Systematic analysis of the causes of NIPS sex chromosome aneuploidy false-positive results

Relationships among maternal monosomy X mosaicism, maternal trisomy, and discordant sex chromosome aneuploidies

OBJECTIVE

To explore the impact of maternal factors on the false-positive fetal sex chromosome aneuploidies (SCAs) results obtained through noninvasive prenatal screening (NIPS).

METHODS

We retrospectively analyzed pregnant women with high-risk SCAs as revealed using NIPS between January 2017 and December 2022. Clinical data such as results of invasive prenatal diagnoses, copy number variation sequencing (CNV-seq) and pregnancy outcomes were analysed.

RESULTS

Overall, 177 (0.6 %) women with SCA-positive results were collected from 27,941 patients who had undergone NIPS. Among them, 110 (62.2 %) pregnant women chose prenatal diagnosis and 39 (35.5 %) cases were confirmed. For the women with monosomy X false-positive results from the NIPS, 53.1 % (17/32) were found to be maternal mosaicism monosomy X. In cases with 47, XXX false-positive results, 60 % (6/10) of them were maternal 47,XXX (5 cases) or maternal mosaicism 47,XXX (1 case). One (1/6, 16.7 %) case of maternal mosaicism monosomy X was detected in the false positive results of 47, XXY/47, XYY revealed. The incidence rate of maternal sex chromosome abnormalities was positively correlated with the Z-score of ChrX. When the Z-score of ChrX ≥ 15, more than 50 % of pregnant women were found to be maternal sex chromosome abnormalities, and when Z-score ≥ 30, the incidence rate was as high as 100 %.

CONCLUSIONS

Maternal monosomy X mosaicism and trisomy X respectively played an important role in the discordance of 45, X and 47, XXX revealed by NIPS. CNV-seq was recommended for the pregnant women at risk of maternal sex chromosome abnormalities, which could help clinicians to provide more accurate and efficient advice during genetic counseling and to guide appropriate prenatal diagnosis strategy for the next pregnancy.

Patient with Mosaic Turner Syndrome and a Derivative X Chromosome with a Variant Triple X Diagnosis in Fetus: A Case Report

Although Turner syndrome is most often sporadic, multigenerational recurrence has been reported more often in the offspring of women with mosaic or variant forms of Turner syndrome. We present a case in which natural conception in a woman with identified 45,X/46,XX mosaicism resulted in a fetus with a gain of a derivative X chromosome. The unexpected fetal finding prompted further cytogenetic evaluation of the patient and subsequent identification of an additional cell line with the same derivative X chromosome, not observed in the initial study. To our knowledge, this is the first case in which further investigation of an abnormal noninvasive prenatal screen resulted in the identification of both maternal and fetal sex chromosome abnormality. We discuss the discordant finding, similar cases, and potential phenotype with respect to skewed X inactivation. We also highlight the use of multiple testing methodologies to characterize the serendipitous identification of a derivative X chromosome.