Trisomy 15 is a genetic disorder caused by the presence of an extra third chromosome in developing diploid cells, which normally only contain two chromosomes. Possessing an extra chromosome leads to developmental disorders, anatomic defects, and sometimes death. There are various clinical syndromes that are trisomy disorders, but trisomy 15 is specifically caused by the additional chromosome being present in the 15th group of the 23 chromosomes that make up human deoxyribonucleic acid (DNA). Trisomy 15 is thought to be relatively rare, but it is difficult to ascertain how many babies would be born with the disorder because most fetuses that have the extra associated chromosome are miscarried in the first trimester. For pregnancies that progress to delivery, two forms of trisomy 15 syndrome are usually present in the children born: Prader-Willi syndrome (PWS) and Angelman syndrome (AS).
During normal conception, the fetus inherits one copy of each chromosome from both his mother and father. In trisomy syndromes, a process of uniparental disomy occurs, which is when two copies of one chromosome come from one parent. This extra chromosome becomes the third chromosome found in trisomy 15. When a child possesses an extra copy of his father’s chromosome, he develops AS, and if the chromosome is from his mother, he develops PWS. Miscarriage results if the fetus inherits double copies from both parents.
Infants born with Prader-Willi syndrome have trouble gaining weight and growing normally. From birth, babies with PS tend to have weak muscles, known as hypotonia, which make it hard to drink from a bottle. Sometimes feeding tubes or other special feeding techniques are used to make sure the baby receives adequate nutrition. As the child grows older, he usually exhibits multiple developmental delays and some develop a symptom unique to the PWS form of trisomy 15, hyperphagia. Hyperphagia is a condition that leads to excessive hunger and, in turn, obesity. If weight is managed and adequate support is established, it is reported that individuals with PWS can lead a fulfilling life.
Infants born with Angelman syndrome exhibit some of the same developmental delays as those with Prader-Willi. With AS, however, there are sometimes more symptoms present that are related to walking and balance. Many people with AS will need care throughout their lives in order to be ambulatory. As infants and toddlers, children with AS seem hyperactive and may seem uncomfortable in social situations. Research shows that with persistent therapy and behavior modification, these behaviors can be lessened or even eliminated.