), programs are being rolled out to fund carrier testing. The current cost is variable depending on the provider ($350-$400). This can be discussed further with your referring doctor.

More information can be found at:

https://www.vcgs.org.au/tests/prepair

https://www.clinicallabs.com.au/patient/our-tests/pregnancy-testing/gene-access-carrier-screen/

https://www.sonicgenetics.com.au/wp-content/uploads/2018/06/PreconceptionCarrierScreening_MS_square-SG_2018revised_final.pdf

by Dr Rhiannon Smith

 

Image from: https://globalgenes.org

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