Population Screening for Reproductive Risk for Single Gene Disorders in Australia: Now and the Future
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Population Screening for Reproductive Risk for Single Gene Disorders in Australia: Now and the Future Martin B. Delatycki1,2 1 Bruce Lefroy Centre for Genetic Health Research, Murdoch Childrens Research Institute, Australia 2 Genetic Health Services Victoria, Australia s the results of the Human Genome Project are Carrier testing programs for genetic disorders began Arealized, it has become technically possible to formally in the early 1970s for Tay Sachs disease (TSD) identify carriers of numerous autosomal and X-linked by measurement of serum hexoseaminidase A (Kaback recessive disorders. Couples at risk of having a child et al., 1997). This was followed by carrier screening with one of these conditions have a number of repro- programs for β thalassaemia by measurement of mean ductive options to avoid having a child with the corpuscular volume and haemoglobin electrophoresis condition should they wish. In Australia the haemo- (Davies et al., 2000). This highlights the fact that globinopathies are the only group of conditions for genetic screening is not always by DNA testing. Indeed which population screening is widely offered and the genetic basis of TSD and β thalassaemia only which is government funded. In some Australian became known some years after screening programs states there are also population screening programs were introduced. If both members of a couple are carri- for cystic fibrosis and autosomal recessive conditions ers of the same autosomal recessive condition or a more common in Ashkenazi Jewish individuals which woman is a carrier of an X-linked condition there is a 1 are generally offered on a user pays basis. It is pre- in 4 (25%) chance of having a child with the condition. dicted that as consumer demand increases and The most often cited rationale for carrier screening testing becomes cheaper, that many people planning is to offer couples reproductive choice by identifying or in the early stages of pregnancy will have carrier those who are at high risk of having a child with a screening for multiple genetic conditions. This will genetic condition (Davies et al., 2000). The reduction have significant implications for genetic counseling, in incidence of genetic conditions through prenatal laboratory and prenatal testing resources. In addition diagnosis and pregnancy termination as well as preim- such screening raises a number of ethical issues plantation genetic diagnosis is often used to measure including the value of lives of those born with genetic conditions for which screening is available. the success of carrier screening (Marteau & Anionwu, 1996). Indeed if few couples identified by screening as Keywords: screening, carrier, cystic fibrosis, thalassaemia, being at high risk of having a child with a particular Tay Sachs disease genetic condition acted on the information, screening programs could not be justified on economic grounds (Haddow, 1997; Zeuner, 1997). Similarly, the uptake There are more than 2,000 autosomal, X-linked and of screening, whilst often used to measure the success mitochondrial disorders whose genetic basis is known of a program, is only one measure by which the value (McKusick, 2008). It is theoretically possible to offer of a program should be assessed (European Society testing for heterozygous mutation status (so called of Human Genetics, 2003). Other important aspects carrier testing) for all autosomal recessive and X- to assess the success of programs are the level of linked disorders whose genetic basis is known and informed consent among those screened and those therefore to provide advice about the risk of any declining screening, the proportion of the target popu- couple having a child affected by these conditions. In lation offered screening, harms in those screened and reality it is only practical to offer population genetic economic outcomes (European Society of Human screening for few conditions largely because of the Genetics, 2003). cost of identifying carriers, the length of time it takes to do testing and the rarity of most genetic disorders in any community. Population screening refers to Received 27 April, 2008; accepted 7 May 2008. testing for carrier status in individuals who are not at Address for correspondence: Associate Professor Martin Delatycki, increased risk of being a carrier because of a family 10th Floor, Royal Children’s Hospital, Flemington Road, Parkville, history of the condition. 