LONDON Scientists in the United States have pinpointed the cause of one of the most common genetic diseases in humans and a leading cause of heart defects.

A team of researchers at Baylor College of Medicine in Texas have identified the gene for digeorge syndrome, which may improve understanding of the disorder and how complex human organs like the heart develop.

Digeorge syndrome occurs in about one in 4,000 live births. Babies born with the disorder have facial deformities, immune system problems and mild retardation. Up to 80 percent of sufferers need surgery to correct serious heart problems.

"It is the most common deletion syndrome in humans," Dr. Antonio Baldini, the leader of the research team, said in a telephone interview.

Many genetic disorders are caused by the mutation of a single gene but deletion syndromes result from the loss of a portion of a chromosome and involves many genes.

"Digeorge syndrome is associated with the loss of a relatively big portion of chromosome 22. There are a number of syndromes like this in humans and this one is the most common," Baldini added.

The researchers pinpointed the problem to a gene called TBX1 using the latest scientific techniques and genetically modified mice. Their research is published in the science journal ***Nature***.

Deletion syndromes pose specific problems for scientists because they involve the loss of several genes but not all of them may be important.

"What we have demonstrated is that the loss of one single gene can cause most of the clinical findings that are seen in this syndrome in mice," said Baldini.

"It is probably the first example of the discovery of a gene involved in a deletion syndrome."

Chromosome 22 was the first human chromosome to be mapped. It is the second smallest chromosome with 545 genes and speeded up the search for candidate genes for digeorge syndrome.

Baldini said the discovery of TBX1 may help scientists develop a preventive strategy because not all mice with the deletion develop symptoms of the disorder. Understanding how this occurs could provide clues to preventing it in humans.

"This gene regulates other genes, like a cascade of events. It is required for the development of certain structures of the heart. So by studying this gene we hope to understand more than we do now about heart development," said Baldini.

"Using this gene we may be able to fish out other genes that are responsible for other birth defects."

(Reuter)