A common type of dwarfism may not be principally caused by a defect in the human growth hormone gene, as previously thought, but rather by a gene that controls the hormone's release into the bloodstream, according to scientists at the Johns Hopkins Children's Center and School of Medicine. Their study is reported in this month's issue of the Journal of Clinical Endocrinology & Metabolism.
Endocrinologists Roberto Salvatori, M.D., and Michael Levine, M.D., studied families in which at least two children had isolated growth hormone deficiency (IGHD) type 1-B, the most common type of growth hormone-deficient dwarfism. These individuals have low but detectable levels of growth hormone (GH) in their blood. The researchers found a faulty releasing hormone receptor gene in three of the families. This gene holds the blueprint for a receptor protein that binds to releasing hormone, thereby instructing cells to produce and release GH into the bloodstream. Each of the three children studied carried a different, previously unidentified mutation in the releasing hormone receptor gene.
"We are learning that many different mutations in the releasing hormone receptor exist out there," says Salvatori, the report's lead author. "Our work suggests also that this kind of dwarfism is not necessarily caused by a defect in the gene that governs the production of human growth hormone."
Using DNA from white blood cells, the Hopkins team sequenced the releasing hormone receptor gene. Of the three releasing hormone receptor mutations they found, all involved the substitution of a single amino acid for another, possibly causing a change in the shape -- and function -- of the entire receptor.
The new findings should also help doctors more easily diagnose the cause of hormone deficiency in children.
Researchers from the Vanderbilt University Department of Pediatrics in Nashville, Tenn., the Hospital Universitario Virgen de las Nieves, the Department of Pediatric Endocrinology in Granada, Spain, and the King Faisal Specialist Hospital Department of Pediatrics in Riyadh, Saudi Arabia, also contributed to the report. It was funded by grants from the National Institutes of Health and the Genentech Foundation for Growth and Development.
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