Lessons from a normal infant with uniparental isodisomy of chromosome 21: A Case Report and Review

Yuying Zhu ^* Ke Wu Cuicui Jiang ^* Qiumin Zhu ^*

• Quzhou Maternity and Child Health Care Hospital, Quzhou, China

Uniparental disomy (UPD) refers to both homologous chromosomes inherited from a single parent. To date, the UPD of all autosomes and the X chromosome has been recorded. Few cases of UPD of chromosome 21 have been documented. At 15 weeks of gestation, the 25-year-old pregnant woman's non-invasive prenatal screening revealed a high risk of trisomy 21. Although the fetal ultrasonography indicated no anomalies, amniocentesis was performed, and the fetal karyotype analysis showed normal. A single-nucleotide polymorphism array (SNP array) revealed that the fetus had the copy-neutral region of homozygosity (ROH) in the long arm of chromosome 21. Subsequently, single whole-exome sequencing was performed due to the risk of recessive gene variants in ROH, and found no homozygous likely pathogenic or pathogenic variants on the long arm of chromosome 21. After genetic counselling, the parent decided to continue this pregnancy. At 37 weeks of gestation, a live male infant was delivered by cesarean section. Copy number variation sequencing showed that the placental tissue was mosaic for trisomy 21. At the final follow-up evaluation, the 6-month-old male had normal phenotype.