Review Question - QID 107000

The karyotype demonstrates trisomy 21 (See Illustration A). Individuals with Down syndrome have an increased risk of developing ALL.

Down syndrome (DS) is the most common chromosomal disorder and cause of congenital mental retardation. Trisomy 21 is typically due to maternal meiotic nondisjunction occurring primarily in meiosis I but also in meiosis II. Other causes may be due to a Robertsonian translocation and, less commonly, mosaicism. Patients with trisomy 21 have an increased risk for developing AML (< 3 y/o), ALL (> 3 y/o), Alzheimer's disease by their fifth decade, Hirschsprung's disease, and duodenal atresia.

Xavier and Taub review acute leukemeia in DS pediatric patients. These patients typically fare worse with ALL than children without DS. Although the mechanism is not well understood, DS patients suffer from greater chemotherapeutic toxicity than their non-DS counterparts. This may be due to genes that are overexpressed on the extra chromosome, such as superoxide dismutase and carbonyl reductase, which are involved in oxygen radical formation. Due to the increased toxicity of chemotherapy, DS patients have a higher morbidity and mortality; they also develop infections and mucositis much more frequently.

Griffith et al. discuss nondisjunction and a possible reason for why nondisjunction occurs more frequently with increasing maternal age. Since oocytes are arrested in prophase I, which is the tetrad stage (4N, diploid) until menstruation, the oocyte must be able to maintain proper chromosomal attachments for many years until the oocyte restarts meiosis. Therefore, over time, the number of possible issues arising only increases, and, as seen, most nondisjunctions occur in anaphase I not anaphase II.

Figure/Illustration A demonstrates the karyotype of Down syndrome, which shows a trisomy on chromosome 21 (Note: red boxes denote the chromosomal abnormality).

Incorrect answers:
Answer 1: Individuals born with Klinefelter syndrome (47,XXY) have an increased risk of developing testicular neoplasms.
Answers 2 & 3: Individuals born with Turner syndrome (45,XO) have an increased risk of developing gonadal blastomas and dysgerminoma ovarian tumors.
Answer 5: Exposure to radiation increases the risk of developing CML.