12 August 2007

Development Primer: 3 Models of Sex Determination and Dosage Compensation, Part 3

Mus (mouse)
Of our three models, the mouse (being a mammal) is most closely analogous to humans. Here, as with the flies, XX animals are female and XY animals are male. This time, however, sex determination depends upon the Y-linked gene Sry instead of the X:A ratio. If you have Sry, you become a male. Note the implications of this for individuals with an abnormal number of sex chromosomes. An XO fly would become a (pseudo)male due to the X:A ratio, whereas an XO human would become a female (with Turner syndrome) due to lack of Sry. An XXY fly would be female, whereas an XXY human would be male (with Klinefelter's syndrome).

Overall sex determination in mammals relies primarily on differentiation of the gonad. The embryonic gonad is bipotential (neither male nor female but capable of differentiating into either) and contains both somatic cells and germ cells (the cells that will give rise to the germ line, i.e. sperm or eggs). The somatic cells of the gonad will differentiate into either Sertoli cells (for testes) or Follicle cells (for ovaries); the hormone production of the gonad will then affect development of the rest of the body.

So we're looking for a cellular mechanism affecting gonad differentiation. As it turns out, there are two primary exogenous growth factors (proteins excreted from a cell that affect growth and development) expressed by the somatic cells: FGF9 and WNT4. FGF9 causes the somatic cells to differentiate into Sertoli cells, and inhibits Wnt4 expression. WNT4 causes the somatic cells to differentiate into Follicle cells, and inhibits Fgf9 expression. Initially, the somatic cells express both factors, and they balance each other out. But as development progresses, the balance gets tipped one way or the other.

In mammals, SRY promotes expression of FGF9, thus tipping the balance toward male development. However, a similar mechanism in other vertebrates might not require a single gene like Sry. For instance, for many animals (such as crocodiles) sex determination is affected by environmental factors like temperature. It's entirely possible that these environmental factors could be affecting a balance such as that between FGF9 and WNT4.

This sex determination mechanism takes place primarily in the gonad; a different mechanism entirely is needed for dosage compensation in all the cells of the body. Unlike the worm mechanism of reducing each X by half in hermaphrodites or the fly mechanism of doubling expression of the male X, dosage compensation in mice and humans works by (almost) completely silencing one of the two female X chromosomes.

In both males and female cells, the autosomes produce enough of a certain blocking factor (BF) to bind to one X chromosome and block expression of the gene Xist. In a cell with only one X chromosome (i.e. a normal male), that' the end of the story. In a cell with two X chromosomes (i.e. a normal female), Xist is expressed on the chromosome that did not receive BF. Xist is a gene that does not code for protein; its end product is untranslated RNA. Xist RNA aggregates to form a region in the nucleus that excludes RNA ploymerase II and transcription factors. The X chromosome migrates into this region, thus silencing it.

The paternal X chromosome carried by the sperm is imprinted so it will always be chosen for inactivation in the zygote. Once the embryo reaches the blastocyst stage of development, X-inactivation is temporarily turned off. As the cells then differentiate, X-inactivation is reinitiated. This time, the decision of which X chromosome to deactivate is random. Thus, the adult animal will be mosaic for X-inactivtion; some of the animal's cells will have the paternal X deactivated, and some cells will have the maternal X deactivated.

The location where BF binds to the X chromosome, since the region runs opposite to Xist, is called Tsix. If Tsix is deleted from one X chromosome, then that chromosome will always be chosen for X-inactivation, since BF cannot bind it. A Tsix deletion in an XY cell will result in ectopic (out-of-place) X-inactivation, which is lethal.

Mouse summary:
Two sex chromosomes -- XY male, XX female. FGF9 and WNT4, mutually inhibitory growth factors, are expressed in the gonad somatic cells. Y chromosome carries Sry, which promotes FGF9 and tips the balance to testis development; otherwise, WNT4 tips balance to ovary development. X-inactivation occurs by transcription of Xist RNA, which forms a nuclear domain excluding transcription factors. Autosomes produce enough blocking factor (BF) to rescue one X chromosome from inactivation, by binding Tsix and thereby blocking Xist. The paternal X is imprinted to be always chosen for inactivation in the zygote; at the blastocyst stage, X-inactivation is reset and randomized.

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