Haploinsufficiency

Haploinsufficiency is a mechanism of action to explain a phenotype when a diploid organism has lost one copy of a gene and is left with a single functional copy of that gene. Haploinsufficiency is often caused by a loss-of-function mutation, in which having only one copy of the wild-type allele is not sufficient to produce the wild-type phenotype. It occurs when an organism has a single functional copy of a gene, and that single copy does not produce enough product to display the wild type's phenotypic characteristics. The genotypic state in which one of two copies of a gene is absent is called hemizygosity. Hemizygosity is not the same as haploinsufficiency; hemizygosity describes the genotype, and haploinsufficiency is a mechanism that may have caused the phenotype. The general assumption is that the single remaining functional copy of the gene cannot provide sufficient gene product (typically a protein) to preserve the wild-type phenotype leading to an altered or even diseased state. As such, haploinsuffiency is typically transmitted with dominant inheritance, either autosomally or X-linked in female humans.

Dominance describes the circumstance in which both alleles in a diploid organism are present but one allele is responsible for the phenotype. That genotypic state is one of heterozygosity (with two different alleles). Co-Dominance is that situation where the effects of both alleles are apparent in the phenotype.

Haploinsufficiency can occur through a number of ways. A mutation in the gene may have erased the production message. One of the two copies of the gene may be missing due to a deletion. The message or protein produced by the cell may be unstable or degraded by the cell.

A haploinsufficient gene is described as needing both alleles to be functional in order to express the wild type. A mutation is not haploinsufficient, but dominant loss of function mutations are the result of mutations in haploinsufficient genes.

The alteration in the gene dosage, which is caused by the loss of a functional allele, is also called allelic insufficiency. An example of this is seen in the case of Williams syndrome, a neurodevelopmental disorder caused by the haploinsufficiency of genes at 7q11.23. The haploinsufficiency is caused by the copy-number variation (CNV) of 28 genes led by the deletion of ~1.6 Mb. These dosage-sensitive genes are vital for human language and constructive cognition.

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