LONDON ¿ Back in the 1970s, Arni Bjornsson was the only plastic surgeon in Iceland, serving a population of 270,000. Working at the University Hospital in Reykjavik, he noticed that many of the people on which he had operated to correct cleft palate and tongue tie were related to each other.
When he discussed his observation with two geneticist colleagues, they quickly realized that the condition was sex linked, and carried on the X chromosome.
Thus began the records of one of the largest known pedigrees of a family affected by a rare inherited condition known as X-linked cleft palate and ankyloglossia (the medical term for tongue tie). Affected males have both cleft palate and a severe form of tongue tie, while carrier females have only tongue tie. In cleft palate, there is a gap in the palate, or roof of the mouth. In tongue tie, the frenulum ¿ the band of tissue that attaches the tongue to the floor of the mouth ¿ extends to the tip of the tongue, so inhibiting its movement.
Bjornsson decided to investigate further, putting together a pedigree that went back seven generations and included more than 300 individuals.
In 1985, Bjornsson met geneticist Bob Williamson, formerly of St. Mary¿s Hospital Medical School in London, who was on a visit to Iceland to discuss a project on the genetics of cystic fibrosis. Williamson sent one of his researchers, Gudrun Moore, to Iceland to collect blood samples from the family members Bjornsson had identified. Moore¿s aim was to identify the mutant gene responsible for the syndrome by positional cloning ¿ a technique that relies on tracking the inheritance of genetic markers.
Sixteen years later, that search has finally ended. Bjornsson, Moore and colleagues report in Nature Genetics that the gene responsible for the syndrome is on the X chromosome and encodes a transcription factor called TBX22. They have been able to find mutations in TBX22 in all tested people affected by the syndrome, and none in those who are not.
The paper is titled ¿The T-box transcription factor gene TBX22 is mutated in X-linked cleft palate and ankyloglossia.¿ Its first author is Claire Braybrook, of the Institute of Reproductive and Developmental Biology at the Hammersmith Campus of Imperial College in London.
Philip Stanier, senior lecturer in molecular genetics at the Institute of Reproductive and Developmental Biology and senior author of the paper, told BioWorld International: ¿This is one of the first genes to be identified that is a major genetic factor for cleft palate. However, most cases of cleft palate are sporadic, and we are now embarking on studies to find out what the frequency of mutations in this gene is in sporadic cases.¿
The finding would allow clinicians to give accurate information on the risks of giving birth to an affected child to people with X-linked cleft palate and ankyloglossia in their family, Stanier said. Prenatal diagnosis would not be offered because both conditions are readily correctable by surgery.
In the long term, it might be possible to offer women carrying an affected fetus therapy to correct the defect during pregnancy, Stanier predicted. He pointed out that in a mouse model of cleft palate, in which the gene for a growth factor called TGFbeta3 is knocked out, giving intravenous TGFbeta3 to female mice during pregnancy results in the birth of mice with normal palates.
The journey from the taking of blood samples in Iceland to the discovery of the gene was not straightforward. In 1987, Moore published a paper in Nature localizing the gene she was searching for to a position on the X chromosome. Soon after, the project was put on hold, but in 1992 she and Stanier decided to resurrect the investigation. They narrowed down the search to a region containing just a few genes, but none of those contained mutations in affected individuals.
Then Stanier attended last year¿s meeting of the American Society of Human Genetics, where he saw a poster by Ana Carolina Marcano of the University of Sao Paulo in Brazil, who had been studying two families also with X-linked cleft palate and tongue tie. Stanier realized that he and Moore should look at the region adjacent to the one they had been studying.
By then, the human genome sequence was available, and they could look up the genes in the new area on a database. Out of seven genes present, four encoded known proteins implicated in other disorders. When they sequenced the remaining three, they found that one ¿ TBX22 ¿ was mutated in the affected individuals of the Icelandic pedigree.
Marcano brought her DNA samples from Brazil and sequencing showed that both had mutations of a different kind in the same gene. Moore and Stanier also had DNA samples that had been collected by Paul Goodfellow, of Washington University School of Medicine in St Louis, from two large pedigrees. One was a Native American family from British Columbia and the other was from an isolated Mennonite community in Manitoba. Again, mutations in TBX22 segregated with the affected individuals in both families.
Finally, Michael Patton, chairman of the British charity The Birth Defects Foundation, which was partly funding Moore and Stanier for this work, asked the researchers if they would test a mother and two sons who were affected. They, too, turned out to have a mutation in TBX22. Tests on numerous unaffected individuals failed to show any mutations in TBX22. Together, Stanier said, these data provided ¿conclusive evidence¿ that they had identified the correct gene.
As well as investigating the role of TBX22 mutations in sporadic cases of cleft palate, Stanier said he intends to try to find out what the TBX22 protein does and on what it acts.