Rare Disease Report

Rare Trisomies Found in Prenatal Screening

AUGUST 30, 2017
James Radke
Humans have 23 pairs of chromosomes. A trisomy is a chromosomal disorder in which the baby has an additional chromosome — 47 instead of 46. The most common is Down syndrome in which children have 3 copies of chromosome 21.

Prenatal screening normally looks for trisomy 21 as well as trisomy 18 (Edward syndrome) and trisomy 13 (Patau syndrome).

A new study published in Science Translational Medicine indicates that testing should be conducted for trisomies on all chromosomes, not just the aforementioned 3. That suggestion by the authors of the study is based on their results showing numerous other rare trisomies being present when they looked at the whole gene.

The whole genome analysis involved 89,817 samples of maternal plasma from the United States (72,972 samples) and Australia (16,885 samples). 
From those samples, 328 samples were found in the US cohort (0.45%) and 71 in the Australian cohort (0.42%) to be considered ‘abnormal’ and flagged for further analysis.

Trisomy 7 was the most common abnormality observed, followed by trisomies 15, 16, and 22.

In the Australian abnormal samples, further analysis revealed that 60 of those 71 samples had trisomies, and of those 60, pregnancy outcome data was available for 52 cases. And among those 52 pregnancies,  22  ended in miscarriages — 13 linked to trisomy 15 and 3 to trisomy 22.

Per a NIH news release,  “We found that pregnancies at greatest risk of serious complications were those with very high levels of abnormal cells in the placenta,” said Mark D. Pertile, Ph.D., co-first author of the study and head of the division of reproductive genetics at Victorian Clinical Genetics Services, part of Murdoch Childrens Research Institute in Melbourne, Australia. “Our results suggest that patients be given the option of receiving test results from all 24 chromosomes.”

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