Friday April 9 2010
Those tested had a family history of deafness
Scientists have discovered a gene “that causes children to be born deaf”, according to The Daily Telegraph. The newspaper also said that the research could lead to new medicines to treat the condition.
The research looked at the DNA of people who had inherited deafness from both sides of their family. It found that they had mutations in both copies of a gene called PTPRQ, which suggests that this gene may normally play a role in hearing or the development of structures within the ear. This study provides impetus for further research into the role of the PTPRQ gene in hearing. Studies will also need to establish what proportion of people with inherited hearing loss have mutations in the gene.
There are many underlying causes of deafness and other gene mutations may also contribute to inherited deafness. For this reason, it unlikely that identification of a single gene that can cause deafness will lead to a cure for all deafness, as suggested in one newspaper headline. Improved understanding of the genetic factors that affect hearing may eventually suggest new ways to treat or prevent hearing loss, but such developments take time and are not guaranteed.
Where did the story come from?
Dr Margit Schraders and colleagues from Redbud University Nijmegen in the Netherlands carried out this research. The study was funded by the Royal National Institute for the Deaf and various Dutch medical research organisations. It was published in the peer-reviewed American Journal of Human Genetics.
The BBC, Daily Express and The Daily Telegraph accurately reported the identification of mutations in the PTPRQ gene, which can lead to an inherited form of deafness. However, the article in the Express says that the study offers “hope of cure for deafness”, which is probably an overly optimistic view of the findings. The coverage in The Daily Telegraph mentions that three genes contributing to deafness have been identified, but it is likely that others are waiting to be discovered.
What kind of research was this?
This was a genetic study that examined the human PTPRQ gene in two families with genetically inherited hearing loss. Hearing loss can be the result of various environmental and genetic factors, and the causes of many cases of deafness cannot be identified. In some cases, hearing loss runs in families, which suggests that the cause in these families is likely to be genetic. Looking at DNA from these families helps identify genes that play a role in hearing and mutations that may lead to hearing problems.
What did the research involve?
The researchers recruited 80 people from 38 families with a history of genetically inherited hearing impairment. The hearing impairment in these families was not caused by illness or injury after birth. The majority of the patients were of Dutch descent. The participants provided blood samples from which the researchers extracted DNA, allowing them to look for single-letter variations (called single nucleotide polymorphisms, or SNPs) throughout the DNA sequences.
From looking at how hearing impairment was passed on through the family tree, the researchers worked out that individuals needed to have two copies of the defective gene in order to have hearing impairment. Therefore, when they analysed the deaf participants’ DNA, the researchers looked for “homozygous regions” (sections of DNA that carried two identical copies of a particular gene).
The researchers found a region on the long arm of chromosome 12 that was homozygous in four individuals with hearing impairment, two from a Moroccan family and two from a Dutch family. They then focused on which of the 12 genes in this region might be associated with deafness in these families. They were particularly interested in the PTPRQ gene, as defects of this gene are associated with deafness in mice.
The researchers also carried out a clinical investigation of the individuals to compare the hearing loss between the families. They also looked at which other parts of the body the PTPRQ gene was active in and assessed whether there were any other clinical effects in organs other than the ear.
What were the basic results?
Genes are made up of regions of DNA (known as exons), which include the blueprints or “code” for making protein. The researchers found that one individual had a single-letter change at position 1491 in exon 19 of the PTPRQ gene. This mutation results in a sequence called a “stop codon” appearing in an inappropriate position. Stop codons are usually found at the end of sequences that code for proteins and they tell the protein-making machinery of the cell that the instructions for making the protein are finished. Proteins created using the DNA carrying the PTPRQ mutation would be incomplete.
In another individual, the researchers found a single-letter change at position 1369 of exon 19 of the PTPRQ gene. This change was predicted to result in a change in one of the amino acids (the building blocks of proteins) in the PTPRQ-produced protein. The researchers predicted that this change was likely to affect the function of the protein. The region that contained this change had the same amino acid sequence in other animals including mice, rats, chimpanzees and cows. If an amino acid sequence is identical in multiple animal species, it suggests that this region is important in the function of the protein it will produce.
The clinical investigations showed no other abnormalities, apart from inner-ear dysfunction, in the participating individuals with hearing impairment and confirmed that both ears were affected. Inner-ear dysfunction had been present in the affected individuals from infancy or early childhood.
The researchers found that the PTPRQ gene was active at low levels in the liver, but at high levels in the lungs and heart. However, there were no clinical abnormalities of these organs in individuals with PTPRQ mutations.
How did the researchers interpret the results?
The researchers concluded that they had found two mutations affecting the PTPRQ gene that lead to deafness. They suggested that, based on previous studies in mice, PTPRQ may be required for the normal maturation of cells in the cochlea, a key inner-ear structure that makes hearing possible.
The researchers found that two different mutations of the PTPRQ gene could result in genetically inherited hearing loss and suggested that this gene is important in inner-ear development and hearing.
This study supports evidence from animal studies, which have found that PTPRQ may have a role in the development of the cochlea. This study provides impetus for further research into how the protein encoded by the PTPRQ gene functions during ear development. This study found mutations in the PTPRQ gene in 2 of the 38 families assessed. Further work will be needed to assess the proportion of individuals in the population whose hearing impairment is related to mutations in this particular gene. It is important to note that other genes have also been associated with inherited hearing loss, and that it is likely that more may be found in the future.
Hearing impairment can be caused by a variety of environmental and genetic factors. For this reason, it is unlikely that the identification of a single gene that can cause deafness will lead to a cure for all deafness, as suggested in one newspaper headline. Improved understanding of the genetic factors that affect hearing may eventually suggest new ways to treat or prevent hearing loss, but such developments take time and are not guaranteed.