Sex chromosomes and sex determination

Transcription

1 Sex chromosomes and sex determination History (1) 1940-ties Alfred Jost embryo-surgical experiments on gonads gonadal sex; male gonadal sex presence of testes; female gonadal sex lack of testes. History (2) 1956 Tjio & Levan 46 chromosomes in humans; 1959 Ford et al. first aneuploidies of sex chromosomes in humans: Turner s. (45,X) and Klinefelter s. (47,XXY) chromosomal sex; male chromosomal sex presence of Y chromosome; female chromosomal sex lack of Y chromosome. History (3) 1949 Barr & Bertram Barr body; Sex Karyotype Barr bodies M 46,XY; 47,XYY 0 47,XXY; 48,XXYY 1 48,XXXY; 49,XXXYY 2 49,XXXXY 3 F 45,X 0 History (4) 46,XX 1 47,XXX 2 48,XXXX 3 49,XXXXX Mary Lyon X chromosome inactivation theory sex chromatin; X chromosome inactivation (1) Course: in females of all known mammals; blastocyst implantation stage;

2 random process, majority of genes; irreversible in all clonal cell descendants, methylation of promoter regions of the genes; controlled by XIST (XIC) gene Xq11.3; X chromosome inactivation (2) Non-random X chromosome inactivation: - decrease of clinical consequences of structural aberrations with X chromosome: structural aberration of one X chromosome (always abnormal X is inactivated); balanced translocations X;autosome (always normal X is inactivated); unbalanced translocations X;autosome (always abnormal X is inactivated). X chromosome inactivation (3) Consequences in females: Barr body presence; dosage compensation of majority of genes; mosaicism of paternal and maternal traits; variable expression of X-linked dominant traits; possible pathological symptoms in heterozygotic carriers of X-linked recessive traits. Y chromosome pseudoautosomal region - 2,6 Mpz; hot recombination; autosomal inheritance; sex determining region; AZF region: DAZ genes; USP9Y gene; heterochromatic region. Sex determination

3 1966 hypothetical testis determining factor (TDF) mapped in the short arms of Y chromosome; 1970-ties and 80-ties gradual narrowing of sex determining region on Y chromosome; different candidate genes for hypothetic TDF: H-Y antigen gene; ZFY gene; SRY gene (1) cloned in 1990; locus: Yp11.3 (1A1 DNA interval) directly next to pseudoautosomal region (possibility of inappropriate recombination); evolutionary conservative: specific for nearly all known mammals; SRY gene (2) single exon, no classical introns; many regulatory motives, but no TATA box and CAAT box (high tissue specificity); distant regulatory sequences (enhancers); unclear course of transcription (different points of start) transcript. SRY the only gene on Y chromosome responsible - approx. 950bp for testes development Sry expression in mice occurs approx. 36h before first morphological signs of testes development; SRY gene mutations cause sex reversal; majority of XX males (90%) has SRY gene on one of X chromosomes; transgenic mice with female chromosomal sex and Sry transgene manifests male sex (but are infertile lack of AZF region). SRY protein = TDF 223aa transcriptional factor; HMG-box 80aa motif analogical to HMG proteins ability to bind with DNA; it specifically binds double-strand DNA with AACAAAG sequence (crucial role of Ile 168 ) HMG-box, not only binds but also induces DNA bending (necessary to start DNA replication and transcription); regulatory influence on male-specific genes.

4 Male-specific genes androgens biosynthesis cascade genes (esp. cytochrome P 450 ); 5a-reductase gene; androgen receptor gene; MIS and its receptor genes; MIS proteolytic enzyme gene; other genes. SRY gene positive or negative regulator? autosomal ZZ (SRYIF) genes hypothesis (McElreavy, 1993) SRY gene as an indirect negative regulator; positive regulation direct on indirect; genes that cooperate in gonad development and function control sex reversal genes. Sex reversal genes (1) SRA1/SOX9 (locus 17q24.3-q25.1) COL2A1 expression and Sertoli cells differentiation; camptomelic bone dysplasia and XY sex reversal; SF1/AD4BP/NR5A1 (locus 9q33) necessary for bipotential gonad development and initiation of its differentiation; acts before SRY gene, together with WT1 gene promotes MIS expression; DSS/SRVX/DAX1 (locus Xp22.11-p21.2) anti-testicular gene antagonizes SRY gene, binds with SF1 gene and inhibits its synergy with WT1 gene; dosage-dependent XY sex reversal (duplication), X-linked recessive adrenal hypoplasia (point mutations). Sex reversal genes (2) WT1 (locus 11p13) tumor suppressor gene, synergy with SF1, antagonism with DAX1 gene; nephroblastoma and Denys-Drash syndrome (male pseudo-hermaphroditism);

