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Human imprinted gene lies within non-imprinted gene
Edward R. Winstead

A gene on human chromosome 20 contains a second gene that is expressed exclusively from the paternal chromosome, according to a new study. This unusual structure—an imprinted gene within a non-imprinted gene—has been documented previously in mice but not in humans.

Imprinting refers to the expression of a gene based on whether it came from the maternal or paternal genome. In humans and other organisms, some genes are marked with the imprint of the parent who contributed them.

The mechanisms that mark and regulate imprinted genes are not well understood. Randy L. Jirtle, of Duke University Medical Center in North Carolina, and colleagues now report that the human genome has at least one 'micro-imprinted' domain. This locus may contain all the elements controlling the expression of the overlapping genes.

"This micro-imprinted domain is uniquely suited for investigating the control elements required for localized regulation of genomic imprinting in the human genome," the researchers write in Genomics.

The study involved several steps. The researchers first determined that the neuronatin gene (NNAT), a regulatory gene expressed in the central nervous system during early development, is imprinted. They mapped its location to within the gene that encodes bladder cancer-associated protein (BLCAP). Finally, they determined that BLCAP is not imprinted—the gene was expressed from both parental chromosomes in multiple tissues.

The researchers focused on the human NNAT gene because its counterpart in the mouse is imprinted. They estimate that mammals may have as many as 200 imprinted genes although only about 45 have been identified in all species. Most imprinting studies involve several of these genes.

Some forms of mental retardation and other developmental disorders are due to a defective or missing copy of an imprinted gene from one parent. With most genes other than those on the sex chromosomes, if the maternal copy is defective, the paternal copy can take its place, and vice versa. But defective imprinted genes, in effect, have no backup.

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Evans, H.K. et al. The neuronatin gene resides in a "micro-imprinted" domain on human chromosome 20q11.2. Genomics 77, 99-104 (September 2001).

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