Gonadal dysgenesis XX; Chapelle syndrome; XX male - SRY gene
The XX gonadal dysgenesis is characterized by a lack of correlation between sexual phenotype and genotype -masculino- said -XX, feminine. Patients differentiate phenotypically into two groups: one normally shows a normal male phenotype and is characterized by a complete development of the gonads as testes without presence of ovarian tissue and, on the other, individuals are grouped with simultaneous tissue ovarian and testicular gonads of the same subject either alone or, more commonly, together (ovotestis). Individuals of this latter group are generally internal and external genitalia ambiguous depending on the amount of functional testicular tissue present in the gonads. In the first group, however, the phenotype varies from full normal male development - more usual to incomplete development as micropenis, hypospadias and cryptorchidism.
Sexing during the embryonic stage is the result of the development of gonadal ridges to form undifferentiated testes or ovaries. The mammalian chromosome contains a gene, known as TDF (Testis Determing Factor), which is required for the formation of the testis and therefore for normal male sexual differentiation. In the absence of the Y chromosome fetal gonads are the sexual development of women. The protein encoded by the SRY gene was equated with TDF, being therefore the
responsible for initiating the male sex determination by activating the gene responsible for the Mullerian inhibiting substance (MIS).
The SRY gene, which is usually located in the distal short arm of the Y chromosome (Yp11.3), encodes a protein called protein sex determining region Y, a transcription factor member of 204 amino acids of high mobility group (HMG: High Movility Group), which is involved in mediating protein binding to DNA, suggesting a regulatory function. In fact, it is believed that it can exert the function of a factor modular architecture of local chromatin, so that responsible for the formation of the transcriptional machinery, regulating the expression of genes involved in sexual differentiation. Specifically, this protein initiates processes that cause a fetus to develop male gonads (testes) and inhibits the development of female reproductive structures (uterus and fallopian tubes).
It has been found, however, on the X chromosome in more than 90% of male patients with genotype XX without sexual ambiguity and 10% of males with XX simultaneous presence of ovarian and testicular tissues in the gonads. The presence of a fragment of the Y chromosome in the short arm of chromosome X is explained by unequal exchange between homologous short arm of the X chromosomes (Xp) and Y (Yp) regions during paternal meiotic division. They described a few cases of males with XX genotype and presence of SRY gene on autosomal chromosomes (chromosome 1 and chromosome 16). The phenotypic variability in those affected by the gonadal dysgenesis XX may be motivated by the differential inactivation of SRY gene on the X chromosome or the presence of point mutations or microdeletions influencing the expression of SRY.
Generally, XX gonadal dysgenesis is not inherited, so it occurs in people with no history of disease in your family. These cases are due to new mutations that occur during the formation of reproductive cells or early embryonic development.
Tests in IVAMI: in IVAMI perform detection of mutations associated gonadal condisgenesia XX, by complete PCR amplification of the exons of the SRY gene, and subsequent sequencing.
Samples recommended: EDTA blood collected for separation of blood leukocytes, or impregnated sample card with dried blood (IVAMI may mail the card to deposit the blood sample).