Other genetic factors, which contribute to male sex differentiation
Apart from the SRY gene, there is a number of other genes that contribute dramatically to the sex determination, as it has been proven from sex reversal disorders. The Wilm's tumor-associated gene (WT1), which is located in the short arm of the chromosome 11, is comprised of 10 exons and can give rise to 24 different protein isoforms. The +KTS and –KTS isoforms arise from an alternative splicing event that incorporates or omits, respectively, three aminoacids.
A research team
27 conducted an isoform-specific knockout. Both male and female mice lacking the WT1 (-KTS) isoform had reduced gonadal size, but cells in the gonads of male mice showed male-specific marker gene expression, although reduced. Mice lacking WT1 (+KTS) had severe kidney defects and male mice were completely XY sex-reversed.
Consistent with the latter study, XY patients with Frasier syndrome develop as females and display urogenital malformations. Analysis of these patients has showed that donor splice mutations in WT1 with loss of the +KTS isoform are associated with this syndrome
28. According to the findings of a study, mutations in WT1 (+KTS) could lead to sex reversal because of a deficiency in the precursors of Sertoli cells
29.
Steroidogenic factor 1 (SF1) is a member of the subfamily of nuclear receptors, the orphan receptors, for which no activating ligand has been found. SF1 gene expression is specifically associated with the gonad and the adrenal as they arise and is a good marker for these cells
30. The gene that encodes for SF1 is located in the long arm of the chromosome 9. Mutation in this gene provoke male to female sex reversal
31.
The gene named SOX9 encodes a transcription factor that contains a SRY-related HMG box
32. It is located in the long arm of the chromosome 17. Expression studies in mice revealed that SOX9 is present in both sexes genital ridges, but after the initial development, is upregulated in males and switched off in females
33. SOX9 is expressed in Sertoli cells throughout life, but not in the ovary. This characteristic is consistent with the upregulation of SOX9 being a direct effect of SRY action. It also suggests that is involved in determination of Sertoli cell fate. Significantly, heterozygotes for mutations in SOX9 show male to female sex reversion
34,
35.
According to the latter study, the protein has to reach a critical threshold to be effective, thus an additional copy of SOX9 can stimulate female to male sex reversal. SRY and SOX9 are functionally related, perhaps ensuring the male development. SOX9 is also necessary for the activation of gene which encodes for the anti- Mullerian hormone (AMH).
Another sex reversal event is observed when the Dosage Sensitive Sex Reversing Adrenal Hypoplasia Congenita critical region on the X (DAX1) gene is partially duplicated in males. The DAX1 is located on the short arm of the X chromosome
36. The single X chromosome in the male is active, but a translocated region from the female X (or a partial duplication of the male X) results in two active alleles of the DAX1 gene when only one occurs normally. This gene dosage feminising effect is sufficient to counteract the effect of SRY
37.
The action of DAX1 gene has resulted to its characterization as an anti-testes gene. Another anti-testes gene is the WNT4. In humans, its duplication causes male to female sex-reversal
38. This effect indicates that both DAX1 and WNT4 can override the effect of SRY.
Finally, there is one more gene that interferes with sex determination, the Doublesex and Mab-3 Related Transcription factor1 (DMTR1). DMTR1 lies on the short arm of the chromosome 9. Mutation or partial deletion of this gene provokes male to female sex reversal
39,
40. Two functioning copies of DMRT1 are therefore required to support testis development in mammals
41.