Genetics 101: understanding transmission and genetic testing of inherited bleeding disorders

Haemophilia is traditionally described as a disorder associated with excessive bleeding or with being a carrier. Those affected by haemophilia are known as haemophiliacs and are usually assumed to be male; carriers are assumed to be female and not affected by excessive bleeding, but have the haemophilia gene. The distinction of bleeding and non-bleeding status is becoming increasingly blurred as it is now recognised that girls and women who have the haemophilia gene can have a bleeding disorder. Although it is very rare for them to be severely affected, many experience heavy menstrual bleeding and other bleeds.

The gene for haemophilia is carried on the X chromosome and may therefore be present in males (who have an X and a Y chromosome) and females (who have two X chromosomes). A man who has the haemophila gene mutation cannot pass it to a son (who receives his Y chromosome), but always transmits it to a daughter (who receives his X chromosome). A woman who has the gene has a 50% chance of transmitting it to a daughter (who receives one X chromosome from each parent) and a 50% chance of transmitting to a son (who receives an X chromosome from his mother) (Figure 1).

Figure 1

Although haemophilia is a rare disorder, many people have heard of it (though they may know little about it). Haemophilia affected several members of the European royal families in the 19th and early 20th centuries through the descendants of Queen Victoria of Great Britain and, perhaps most famously, Alexei, son of Tsar Nicholas II of Russia. The royal family tree of Queen Victoria shows how this pattern of genetic inheritance can affect successive generations (Figure 2).

Figure 2

Not all cases of haemophilia have a known family history. Spontaneous gene mutations of any type are very common (it is estimated that every adult has approximately 1013–10¹⁴ mutations) and can occur in several ways (Table 1). Both haemophilia A and B occur spontaneously in families with no history of the disorder, affecting only one (isolated case) or two or more (sporadic cases) individuals in a single generation. These isolated and sporadic cases each account for 20–25% of haemophilia. A higher proportion of familial haemophilia is of mild to moderate severity compared with isolated and sporadic cases ^[1].

A mutation that causes what appears to be an isolated or sporadic case does not necessarily originate in the individual who has come to clinical attention. Studies of family trees in people with haemophilia A or B have shown the mutation to be present in apparently unaffected mothers in about 60% of cases, who in turn had acquired the mutation from their father or mother ^[2,3]. It is therefore important to carry out genetic testing in the family of a person newly diagnosed with haemophilia in order to identify other family members affected. The genes responsible for genetic transmission of haemophilia are mutated forms of the F8 (for haemophilia A) and F9 (for haemophilia B) genes. Both molecules present many different mutations that can cause haemophilia. In haemophilia A, the most common are the Intron 22 inversion, found in about 40% of people, and the Intron 1 inversion, which occurs in 3–6%. This information is useful when carrying out genetic testing. Knowing the genetic mutation in a family may also help to assess the risk of inhibitor (antibody) formation.

Familial rare bleeding disorders (RBDs), excluding platelet function disorders, may be mild or severe, depending on the deficiency of a particular clotting factor; severe RBDs are very rare. The genes responsible for these disorders are not carried on the X chromosome, and transmission is therefore not sex-linked. For example, factor VII deficiency is inherited in an autosomal recessive pattern: acquiring the mutated gene from one parent means the child is a carrier and may have a slightly reduced level, but a child with abnormal genes from both parents will have a severe bleeding disorder. The sons and daughters of a father with factor VII deficiency and an unaffected mother therefore each have a 50% chance of carrying the gene but are unlikely to have symptoms. The offspring of a father and mother who were both carriers would have a 25% chance of being unaffected, a 50% chance of being a carrier and a 25% chance of a bleeding disorder. Consequently, consanguineous marriage increases the risk that children will inherit a severe RBD, and as marriage between near-relatives is common in some communities, the occurrence of RBDs is often culturally determined.

Where an individual is found to have an RBD, permission should be sought to inform all relatives and invite them to have an assessment. Having a normal level of the coagulation factor does not exclude the possibility of being a carrier. Genetic testing in the person first identified as having the RBD can require time and it is important not to delay this analysis.

