Non-invasive prenatal testing for sex chromosome abnormalities: a source of confusion

Maternal Sex Chromosome Aneuploidy Identified through Noninvasive Prenatal Screening: Clinical Profile and Patient Experience

OBJECTIVE

Noninvasive prenatal screening (NIPS) may incidentally identify maternal aneuploidies that have health implications. We evaluated patients' experience with counseling and follow-up diagnostic testing after NIPS flags a potential maternal sex chromosome aneuploidy (SCA).

STUDY DESIGN

Patients who underwent NIPS at two reference laboratories between 2012 and 2021 and had test results that were consistent with possible or probable maternal SCA were contacted with a link to an anonymous survey. Survey topics included demographics, health history, pregnancy history, counseling, and follow-up testing.

RESULTS

A total of 269 patients responded to the anonymous survey, and 83 of these individuals also completed one follow-up survey. Most received pretest counseling. A total of 80% were offered fetal genetic testing during the pregnancy, and 35% of patients completed diagnostic maternal testing. Monosomy X-related phenotypes such as short stature or hearing loss prompted follow-up testing that led to a diagnosis of monosomy X in 14 (6%) cases.

CONCLUSION

Follow-up counseling and testing after a high-risk NIPS result suggestive of maternal SCA is heterogenous in this cohort and may be frequently incomplete. Health outcomes may be affected by these results and additional research could improve the provision, delivery, and quality of posttest counseling.

KEY POINTS

· NIPS results showing potential SCA could have maternal health implications.. · Variations in counseling and testing after NIPS were observed for women with suspected SCA.. · Comprehensive counseling and diagnostic testing strategies are critical for these patients..

Diagnostic cytogenetic testing following positive noninvasive prenatal screening results of sex chromosome abnormalities: Report of five cases and systematic review of evidence

BACKGROUND

Follow-up cytogenetic analysis has been recommended for cases with positive noninvasive prenatal screening (NIPS) results. This study of five cases with numerical and structural sex chromosomal abnormalities (SCA) and a review of large case series of NIPS provided guidance to improve prenatal diagnosis for SCA.

METHODS

Following positive NIPS results for SCA, karyotype analysis, chromosomal microarray analysis (CMA), fluorescence in situ hybridization (FISH), and locus-specific quantitative PCR were performed on cultured amniocytes, chorionic villi cells, and stimulated lymphocytes. Review of large case series was performed to evaluate the NIPS positive rate, follow-up rate of cytogenetic analysis, positive predictive value (PPV) for major types of SCA, and relative frequencies of subtypes of major SCA.

RESULTS

Of the five cases with positive NIPS for SCA, case 1 showed a mosaic pattern of monosomy X and isodicentric Y; case 2 showed a mosaic pattern of monosomy X confined to the placenta; cases 3 and 4 had an isochromosome of Xq, and case 5 showed a derivative chromosome 14 from a Yq/14p translocation of maternal origin. Review of literature showed that mean positive rate of NIPS for SCA was 0.61%, follow-up rate of cytogenetics analysis was 76%, and mean PPV for SCA was 48%. Mosaic patterns and structural rearrangements involving sex chromosomes were estimated in 3%-20% and 3% of SCA cases, respectively.

CONCLUSION

These five cases further demonstrated the necessity to pursue follow-up cytogenetic analysis to characterize mosaic patterns and structural abnormalities involving sex chromosomes and their value for prenatal genetic counseling. A workflow showing the performance of current NIPS and cytogenetic analysis for SCA was summarized. These results could facilitate an evidence-based approach to guide prenatal diagnosis of SCA.

Current genetic counseling practice in the United States following positive non-invasive prenatal testing for sex chromosome abnormalities

The purpose of this study was to describe current genetic counseling practice in the United States following a non-invasive prenatal testing (NIPT) result positive for a sex chromosome abnormality (SCA). Screening for SCAs can be confounded by confined placental mosaicism, natural loss of the X chromosome from maternal cells during aging, and undiagnosed maternal SCA or copy number variant (CNV). Furthermore, with the exception of 45,X, individuals with SCAs usually have no ultrasound or postnatal findings. This makes follow-up for unresolved positive NIPT necessary; however, there are currently no clinical guidelines. This study used a cross-sectional design with an anonymous questionnaire to survey 176 genetic counselors. The majority of prenatal respondents always offered diagnostic testing (>88%) and anatomy ultrasound (~90%), but the percent consistently offering maternal karyotype (22%-52%) and postnatal evaluation (28%-87%) varied. Maternal karyotype was offered more often when NIPT was positive for 45,X or 47,XXX and patients had normal prenatal diagnostic testing (p < 0.02) or declined testing (p < 0.02). Offer of postnatal evaluation was more likely when diagnostic testing was declined (p < 0.001). The majority of pediatric providers always offered a postnatal karyotype for the newborn (>72%) but the percent offering maternal karyotype (6%-46%) varied widely. With the current inconsistencies, many newborns with undiagnosed SCAs who could benefit from growth hormone therapy, early intervention, and/or targeted surveillance may be missed. Therefore, there is a need for professional guidelines to help improve the consistency of clinical care for patients with NIPT results positive for SCAs.

Non-invasive prenatal aneuploidy testing: Critical diagnostic performance parameters predict sample z-score values

OBJECTIVES

Non-invasive prenatal aneuploidy testing (NIPT) by next-generation sequencing of circulating cell-free DNA in maternal plasma relies on chromosomal ratio (chr_ratio) measurements to detect aneuploid values that depart from euploid ratios. Diagnostic performances are known to depend on the fraction of fetal DNA (FF) present in maternal plasma, although how this translates into specific quantitative changes in specificity/positive predictive values and which other variables might also be important is not well understood.

DESIGN & METHODS

To explore this issue, theoretical relationships between FF and various measures of diagnostic performances were assessed for a range of parameter values. Empirical data from three NIPT assays were then used to validate theoretical calculations.

RESULTS

For a given positivity threshold, dramatic changes in specificity and positive predictive values (PPV) as a function of both FF and the coefficient of variation (CV) of the chr_ratio measurement were observed. Theoretically predicted and observed chr_ratio z-scores agreed closely, confirming the determinant impact of small changes in both FF and chr_ratio CV.

CONCLUSIONS

Evaluation of NIPT assay performances therefore requires knowledge of the FF distribution in the population in which the test is intended to be used and, in particular, of the precise value of the assay chr_ratio CV for each chromosome or genomic region of interest. Laboratories offering NIPT testing should carefully measure these parameters to ensure test reliability and clinical usefulness in interpreting individual patients' results.

Sex chromosome aneuploidy screening in a general population

Cell-free fetal DNA testing is being used in parallel or in contingency screening as part of the first trimester screen. The test has high sensitivity and specificity for the trisomies 21, 18 and 13. The test also offers the option of assessing sex chromosome aneuploidies (SCA) which are recognised to be the next most common group of aneuploidies in the live birth population. Companies that offer the sex chromosome assessment report an accuracy rate of above 99% and a significant number of high-risk results have been detected in a multi-site Australian ultrasound practice. A high proportion of these women underwent prenatal testing to further assess the sex chromosomes. This study reports the results of these invasive investigations and results show that many of the high-risk SCA results appear to be false positives. This study reports the clinical experience of cell-free fetal DNA (cfDNA) testing with regard to sex chromosome aneuploidies in singleton pregnancies for a multi-site Sydney specialist O&G Ultrasound practice.