Review Question - QID 107012

The newborn was born with the physical features of Edwards syndrome, which is due to a meiotic nondisjunction resulting in trisomy of chromosome 18, most commonly of maternal origin.

Edwards syndrome is the most common cause of trisomy resulting in live birth after Down syndrome. The trisomy is a result of meiotic nondisjunction and the risk of it occurring rises with increasing maternal age. Infants are born with mental retardation and present with micrognathia (small jaw), malformed low-set ears, rocker-bottom feet (Figure A), clenched hands with overlapping fingers (Illustration A), and a prominent occiput. Infants will usually die before the age of 1 year and many are born with congenital heart disease such as a VSD.

Ramesh and Verma perform a retrospective study to determine the parental origin of the extra chromosome in Edwards syndrome. They found that out of 171 cases of trisomy 18, 93% were due to maternal nondisjunction and 7% were of paternal origin. They also determined where the nondisjunction occurred: 16% occurred in meiosis I, 31% occurred in meiosis II, and 53% occurred in postzygotic mitosis or the stage could not be determined.

Rasmussen et al. perform an analysis to determine the survival of patients using the Metropolitan Atlanta Congenital Defects program using records of those born between 1968-1999 with trisomy 13 (n = 70) and 18 (n =114). They found the median survival to be 7 days for trisomy 13 and 14.5 days for trisomy 18, and 91% died within one year. Female infants and races other than Caucasian survived longer for trisomy 18. For trisomy 13, race and gender did not affect survival.

Figure A demonstrates "rocker bottom" feet which is due to a prominent calcaneus and a convexity of the foot producing the appearance of a rocking chair. Illustration A depicts a karyotype of Edwards syndrome. Illustration B demonstrates the clenched hands with overlapping fingers of an infant with Edwards syndrome.

Incorrect answers:
Answer 1: Patau's syndrome is due to a meiotic nondisjunction resulting in trisomy of chromosome 13.
Answer 3: Turner syndrome is due to a meiotic nondisjunction such that a gamete loses a sex chromosome, resulting in a karyotype of 45,XO.
Answer 4: Cri-du-Chat syndrome is due to a microdeletion of the short arm on chromosome 5.
Answer 5: Williams syndrome is due to a microdeletion of the long arm on chromosome 7.