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Aicardi Syndrome and Klinefelter Syndrome
Published in Dongyou Liu, Handbook of Tumor Syndromes, 2020
Skewed X-inactivation refers to the preferential, non-random inactivation of one of the two X chromosomes in females. While random X-inactivation keeps each X chromosome active in about half of cells, skewed X-inactivation keeps one X chromosome active in more than half of cells. Significant correlation is observed between skewed X inactivation and smaller gray matter volume in the left insula of the brain, which is involved in social, emotional, and mental processing.
Genetics
Published in Rachel U Sidwell, Mike A Thomson, Concise Paediatrics, 2020
Rachel U Sidwell, Mike A Thomson
This is a random inactivation in females of one of the X chromosomes in each cell. The inactivated chromosome is a dense chromatin mass called a Barr body. The theory of X inactivation is the Lyon hypothesis.
Genetic counselling in Mendelian disorders
Published in Angus Clarke, Alex Murray, Julian Sampson, Harper's Practical Genetic Counselling, 2019
The pseudoautosomal region of Xp has homology with part of the Y chromosome and is involved in pairing between the sex chromosomes at meiosis. The process of X inactivation is itself controlled by a different region of the X chromosome, the X-inactivation centre including the Xist locus on the proximal long arm. The Xist transcript does not encode a protein but functions as a large, non-coding RNA molecule.
Combined X-linked familial exudative vitreoretinopathy and retinopathy of prematurity phenotype in an infant with mosaic turner syndrome with ring X chromosome
Published in Ophthalmic Genetics, 2023
Sandra Hoyek, Marlene Wang, Audina M. Berrocal, Ashley Wong, Emily M. Place, Heather Mason-Suares, Angela E. Lin, Shizuo Mukai, Nimesh A. Patel
The clinical suspicion of atypical ROP in this case was the main reason for the workup and for the FEVR diagnosis. Our patient had a deletion including the entire NDP gene on the ring X chromosome. Pathogenic variants of the NDP gene are known to cause X-linked FEVR phenotype in males (13,14). Many genetic abnormalities on the X chromosome are well tolerated in females because of the preferential inactivation of the abnormal X which may balance the genetic material. In cases of ring X chromosome, the phenotype can be more severe compared to those with non-mosaic Turner syndrome (45,X). In our case, the ring X chromosome paradoxically included the XIST gene responsible for the initiation of X inactivation. Severe phenotype can still occur in the context of ring X chromosome with XIST expression due to the partial expression of the XIST gene leading to incomplete inactivation of r(X) or the inactivation of the normal X chromosome during embryogenesis (15). This latter phenomenon is called skewing of X inactivation and results in phenotypic expression of X-linked recessive diseases in females (16). It remains unclear why individuals with ring X have an increased chance of having more significant developmental delays and possibly intellectual disability compared to those with TS without ring X. Although exceptions exist in the literature and in our clinical experience, this patient has typical developmental delays, and longitudinal follow-up is needed to determine if there will be intellectual disability.
Premature ovarian insufficiency: a toolkit for the primary care physician
Published in Climacteric, 2021
I. Lambrinoudaki, S. A. Paschou, M. A. Lumsden, S. Faubion, E. Makrakis, S. Kalantaridou, N. Panay
If the diagnosis of spontaneous POI appears likely, chromosomal analysis should be offered to all women on the basis that chromosomal abnormalities are highly prevalent in POI (Figure 1(a)). When primary amenorrhea occurs, 21% will have a karyotypic abnormality compared to 11% with secondary amenorrhea13. Karyotyping is the gold standard to assess chromosomal abnormalities, although newer techniques exist. Turner syndrome (XO) is the commonest chromosomal abnormality in POI; it occurs in 1 in 2500 births and involves the complete or partial loss of one X chromosome. Turner syndrome mosaicism can also occur, leading to less severe POI with the possibility of pregnancy. The loss of X-linked genes results in X inactivation of important X-related gene products that escape inactivation by the second X14. If Y chromosomal material has been detected, these women should be counseled about the possibility of developing a gonadal tumor and gonadectomy should be offered.
Premature ovarian insufficiency: an International Menopause Society White Paper
Published in Climacteric, 2020
N. Panay, R. A. Anderson, R. E. Nappi, A. J. Vincent, S. Vujovic, L. Webber, W. Wolfman
Turner syndrome occurs in 1 in 2500 births and involves the complete or partial loss of one X chromosome (deletions, translocations, inversions, isochromosomes, and sometimes mosaicisms)17. The loss of X-linked genes results in X inactivation of important X-related gene products that escape inactivation by the second X18. These women are usually born with a normal number of primordial follicles that undergo accelerated atresia19. Some women with primary amenorrhea, especially those with Y material in the karyotype, may have streak gonads. Women with a mosaic X pattern are more likely to present at variable times after menarche20. They may have phenotypic characteristics including short stature, lymphedema, webbed neck, visual impairment, strabismus, otitis media, high arched palate, wide-spaced nipples, shield chest, multiple nevi, cubitus valgus, a short fourth metacarpal, as well as cardiac (coarctation or aortic anomalies) and renal tract abnormalities. Women with Turner syndrome are best managed in multidisciplinary clinics due to possible long-term health issues including potential pregnancy risks, hearing and learning difficulties, diabetes, celiac disease, hypothyroidism, hepatic dysfunction, dyslipidemia, coronary artery disease, and cerebrovascular disease21. If Y-chromosome material is present in some cells, gonadectomy is recommended22.