Turner Syndrome Chromosome
Turner syndrome results from complete or partial loss of the second X chromosome, occurring in females with either a 45,X karyotype (monosomy X) or various structural abnormalities and mosaicism involving the X chromosome. 1, 2, 3
Chromosomal Abnormalities in Turner Syndrome
The chromosomal composition varies significantly among affected individuals:
Classic Monosomy X (45,X)
- Complete absence of the second sex chromosome occurs in 40-50% of cases, representing the most recognizable karyotype 4, 5
- The paternal X chromosome is absent in approximately 75% of patients, though this has no clinical utility in routine care 4
Mosaic Karyotypes
- Monosomy X mosaicism (45,X/46,XX or other combinations) accounts for 3-25% of cases 4, 5
- Mosaic patterns can include normal 46,XX cells alongside 45,X cells, with cardiovascular risk determined by the presence of any 45,X cell line 6
- Even a minority of 45,X cells (such as 21%) mandates full Turner syndrome cardiovascular surveillance protocols 6
Structural X Chromosome Abnormalities
- Isochromosome Xq (duplication of long arm, loss of short arm) occurs in 10-18% of cases 4, 5
- Ring X chromosome is present in 10-16% of patients 4
- Deletion of Xp (short arm) affects less than 5% 4
- Unbalanced X-autosome translocation represents less than 2% of cases 4
Y Chromosome Material
- Mosaicism involving Y chromosome material (45,X/46,XY or variants) occurs in 6-12% of cases, requiring specific surveillance for gonadoblastoma risk 4, 5
Clinical Implications of Chromosomal Variants
Despite extensive research, no clear genotype-phenotype correlation has been definitively established between specific karyotypes and clinical severity 5. However:
- The presence of any 45,X cell line confers Turner syndrome-related risks, particularly cardiovascular complications including aortic dissection 1, 6
- Approximately 50% of patients have cardiovascular defects: bicuspid aortic valve (15-30%), aortic coarctation (7-18%), and ascending aortic dilation (33%) 1, 7
- Pregnancy loss occurs in approximately 99% of Turner syndrome conceptions, making liveborn cases relatively rare at 1 in 2,500-3,000 female births 4, 5
Genetic Mechanisms
The phenotype results from haploinsufficiency of genes that normally escape X-inactivation and are expressed from both X chromosomes in typical females 1, 3. The loss of one functional copy of these genes leads to the multisystem manifestations characteristic of Turner syndrome 8.