BioWorld International Correspondent

LONDON - A rare syndrome of hypoparathyroidism, mental retardation and facial abnormalities has been tracked to mutations in a gene encoding a protein that helps tubulin to fold. The disease is believed to be the first to be linked to abnormalities in the pathway for the folding and assembly of tubulin, which forms part of the cellular cytoskeleton.

An international consortium of physicians and scientists from Israel, Belgium, the UK, Saudi Arabia, Kuwait and the U.S. collaborated to track down the gene responsible for the disease, which is found almost exclusively in Middle Eastern populations. They report their findings in a paper in the Oct. 21, 2002, online issue of Nature Genetics titled "Mutation of TBCE gene causes hypoparathyroidism-retardation-dysmorphism and autosomal recessive Kenny-Caffey syndrome."

Individuals with hypoparathyroidism-retardation-dysmorphism (HRD), or Kenny-Caffey, syndrome have no parathyroid function, very small head circumference, extreme growth failure and mental retardation. Males have small testes. Those affected by Kenny-Caffey syndrome also have osteosclerosis (an abnormality of skeletal calcification) and recurrent bacterial infections.

Ruti Parvari, senior lecturer in the department of molecular developmental genetics at Ben Gurion University of the Negev in Beer Sheva, Israel, told BioWorld International that apart from the scientific interest in identifying the gene that caused the syndromes, her physician colleagues wanted to be able to help affected families. "Patients with these syndromes are very sick and many die at a very early age. If we could find the genetic defect, then clinicians would be able to offer prenatal diagnosis," she said.

The syndromes are extremely rare outside of the Middle East. Doctors in Saudi Arabia know of more than 100 patients of Bedouin origin who are affected, and several tens of patients have been identified in southern Israel. To try to track down the gene, therefore, Parvari and her colleagues combined forces with the U.S. group, which studied patients from Kuwait and Saudi Arabia. Their search speeded up further when a physician in northern Israel put them in touch with an additional patient, who was genetically highly informative, and a team in Belgium also contributed an affected family.

"Once we had combined all the information from all these patients, we were able to narrow the search down to a region which contained four genes and sequence these genes in our patients," Parvari said.

They found that all the Bedouin individuals in their study had the same mutation in the tubulin-specific chaperone E (TBCE) gene. The researchers believe that this had been inherited from a common ancestor.

"When we found a different mutation in the same gene, causing the same disease in the Belgian patients," Parvari said, "this provided us with proof that this really was the gene responsible for the disease."

Their next step was to look for evidence that the mutation caused a failure in function of the gene's product. The protein encoded by TBCE is one of five chaperones involved in helping alpha-tubulin fold correctly, and in helping dimers of alpha-beta tubulin to form, prior to the formation of microtubules.

Members of the consortium therefore examined the cytoskeleton of fibroblasts and lymphoblastoid cells taken from people with the syndromes, using antibodies to tubulin, in order to detect abnormalities.

"We saw that the microtubule organizing center was disrupted and that the formation of microtubules was diminished, even though the amount of tubulin remained unchanged, compared to normal controls," Parvari said. The Golgi apparatus and late endosomes also showed abnormal staining with antibodies to tubulin.

Electron microscopy studies showed that cells from affected individuals had dilated endoplasmic reticulum, unusual areas of lamellae contained in intracellular membranes and large vacuoles. "These are not features of normal cells," Parvari said.

"We were quite amazed by these findings," Parvari added. "The cytoskeleton of the cell is a major factor that holds the cell together and we find it surprising that cells can survive such a defect in their organization."

It is possible that different tissues are affected to different degrees, she said. "We now want to find out how the disturbance in the cytoskeleton affects different body systems. These children have smaller head circumference and suffer mental retardation. Perhaps this is the effect of the cytoskeleton disturbance on the neurons."

The researchers are, however, puzzled by the apparent absence of the parathyroid glands in affected patients. "Quite how global disturbance of tubulin, which is everywhere and a major component of all cells, manages to specifically affect the development of the parathyroid glands, we can't imagine," Parvari said. "It will be very interesting to find out why this is."

She and her colleagues are planning further investigations into the function of chaperone E, and the proteins with which it interacts.