5 SRA2/DMRT1/TDFA (locus 9p24.3) analogue sex dimorphism gene in D. melanogaster and C. elegans (model organisms) autosomal TDF; del(9)pter-p22 (DMRT1 i DMRT2) true hermaphroditism, XX or XY sex reversal; Sex reversal genes (3) WNT4 (locus 1p35) stimulates expression of DAX1 anti-testicular gene total XY sex reversal, in females Müller ducts aplasia and hyperandrogenism. RSPO1 (locus 1p34.3) homozygotic mutations cause palmary and plantary hyperkeratosis syndrome with total sex reversal or true hermaphroditism (46,XX); in 46,XY males normal fertility; RSPO1 gene acts synergically with WNT4 gene in XX gonads stabilization (one of the most important genes in ovaries development control). Disorders of Sex Determination (DSD) Pure gonadal dysgenesis XY (Swyer syndrome, XY sex reversal, XY females) phenotypic females with normal external genitalies, but with ovarian dysgenesis; 10-15% - SRY gene defects (microdeletions, point mutations, esp. in HMG-box); SRY gene promoter or regulatory motives mutations; sex reversal genes; Pure gonadal dysgenesis XX (XX sex reversal, XX males) phenotypic males with symptoms similar to Klinefelter syndrome; 90% - SRY gene translocated from Y chromosome is present on one of the X chromosomes (inappropriate recombination out of pseudoautosomal region); sex reversal genes; True hermaphroditism histological tissue of testis and ovary in one person; karyotype: in 80% - 46,XX, rarely - 46,XY or 46,XX/46,XY variable phenotypes

6 Male pseudo-hermaphroditism normal male karyotype: 46,XY and histologically normal testes (frequent cryptorchidism); very variable external phenotypes: androgens insensitivity syndrome (CAIS, PAIS); persistent Müllerian ducts syndrome; 5a-reductase defect; Leydig cells aplasia; congenital adrenal hyperplasia (some forms); sex reversal genes. Androgen insensitivity syndrome X-linked recessive inheritance; androgen receptor gene mutations (esp. C-domain); locus Xq13, 8 exons, 919 AA proteins; C domain (250 AA) binds androgens; central domain binds DNA. partial (PAIS) or complete (CAIS) receptor activity; CAIS tall females, with blind vaginal recess, no uterus, no ovarian ducts, and with testes in abdomen; PAIS variable hermaphroditic phenotypes; Persistant Müllerian ducts syndrome (PMDS) phenotypical males with limited fertility, usually with cryptorchidism, and uterus with oviducts in abdomen; 50% - MIS gene; 50% - MIS receptor gene or MIS proteolytic enzyme. 5a-reductase defect variable hermaphroditic phenotypes, depending on residual enzyme activity; two functional genes: expression in liver (5p) and in prostate (2p23) and one pseudo-gene (Xq24);

7 5 exons, 29kDa protein; mutations in co-enzyme or substrate binding sites Congenital adrenal hyperplasia (CAH) boys with different incomplete masculinization (practically always with cryptorchidism); 20,22-cholesterol desmolase deficiency P450SCC gene (locus: 15q21.1, 9 exons); 3b-hydroxysteroid dehydrogenase deficiency locus: 1p13; 17a-hydroxylase or 17,20-liase deficiency P450C17 gene (chromosome 10); 17b-ketosteroid reductase deficiency. Female pseudo-hermaphroditism normal female karyotype: 46,XX and histologically normal ovaries; different degree of external virilisation caused by androgens excess; congenital adrenal hyperplasia AR; placental aromatase defect AR; sex reversal genes. Congenital adrenal hyperplasia (CAH) girls with variable hermaphroditic symptoms (virilisation), sometimes with salt loss syndrome; 21a-hydroxylase deficiency (85% cases of CAH) P450C21 gene (chromosome 6, 10 exons, 494aa protein); 11b-hydroxylase (10% cases of CAH) P450C11 gene (locus: 8q21, 503aa protein); 3b-hydroxysteroid dehydrogenase deficiency locus: 1p13