In most cases of BBS, both parents carry a normal gene and a faulty, recessive gene. Although the parents have one copy of the faulty gene and are called carriers of the disease, they are unaffected by the presence of the faulty gene. For a recessive disease to occur, a child has to inherit two faulty copies of the gene; one from each parent. The child from each pregnancy has a 1 in 4 chance of being affected. If a new born child is not affected then there is a 2 in 3 chance that he/she will be a carrier of the faulty gene for BBS. As the syndrome is rare, a gene carrier is unlikely to have affected children unless their partner is also a carrier. This risk of encountering another carrier increases if people marry close relatives. As some BBS genes are more common than others, there is variation in the frequency with which they are being carried, known or unknown, in the population. For the most common BBS genes, BBS1 and BBS10, the frequency is estimated to be 1 in 250, whereas for a rarer gene such as BBS9, the frequency is closer to 1 in 820.
To date, (2015) mutations in 19 BBS genes have been identified in 85% of BBS patients (see www.ncbi.nlm.nih.gov/books/NBK1363/ for updates on new genes). It is known that there are still more genes to find since not all patients have an identified mutation in any of these identified BBS genes, indicating that these patients must have mutations in other genes. Some genes are more common than others; a quarter of patients have mutations in BBS1 and another quarter have mutations in BBS10. However, patients who carry mutations in the same BBS gene can display quite different symptoms of the syndrome: one might have extra digits at birth whereas another person with an identical mutation may not have extra digits at all. It is hoped that comprehensive genetic testing will improve predictions about disease progression in the future.
Where BBS is confirmed genetically, a simple carrier test is possible in at-risk adult relatives (e.g. siblings) to help determine their own risk of having affected children if their partner is also a carrier. Knowledge of the BBS mutations can also provide the basis for prenatal screening tests, should parents need to know in early pregnancy if the foetus is affected.
Where both parents are carriers of a change in the same BBS gene, every pregnancy has a 1 in 4 risk of the child having Bardet-Biedl Syndrome. If the genetic changes are known in the parents, there are several options available to them, should they wish to have more children.
Couples who are already pregnant may consider prenatal testing (e.g. chorionic villus sampling/amniocentesis). It involves testing either the tissue or fluid around the baby in early to mid pregnancy (10-16 weeks). There is a small risk to the pregnancy with prenatal testing; couples who wish to pursue this option should discuss it with their geneticist and obstetrician.
A couple can be referred to be considered for PGD via a clinician provided that they fulfil a number of criteria. These can vary between NHS centres, however common requirements are that they do not already have a healthy child and that they do not smoke. Couples who wish to pursue this route should contact their GP or geneticist.
Couples who are not interested in either of these options, or do not know the genetic change that has caused BBS may opt to have a detailed scan throughout pregnancy to assess if there is evidence that the baby is affected. This should be done in a specialist centre by an ultrasonographer who is familiar with the features associated with BBS. Features of BBS may be difficult to pick up before 20 weeks gestation and, as already mentioned, may not be present in all babies with BBS.
Those who have BBS, who wish to have children, may also have several options available to them, including the options outlined above. The first step is for the partner of the person who has BBS to be tested for carrier status, to determine whether he or she carries the same change in the same gene.
The children of a BBS affected parent will all be carriers of the affected gene, but will not have the syndrome provided that the other parent does not have any changes in the same gene. If the other parent is a carrier of a change in the same gene, then each child has a 50% chance of having the syndrome. If both parents have BBS, then all children will be affected with BBS.