Ob/Gyn Medical News about Prader-Willi syndrome

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Is gestation in Prader-Willi syndrome affected by the genetic subtype
Journal of Assisted Reproduction and Genetics, 09/28/09

Butler MG et al. – Maternal disomy 15 in PWS may contribute to disturbances in gestational age and delivery by impacting on placental structure or function secondary to the abnormal chromosomal number in the placental cells or in mechanisms leading to the maternal disomy status in PWS infants.

Exclusive Author Commentary
Merlin G. Butler, 09/30/09

Prader-Willi syndrome (PWS) is a complex neurogenetic disorder due to the lack of expression of paternally expressed genes in the chromosome 15q11-q13 region. PWS and Angelman syndrome, an entirely different clinical disorder but also involving chromosome 15, were the first examples in humans of genomic imprinting or due to differences in gene expression depending on the parent of origin. About 1/4 of PWS subjects show maternal disomy 15 (both chromosome 15s from the mother) as the cause due to fertilizaton of an egg with two number 15 chromosomes by a normal sperm. In early pregancy a trisomy rescue event occurs whereby the trisomy 15 embryo loses the paternal chromosome 15. Trisomy 15 is a common cause of miscarriages. Therefore, the embryo now has a normal 46 chromosome count but with both 15s from the mother. Maternal disomy 15 in PWS is of two types: heterodisomy or isodisomy; the disomic type may also impact on the pregnancy and clinical outcome in PWS. Most PWS subjects have maternal heterodisomy 15 when the baby inherits each of the mother's chromosome 15s while in maternal isodisomy 15 the two identical chromosome 15s are inherited from the mother due to nondisjunction in meiosis II or possibly from a somatic recombination in early pregnancy producing a segmental or partial form of isodisomy. Therefore, maternal isodisomy may also lead to other genetic disorders in the PWS individual. For example, if the mother carries a recessive allele for a genetic condition on chromosome 15 then the PWS offspring could have inherited both recessive alleles for another genetic condition along with PWS. Although there are no recognized growth factor genes in the proximal long arm (15q11-q13 region) that should affect placental growth or other imprinted genes in humans on chromosome 15, the insulin-like growth factor receptor (IGF1R) gene is located on the distal long arm of chromosome 15 and involved with growth and development. IGF1R is a member of a gene family of growth factors and receptors known to be imprinted in humans and can impact on pre- and post-natal growth including the placenta possibly altering gestational age in PWS. Furthermore, mosaicism of chromosome 15 in the placenta could impact on placental growth and function in the maternal disomy 15 pregnancies and PWS. The trisomy rescue event in the developing embryo may lead to a mixture of normal (46 chromosomes in the developing embryo) and abnormal (47chromosomes in the placental tissue) chromosome complements in the PWS fetus with maternal disomy 15. Imprinted gene products are critical regulators of growth and more research is needed to further address the observations of abnormal gestations in pregnancies with PWS and in other genomic imprinting disorders in humans.

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