Reproductive Carrier Screening

Carrier screening
In 2017, Sydney couple Rachael and Jonathan Casella faced every parent’s worst nightmare when they lost their 7-month old baby, Mackenzie.  Mackenzie had Spinal Muscular Atrophy (SMA) Type 1, a rare genetic condition that causes muscle wasting and severe weakness.  Most babies with SMA Type 1 die before they reach two years of age.  It’s a cruel disease for which there is currently no cure.

SMA is a “recessive” genetic disorder.  Recessive disorders happen when a baby inherits two copies of a damaged gene, one from each parent.  The parents themselves are healthy and usually have no idea they carry a damaged gene.

It is possible to test couples for the damaged genes involved in SMA and hundreds of other genetic conditions.  This technology has existed for several years, but it has not been widely available and it is quite expensive.  Also, most of these genetic conditions are very rare.  Therefore, couples weren’t routinely offered these tests.

When Mackenzie’s parents learned that a blood test would have identified them as carriers for SMA, they started a campaign to make it available to all couples.  The Australian Government has announced a trial of free testing and follow-up for selected couples planning a pregnancy.  This program will start in 2019.  In the meantime, the tests are available to patients willing to pay privately.

A bit about recessive inheritance

A gene is like a little instruction that has been written down.  Each gene tells our body to do something – for example, to make a particular protein.  When a gene is damaged, our body gets the wrong instruction.  This causes illness.

We inherit most genes in pairs – one from our mother and one from our father.  For some genes, we only need one of that pair to work properly.  Even if we inherited one damaged gene, the other working gene will take over and we will be healthy.  But we can pass the damaged gene on to our children.  A “carrier” is a healthy person who carries a damaged gene and can pass it to their child.

The problem comes when two carriers get together and have a baby.  In this situation, each parent has a damaged gene they can pass on to the baby.  If a child inherits the same damaged gene from both parents, that child will have two damaged genes and no working gene.  That child will be sick.  When two carriers have children, there is a 25% chance of each child being sick.  There is also a 50% chance that any children will be carriers like their parents.

This is called recessive inheritance.  There are other ways to inherit genetic conditions.  However, most of the conditions listed for reproductive carrier screening are recessive conditions.

What is carrier screening?

Screening is when we test for a disease, or the precursor to a disease, in people who are healthy and low-risk.  A good example is breast cancer.  We do mammograms on all women of a certain age, even when the women have no family history or symptoms.  We do this because we know picking up breast cancer early on gives us lots more options for successful treatment.

Reproductive carrier screening looks for a range of genes that we know cause serious diseases.  It tests healthy people to see if they are unknowingly carrying a damaged gene.  The aim is to identify couples who carry the same damaged gene and are at risk of having affected children.

It’s important to note that screening is different to testing for known conditions.  If you know there is genetic condition in your family, you should not rely on carrier screening tests.  Instead, you should be referred to a genetic service for targeted testing.

What’s available in Australia now?

Several laboratories offer carrier screening for a private fee.  The tests are expensive and there is no Medicare rebate, so you have to pay the full cost yourself.

The most widely available test looks at three of the most common serious genetic conditions that affect young children.  These are Spinal Muscular Atrophy (SMA), Cystic Fibrosis (CF) and Fragile X syndrome (FXS).  Fragile X syndrome is a special case, because it can also cause health problems for carriers.

Extended tests are also available.  For example, one laboratory in NSW offers a test that screens for 327 conditions.  Individually, all of these conditions are very rare.  However, the laboratory states that around 1 in 30 couples will be identified as high-risk for having a child affected by one of these disorders.

What happens if my partner and I get a positive result?

Your doctor will refer you to a genetics service.  The genetics service will counsel you about your options and help to organize further care.

Some couples may choose to do IVF and have the embryos tested for the disease – this is called pre-implantation genetic diagnosis (PGD).  Embryos are not implanted if they are affected.

Other couples, including those who are already pregnant, may choose to have their unborn baby tested.  A Maternal-Fetal Medicine specialist uses a needle to take some tissue or fluid from around the baby.  This is called amniocentesis or CVS.  Couples have the option of pregnancy termination if their baby is affected.

I’m thinking of doing the carrier screening test – what should I be aware of?

Before you have any medical test, you should stop and ask yourself

  • What exactly will this test tell me?
  • What does the test NOT tell me?
  • How will I use this information?
  • Does this information have any other implications?

It’s extremely important to go through this thought process if you are considering carrier screening.  Carrier screening is not mandatory and there are some areas of controversy.  Each couple should feel absolutely free to accept or decline the test based on their personal preferences.  You should also discuss screening with a doctor or genetic counsellor who is familiar with these tests.

Different laboratories offer slightly different test panels.  It’s important to know which conditions are on your panel.  For example, the most common carrier screening test looks for three specific conditions, whereas some other panels test for hundreds of conditions.

It’s also really important to remember that not all conditions can be identified on a blood test.  There is no such thing as a test that checks for “everything” or guarantees you a healthy baby.

The most important question to ask yourself is what you would do with a positive result.  Would it change anything for you?  Would you be prepared to do IVF, or have an amniocentesis with a small risk of miscarriage, or terminate a pregnancy?  These are very personal questions for which there are no right or wrong answers.  If you feel the information would be helpful to you, then go ahead.  If you would not act on a positive test result, you may want to reconsider the test.

It’s also important to understand that a positive result may have implications for your existing children and other family members. Finally, a positive result may have unexpected effects on things like your insurance premiums.  Because this is a very new field, we are only just working out some of the non-medical implications of having this genetic information available.

When should I be tested?

Ideally, have the test before you start trying to conceive.  Before pregnancy, there are more options available to reduce your chance of having an affected child.

If you are already pregnant, you can still have the test.  If you have a positive result, you will need to talk to a Maternal-Fetal Medicine specialist.  The MFM specialist will discuss the options for testing your unborn baby and how the result might affect your pregnancy.