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Cytogenetics Labs ID New Genetic Disorder by Array CGH

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By Justin Petrone

Four US laboratories that perform diagnostic genetic testing of chromosome abnormalities in individuals with unexplained physical and developmental disabilities have used microarray-based comparative genomic hybridization to identify a previously unrecognized genetic disorder.

In a study published this week online in the American Journal of Human Genetics, researchers at Signature Genomic Laboratories, Nationwide Children's Hospital, Emory University School of Medicine, and Baylor College of Medicine use array-CGH to independently identifty a total of seven individuals with nearly identical-sized deletions on one of their two copies of chromosome 17.

The individuals in the study had common features, including mild to moderate developmental delay, microcephaly, postnatal growth retardation, heart defects, and hand, foot, and limb abnormalities. "Although all individuals had at least mild dysmorphic facial features, there was no characteristic constellation of features that would elicit clinical suspicion of a specific disorder," the authors wrote.

None of the children's parents had the deletions, which suggests the anomaly is causative of the children's physical and developmental disabilities, according to the authors. The researchers estimated the incidence of this genetic abnormality in the general population to be approximately 1 in 115,000.

According to the researchers, two of the genes in the deleted region were of particular interest because of their potential role in the heart and limb anomalies in the children. They wrote that TBX2 and TBX4 are members of a closely related gene family, the T-box genes, that is involved in development.

Mutations of TBX4 have been identified in individuals with small patella syndrome, characteristic features of which include delayed tissue formation of the pelvic bones and femur and foot anomalies. Several other genetic disorders for which heart and limb defects are characteristic features have been associated with disruption of T-box genes, which led the researchers to suggest the newly reported disorder is a similar syndrome.

Blake Ballif, director of product development research at Spokane, Wash.-based Signature, noted in a statemen that "in a substantial proportion of children with mental retardation and developmental delays, the genetic basis is unknown."

He said the "identification of a previously unknown genetic aberration sheds further light on the genetic basis of human disease, in addition to providing an answer for these children and their families."

Bailif also noted that the study represented a "collaborative academic effort by several otherwise competing diagnostic laboratories."

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