Prenatal diagnostic testing may be performed in early pregnancy to diagnose an affected male foetus in time to consider termination, or at a later stage to support safe obstetric management. Current options include chorionic villus sampling (CVS), amniocentesis (sampling amniotic fluid), and cordocentesis (sampling cord blood). CVS can be carried out at 11–14 weeks’ gestation, whereas amniocentesis is performed at 15–20 weeks; both are associated with a one percent risk of miscarriage. Cordocentesis is performed after 20 weeks’ gestation to assess foetal factor level when the causative mutation is unknown or genetic analysis is not available. It is associated with a higher risk of miscarriage compared to CVS and amniocentesis, and is little used for prenatal diagnosis in Western Europe.

Most women with haemophilia consider non invasive prenatal diagnosis to determine the foetal sex ^[16]. Studies have shown that women with haemophilia are less likely to choose prenatal diagnosis than women whose foetus is at risk of Down’s syndrome (23% vs 71%) ^[12]. About one in three women decline an invasive procedure due to the risk of pregnancy loss ^[16].

Foetal sex determination allows female foetuses to be excluded from subsequent invasive testing. Options for non-invasive testing to assess foetal sex include ultrasound examination of foetal genitalia, with 98–100% accuracy in the second trimester. In the first trimeste, analysis of free foetal DNA in maternal plasma for the detection of Y chromosome sequence offers 99–100% accuracy for foetal sex determination from 8–10 weeks’ gestation ^[17,18]. Experience at the Royal Free Hospital in London shows that uptake of prenatal diagnosis in 2005–2018 was twice that in 1985–1995. This was associated with a corresponding increase in knowledge of foetal sex to the attending obstetrician; uptake of invasive testing declined over the same period.

Decisions about the management of labour and the mode and place of delivery can be supported by amniocentesis during the third trimester; confirmation that the pregnancy is unaffected means delivery can take place in the local maternity unit. This timing avoids the risk of miscarriage but there is a one percent chance of early labour, raising the possibility that delivery could occur before the results of the test are known. In one London centre, late amniocentesis in nine pregnancies identified three males with haemophilia, of whom two were delivered vaginally and one by emergency Caesarean section (for foetal bradycardia). Five foetuses were confirmed normal, of whom four were delivered vaginally and one assisted by vacuum extraction. One amniocentesis failed and an unaffected boy was delivered vaginally ^[19].

Analysis of free foetal DNA in maternal plasma has been shown to determine non-invasively whether a male foetus has inherited a haemophilia gene from his mother by identifying an imbalance between the mutant and wild-type gene in maternal plasma using quantitative PCR technology. In one study, haemophilia mutations were correctly identified in all of 12 at-risk pregnancies as early as 11 weeks’ gestation ^[20]. In another study, foetal DNA analysis in maternal plasma was also used successfully for prenatal diagnosis of haemophilia caused by gene mutations most frequently associated with severe haemophilia ^[21]. It is hoped that these non-invasive approaches are soon validated for use in clinical practice in the near future and can be used to identify affected male pregnancies after early determination of sex, offering a potentially safer alternative to CVS and amniocentesis, and avoiding the risk of miscarriage associated with these procedures.

Preimplantation genetic diagnosis is a very early form of prenatal diagnosis, in which embryos created in vitro are analysed for the causative genetic mutation and only unaffected embryos are transferred to the uterus.

Preimplantation genetic diagnosis is a challenging process for potential parents, both emotionally and physically. It involves hormone therapy to induce ovulation followed by a procedure to obtain eggs, which are then fertilised. The embryo is biopsied to obtain genetic material; embryos found to be female or unaffected by a gene mutation are then returned to the uterus. This procedure has largely been used to select female embryos for embryo transfer, but has recently been used to identify male embryos without haemophilia. In the UK, the application of preimplantation diagnosis for women with X-linked disorders is associated with a pregnancy rate of 29% and a live birth rate of 27%; 9% of pregnancies have been twins. There are concerns about safety because the removal of genetic material for analysis at such an early stage may affect foetal viability and development, and there is a risk of misdiagnosis.

Comprehensive chromosome screening can be used to identify embryos with chromosomal abnormalities; this technique, along with selective single embryo transfer, increases the rates of implantation and live birth ^[22], and reduces multiple pregnancies.

All options should be considered when counselling potential parents affected by haemophilia about their reproductive choices. These include assisted fertility techniques, such as egg or embryo donation, offering more approaches for parents affected by haemophilia. There is also the possibility of adoption and fostering, or of having no children. All the procedures for prenatal diagnosis and assisted pregnancy are challenging and may affect mental and physical health. Counselling and support should be available so that women in particular have opportunities to talk about their feelings.