Trisomy Xp and partial tetrasomy Xq resulting from gain of a rearranged X chromosome in a female fetus: pathogenic or not?

Maria Yiu, Zhongxia Qi, Anita Ki, Jingwei Yu

Cytogenetic analysis of chorionic villous sampling revealed a mosaic karyotype with gain of a rearranged X chromosome. Microarray and additional studies indicated that the rearranged X carried an inverted duplication, a deletion and a satellited Xqter. Gain of this rearranged X was confirmed by follow-up amniocentesis and postnatal cord blood sample. A full-term infant girl was delivered and showed normal physical findings at both birth and 21-month follow-up examinations. Late replication studies demonstrated that the rearranged X was inactivated in all abnormal cells analyzed. Skewed X-inactivation may suppress the potentially deleterious effects of genomic imbalance; however, gain of X chromosomes, particularly rearranged X chromosomes, often presents challenges for prenatal genetic counseling. The gradation of clinical phenotype severity generally correlates with the number of additional X chromosomes. However, the X chromosome regions responsible for the abnormal phenotypes are poorly understood. This case will further elucidate the phenotypic effects of X inactivation and X chromosome abnormalities.