In recent years, several genes that can cause hearing loss and deafness have been discovered. Today, genetic carriers of mutations in these genes can easily be diagnosed using simple genetic tests.
The most common mutations causing hereditary hearing loss and deafness are mutations in the genes connexin 26 and connexin 30, which account for around forty percent of cases of congenital deafness. In addition, mutations to the gene TMC1 are also linked to congenital hearing loss.
People can be carriers of these mutations without being deaf since this gene has an extra healthy copy. If both parents are carriers of these mutations, they can pass two mutated gene copies to their child, who in these cases will be born deaf.
There is an ethical dispute regarding the subject of pregnancy termination following a diagnosis of deafness. On the one hand, deafness is considered a significant condition, and on the other hand, deaf people can have a high quality of life.
For this reason, prenatal screening tests for diagnosing deafness are not always recommended by professionals but can be performed according to parents’ wishes.
Populations at risk for hereditary hearing loss
Generally, congenital hearing loss is not common, but it is a diagnosable condition. For this reason, many people turn to genetic counselling to rule out being carriers of known gene mutations associated with deafness.
The main populations at risk for congenital hearing loss are people with a family history of deafness. If one family member has congenital hearing loss, there is a risk for carrying the gene mutations that may cause deafness.
As mentioned, two mutated copies of the genes associated with deafness are required for hearing loss to occur. Therefore, both parents must be carriers for their child to be born with hearing loss. The most significant risk is for couples with a history of hearing loss on both sides of the family.
The prevalence for carrying mutations in the genes connexin 26 and connexin 30 is 1 in 40 people.
Apart from these two genes, hearing impairments can occur due to mutations in the gene TMC1. This gene accounts for thirty percent of hearing loss in populations at risk.
Prenatal and preconception diagnosis
In general, carrier screening for gene mutations associated with congenital hearing loss is available as part of preconception screening, which is recommended to all couples planning on having children. Since hearing loss is not a severe disorder and most people with hearing loss enjoy a good quality of life, healthcare professionals do not usually recommend routine testing for hearing loss.
In preconception screening, the mother is first tested for being a gene mutation carrier, if she is a carrier, her partner will also be tested. If both parents are found to be carriers of the relevant mutations, the fetus can also be tested for carrying gene mutations through chorionic villus sampling, performed around 10-13 weeks of gestation.
In addition, preimplantation genetic diagnosis (PGD) can be performed as part of the in-vitro fertilization process. In this procedure, the embryos are tested for relevant mutations and only the genetically healthy embryos are chosen.
It should be noted that apart from the mentioned genes, there are most likely other gene mutations that can cause congenital deafness that have not yet been interpreted. Therefore, positive results in genetic testing for hearing loss do not fully guarantee that the fetus will have normal hearing.