Reproductive carrier screening

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PRACTICE POINT

All women/ couples planning a pregnancy, or who are already pregnant, should have a comprehensive family history recorded.

Women/ couples who are known carriers Recessive genetic conditions such as cystic fibrosis (CF) occur when a person inherits a particular genetic variant from each parent. A carrier is an individual who only has one copy of the gene variant and generally does not have symptoms, but can pass the variant to their children. Some conditions are due to a pathogenic variant in a gene on the X chromosome (X-linked inheritance). Typically, these conditions affect more males (who have the sex chromosomes XY) than females (who have the sex chromosomes XX). A woman who is a carrier of an X-linked condition has the variation on one of her X chromosomes, which she can pass on to her children. However, if the biological male has a pathogenic variant in an X chromosome gene, he will not pass it to his sons, but will pass it to all of his daughters. of a genetic condition or have a relevant family history should be made aware of the availability of carrier screening Carrier screening is a test to determine whether an individual carries a genetic variant that does not generally affect that individual’s health, but increases his or her chance of having children with the condition in question. The outcome of such testing can influence future reproductive decisions. Carrier screening is performed on individuals who are not necessarily known to be at increased risk for a particular genetic condition. Screening tests can be conducted on individuals from specific groups such as those from a common ethnic background (eg: screening for Tay-Sachs disease carrier status in the Ashkenazi Jewish community) or entire populations. Carrier screening and offered referral to specialist services (ie genetics or obstetrics).

Information on carrier screening for at least the more common genetic conditions that affect children (eg cystic fibrosis <link to chapter> [CF], spinal muscular atrophy [SMA], fragile X syndrome [FXS]) should be offered to low-risk women and couples (ie regardless of family history and ethnicity). Women/couples can be offered larger panels of genes for carrier screening (expanded carrier screening, ECS).

The decision to have screening is a personal choice to be made by the individual/couple.

What do I need to know?

Reproductive carrier screening is used to identify carriers of genetic conditions with an autosomal recessive inheritance Autosomal recessive conditions affect either sex, and often occur in the absence of any family history. Recessive conditions or traits appear when an individual inherits two copies of pathogenic variants in the same gene (one from each parent). Parents of a child with an autosomal recessive condition are usually asymptomatic carriers. The affected child has two copies of the particular gene change. The recurrence risk of autosomal recessive conditions is one in four for each pregnancy. Wide variability in clinical expression is common in many autosomal recessive conditions. Autosomal recessive conditions are more common when the parents are consanguineous. or X-linked inheritance pattern.

Information about carrier screening should be offered to all women or couples during pre-conception and early in the pregnancy (ie first trimester). Identifying carrier couples before pregnancy provides greater reproductive options. For example:

in-vitro fertilisation (IVF) with pre-implantation genetic testing Pre-implantation genetic testing is testing performed on embryos produced by IVF. Prenatal testing of successful pregnancies may be undertaken as pre-implantation genetic testing is less than 100% accurate.
use of donor gametes
prenatal diagnostic testing

Traditionally, reproductive carrier screening for inherited recessive conditions was offered on the basis of ethnicity. However, this is known to lead to significant under-identification of carrier couples. Given the multicultural nature of society and people of different ethnic backgrounds having children, ethnicity is poorly predictive of carrier frequency in Australia.

The decision to undertake carrier screening is a personal choice to be made by the individual/ couple. Women/ couples should be informed of the benefits, limitations and cost of screening. Ideally, this information is provided pre-pregnancy.

Genetic testing

A number of pathology services offer carrier screening, which can be ordered through general practice. There are MBS items available for carrier screening, fragile X syndrome, SMA and cystic fibrosis. Depending on the test provider:

screening may look for a limited number of conditions (eg CF, fragile X syndrome [FXS], spinal muscular atrophy) or screen for an expanded range of conditions (ie >100)
genetic counselling may or may not be available from the pathology provider.

Note: Carriers of haemoglobinopathies may be initially identified through a routine full blood examination (FBE). Mackenzie’s Mission, a national research project funded by the Federal Government, is in progress but closed for further recruitment.

It aims to identify how reproductive carrier screening for an expanded range of conditions can best be made available to Australian couples who want it.

When should I refer?

Couples identified as high risk of having a child with a genetic condition should be offered referral to specialist services (ie genetics or obstetrics), that is couples where both are carriers of the same autosomal recessive condition and/or where the woman is a carrier of an X-linked condition.

Other considerations

Carrier screening needs to occur in a timely manner to provide women or couples with reproductive options. The testing of biological male partners of pregnant female carriers is of particular importance.

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