Imprinted Genes

Imprinted genes are genes whose expression is determined by the parent that contributed them.

Imprinted genes violate the usual rule of inheritance that both alleles in a heterozygote are equally expressed.

Examples of the usual rule:

But there are a few exceptions to this rule. A small number of genes in mammals (~80 of them at a recent count) and in angiosperms have been found to be imprinted. Because most imprinted genes are repressed, either

The process begins during gamete formation when

All the cells in a resulting child will have the same set of imprinted genes from both its father and its mother EXCEPT for those cells ("germplasm") that are destined to go on to make gametes. All imprints — both maternal and paternal — are erased in them.

Three Examples

1. IGF2

— the gene encoding the insulin-like growth factor-2

In humans (and other mammals like mice and pigs) the IGF2 allele inherited from the father (paternal) is expressed; the allele inherited from the mother is not.

If both alleles should begin to be expressed in a cell, that cell may develop into a cancer.

2. IGF2r

— the gene encoding the cell receptor for Igf-2

In mice the IGF2r allele inherited from the mother is expressed; that from the father is not. Differential imprinting accounts for this, and the mechanism is described below.

3. XIST

— the gene encoding the RNA that converts one of the X chromosomes in a female cell into an inactive Barr body. This process is random in the cells of the female fetus and thus is NOT an example of imprinting. However, all the cells of her extraembryonic membranes (which form the amnion, placenta, and umbilical cord) have the father's X chromosome inactivated. Imprinting of the XIST locus accounts for this.

Mechanism of parental imprinting

The process of imprinting starts in the gametes where the allele destined to be inactive in the new embryo (either the father's or the mother's as the case may be) is "marked". The mark appears to be methylation of the DNA in the promoter(s) of the gene.

Methyl groups are added to cytosines (Cs) in the DNA. When this occurs at stretches of alternating Cs and Gs called CpG islands in a promoter, it prevents binding of transcription factors to the promoter thus shutting down expression of the gene.

Methylation — and thus inactivation — of the promoters of tumor suppressor genes is frequently found in cancer cells.

Although methylation seems to be the imprinting signal, keeping the gene shut down may require the production of RNA.

Example 1: the IGF2r gene

A report in Nature (16 October 1997) by Wutz et al, reveals that:

In the mother's (maternal) copy of the gene,
In the father's (paternal) copy of the IGF2r gene (the imprinted version)

Example 2: XIST

The XIST locus on the X chromosome encodes a long noncoding RNA that shuts down all (or almost all) of the other genes on the chromosome, converting it into an inactive Barr body. [More].

Is imprinting important?

Yes.

Imprinting and Parthenogenesis

Imprinting is the reason that parthenogenesis ("virgin birth") does not occur in mammals. Two complete female genomes cannot produce viable young because of the imprinted genes.

For example, the embryo needs the father's Igf2 gene because the mother's copy has been imprinted and is inactive. However, two healthy laboratory mice have been produced by parthenogenesis; that is, containing two female (haploid) genomes. (See Kono, T. et al., Nature, 22 April 2004.)

This was done by fusing two oocytes (thus each cell haploid):

Out of several hundred attempts, two resulting blastocysts not only implanted successfully in a surrogate mother but went on to be born normally. One even grew up and had babies of her own.

Imprinting in Plants

Some genes in the endosperm of angiosperms are imprinted by the addition of methyl groups. For some, both maternal copies (endosperm is 3n) are expressed (demethylated) while the male allele remains shut down. For other genes, it is the female alleles that are imprinted and thus not expressed while the male allele is functional.

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20 March 2013