Loss of heterozygosity

Loss of heterozygosity (LOH) is a gross chromosomal event that results in loss of the entire gene and the surrounding chromosomal region.

Most diploid cells, for example human somatic cells, contain two copies of the genome, one from each parent (chromosome pair); each copy contains approximately 3 billion bases (adenine (A), guanine (G), cytosine (C) or thymine (T)). For the majority of positions in the genome the base present is consistent between individuals, however a small percentage may contain different bases (usually one of two; for instance, ‘A’ or ‘G’) and these positions are called ‘single nucleotide polymorphisms’ or ‘SNPs’. When the genomic copies derived from each parent have different bases for these polymorphic regions (SNPs) the region is said to be heterozygous. Most of the chromosomes within somatic cells of individuals are paired, allowing for SNP locations to be potentially heterozygous. However, one parental copy of a region can sometimes be lost, which results in the region having just one copy. The single copy cannot be heterozygous at SNP locations and therefore the region shows loss of heterozygosity (LOH). Loss of heterozygosity due to loss of one parental copy in a region is also called hemizygosity in that region.

The loss of heterozygosity is a common occurrence in cancer, where it indicates the absence of a functional tumor suppressor gene in the lost region. However, many people remain healthy with such a loss, because there still is one functional gene left on the other chromosome of the chromosome pair. However, the remaining copy of the tumor suppressor gene can be inactivated by a point mutation, leaving no tumor suppressor gene to protect the body. Loss of heterozygosity does not imply a reversal to the homozygous state.

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