3052, Victoria, Australia. E-mail: [email protected] 422 Twin Research and Human Genetics Volume 11 Number 4 pp. 422–430 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.42, on 01 Oct 2021 at 12:06:06, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1375/twin.11.4.422 Reproductive Screening for Single Gene Disorders (Barlow-Stewart et al., 2003; Gason et al., 2005). These Table 1 will therefore be discussed in some detail. Reproductive Options Available to Couples Where Both are Carriers of the Same Autosomal Recessive Condition or the Female is a Carrier of Haemoglobinopathies an X-linked Condition The haemoglobinopathies are a group of autosomal Reproductive option Nonpregnant Pregnant recessive conditions that affect the function of haemo- Prenatal testing with pregnancy Yes Yes globin. They are the most common global recessive termination of affected fetuses conditions with about 270 million people (4.5% of the Pre-implantation genetic diagnosis Yes No world’s population) being carriers and at least 300,000 Donor gamete or embryo Yes No individuals are born each year who are affected by one Adoption Yes No of the haemoglobinopathies (Angastiniotis & Modell, Note: The pregnancy status refers to whether the female is pregnant at the time the 1998). Haemoglobinopathies can be divided into struc- couple’s carrier status is identified and whether the specific reproductive option tural variants, the commonest of which is sickle cell is available for that pregnancy. disease, and those that have quantitative effects on haemoglobin chains. The quantitative haemoglo- Couples may wish to know their carrier status to binopathies are the thalassaemias: the globin proteins be prepared for the birth of a child with a genetic are structurally normal but there are insufficient α- or condition or to avoid having a child affected by the β-globin chains (Old, 2007). condition. Couples at high risk have a number of The two main forms of thalassaemia, α and β, reproductive options to enable them to avoid having a result from mutations in the α- and β-globin genes, child affected by the condition. These are outlined in respectively. The genetics of α-thalassaemia is compli- Table 1. cated by the fact that there are four rather than two α-globin genes. Thalassaemia is most common in Criteria for the Introduction people from Mediterranean countries, including the of Carrier Screening Middle East, India, Africa and south-east Asia (Davies The decision to offer population carrier screening for et al., 2000). a genetic condition is generally arrived at because the β thalassaemia major results in chronic anaemia condition fulfils a number of criteria. The basic prin- requiring regular blood transfusions and iron chela- ciples of population screening were developed by tion therapy (Birgens & Ljung, 2007). Because there Wilson and Junger in 1968 (Wilson & Junger, 1968). are four α-globin genes, an individual may have 0–4 A significant recent addition to the principles of popu- functioning α-globin genes. If a fetus has no func- lation screening is that appropriate education should tional α-globin genes, they will have haemoglobin be provided so that individuals can make informed Bart’s hydrops fetalis syndrome and will usually not decisions about having testing and that the individ- survive (Birgens & Ljung, 2007). The presence of a ual’s decision is respected and they are protected from single functional α-globin gene results in haemoglobin stigmatisation and discrimination (Andermann et al., H disease and may require regular blood transfusions 2008). The principles of population genetic screening and iron chelation if the affected individual has a are listed in Table 2. moderately severe haemolytic condition. Individuals with two or three functional α-globin genes are Cascade Testing healthy carriers of α-thalassaemia. Their partner Cascade testing refers to testing for carrier status of relatives of individuals with a genetic condition or where an individual is identified as a carrier of such a condition (Barlow-Stewart et al., 2007). The rationale Table 2 is that genetic relatives are at much higher than back- The Principles of Population Genetic Screening (Khoury et al., 2003; ground risk of being a carrier of a genetic condition in Wilson & Junger, 1968) their family. If the causative mutation(s) is known in Principle the family, carrier testing is available and can be 1 It screens for an important problem offered to relatives irrespective of the population fre- 2 There is acceptable treatment quency of the condition. 3 There are facilities for diagnosis and treatment Population Genetic Screening in Australia 4 There is a recognized latent or early symptomatic stage 5 The natural history is understood Population genetic carrier screening is widely conducted 6 There is a suitable test acceptable to population in Australia for haemoglobinopathies (Metcalfe et al., 2007). Population screening is offered in some places 7 There is an agreed policy on who to treat for cystic fibrosis (Barlow-Stewart et al., 2003; Christie 8 The cost of case finding is balanced against total expenditure et al., 2006; Massie et al., 2005) and autosomal reces- 9 There is a continuous process of case finding sive conditions more common among Ashkenazi Jews 10 Appropriate education should be provided Twin Research and Human Genetics August 2008 423 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.42, on 01 Oct 2021 at 12:06:06, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.