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Prenatal Screening for Chromosome Disorders

Prenatal screening for chromosome disorders by maternal serum screening, ultrasound and non-invasive prenatal tests, such as Harmony®, is an established part of reproductive care in Australia.

The overall risk of chromosome disorders rises markedly with maternal age, as shown in Figure 1. (There are two exceptions: Monosomy X, also known as Turner syndrome, and microdeletions, such as 22q11.2, occur independently of maternal age). This does not mean that chromosome screening should be restricted to older mothers.

Younger mothers have more babies than older mothers, and the overall outcome is that the majority of pregnancies with a serious chromosome disorder occur in mothers under 35 years of age. For this reason, screening for chromosome disorders in pregnancy should be offered to mothers of all ages.

The great majority of these chromosome disorders are new abnormalities that have happened for the first time in this pregnancy. They are not inherited disorders, and genetic testing of the parents provides no information about the risk of such an abnormality. This provides another reason for offering screening for chromosome disorders to all mothers, irrespective of family history.

 

The frequency of single-gene disorders at birth

Chromosome disorders are not the only type of genetic condition which can affect the developing foetus. Many serious childhood disorders are due to recessive mutations that have been inherited from parents, with the parents being unaffected by these mutations. A parent who is a carrier of a recessive mutation, that is, having one normal and one abnormal copy of a gene, will not be affected by the abnormal gene. Everyone is a carrier for one or more disorders; this is of no immediate consequence and there usually is no family history of the disorder.

The situation changes if both parents are carriers of mutations in the same gene located on one of the autosomes (chromosomes 1-22). The chance of their child inheriting the abnormal gene from each parent, and so developing an autosomal recessive disorder, is 25%. The situation is a little different for a woman with a recessive mutation on an X-chromosome: each of her sons is at 50% risk of inheriting the abnormal gene and being affected, and half of her daughters will be carriers. Overall, the risk of a woman who is an X-linked carrier having an affected child is approximately 25%.

There are hundreds of inherited autosomal and X-linked recessive disorders that present in infancy and early childhood. These disorders are individually rare but, together, they are more common than the chromosome disorders for which prenatal screening is widely available and accepted. Further, the risk of these recessive disorders does not vary with maternal age (Figure 1). For mothers under 35 years of age, the risk of having a child with a serious childhood-onset recessive disorder is greater than the risk of having a child with a chromosome disorder.

 

Screening potential parents for recessive disorders

These disorders are inherited but there is usually no family history to provide a clue.

Until recently, the only way of identifying a carrier of a rare recessive disorder was to diagnose the disorder in their affected child.

This has now changed.

It is possible to screen a couple for mutations in autosomal genes, and a woman for mutations in X-linked genes, to determine whether they are at 25% risk of having an affected child. This screening test is called ’reproductive carrier screening’.

From both a technical and clinical perspective, the challenge lies in choosing which genes to analyse. A number of providers, including Sonic Genetics, offer reproductive carrier screening for mutations responsible for three common disorders: cystic fibrosis and spinal muscular atrophy (both autosomal recessive) and Fragile X syndrome (X-linked recessive). Approximately 6% of people are carriers of one or more of these conditions, and 0.6% (one in 160) couples are at 25% risk of having an affected child.

Those couples who are identified as carriers can consider a variety of options, including IVF with a donor gamete, pre-implantation genetic diagnosis, prenatal diagnosis by CVS, or they may make an informed decision to accept the risk.

RANZCOG recommends that couples be offered such screening.

The cost of this three-gene panel is approximately $400* per person. There is no Medicare rebate for carrier screening; there are exceptions (and restrictions) for people with a documented family history of cystic fibrosis or Fragile X syndrome.

 

Expanded reproductive screening

If we were to screen more genes, we would identify more carriers.

Sonic Genetics offers a screen of over 300 genes (autosomal and X-linked) which cause serious recessive childhood disorders. We estimate that approximately 70% of Australians are carriers for one or more conditions included in this screen and 3% (one in 30) couples are at 25% risk of having an affected child. This amounts to five times more information than is provided by the three-gene panel.

This screen, the Beacon Expanded Carrier Screen, currently costs $995* per person or $1,750* for couples tested together.

It is tempting to think that ‘more genes tested = more information for a couple’. This is not the case because the information provided by a carrier screen is also determined by the carrier frequency, mode of inheritance and detection rate of the assay for each gene.

Some currently available screens of more than 100 genes provide less information than the three-gene screen described earlier.

 

Implementing reproductive screening

Before offering reproductive carrier screening to your patients, it is important to consider some of the nuances, particularly in relation to the Fragile X syndrome (some carriers will develop premature ovarian failure or a tremor/ataxia syndrome in later life) and when there is a family history of a recessive disorder (seek expert advice; do not rely on screening).

It is also important to recognise that some couples will not want this carrier information – and others will demand it. Each person needs to be free to make their own decision about what information they wish to have.

We provide information about the three-gene and Beacon screens for both requestors and patients on our website.

Sonic Genetics also offers genetic counselling free-of-charge for couples who are identified by either of these reproductive carrier screens as being at high risk of having an affected child (see www.sonicgenetics.com.au/rcs/gc).

 

Conclusion

It is accepted practice that every woman is offered screening for chromosome disorders in pregnancy, irrespective of age and family history.

In a similar vein, every couple should be offered reproductive carrier screening for recessive disorders, irrespective of age and family history. For women under 35 years, the risk of their child having a recessive disorder is greater than the risk of a chromosome disorder. Offering reproductive carrier screening simply represents good medical practice.

 

References

RANZCOG. Prenatal screening and diagnostic testing for fetal chromosomal and genetic conditions. 2018 Aug. 35 p. Available from: https://www.ranzcog.edu.au/RANZCOG_SITE/media/RANZCOG-MEDIA/Women%27s%20Health/Statement%20and%20guidelines/Clinical-Obstetrics/Prenatal-screening.pdf?ext=.pdf

Archibald AD, Smith MJ, Burgess T, Scarff KL, Elliott J, Hunt CE, et al. Reproductive genetic carrier screening for cystic fibrosis, fragile X syndrome, and spinal muscular atrophy in Australia: outcomes of 12,000 tests. Genet Med. 2018; 20(5): 513-523 Available from https://www.ncbi.nlm.nih.gov/pubmed/29261177 doi:10.1038/gim.2017.134.

Sonic Genetics [Internet]. c2015. Reproductive Carrier Screening; 2018. Available from: www.sonicgenetics.com.au/rcs

 

General Practice Pathology is a new regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs.

The authors provide this editorial, free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.