Discordant NIPT result in a viable trisomy-21 pregnancy due to prolonged contribution to cfDNA by a demised trisomy-14 cotwin

Trisomy 21 with Maternally Inherited Balanced Translocation (15q;22q) in a Female Fetus: A Rare Case of Probable Interchromosomal Effect

Chromosomal rearrangements can interfere with the disjunction and segregation of other chromosome pairs not involved in the rearrangements, promoting the occurrence of numerical abnormalities in resulting gametes and predisposition to trisomy in offspring. This phenomenon of interference is known as the interchromosomal effect (ICE). Here we report a prenatal case potentially generated by ICE. The first-trimester screening ultrasound of the pregnant woman was normal, but the NIPT indicated a high risk for three copies of chromosome 21, thus suspecting trisomy 21 (T21). After a comprehensive clinical evaluation and genetic counseling, the couple decided to undergo amniocentesis. The prenatal karyotype confirmed T21 but also showed a balanced translocation between the long arm of chromosome 15 (q22) and the long arm of chromosome 22. The parents' karyotypes also showed that the mother had the 15;22 translocation. We reviewed T21 screening methods, and we performed a literature review on ICE, a generally overlooked phenomenon. We observed that ours is the first report of a prenatal case potentially due to ICE derived from the mother. The recurrence risk of aneuploidy in the offspring of translocated individuals is likely slightly increased, but it is not possible to estimate to what extent. In addition to supporting observations, there are still open questions such as, how frequent is ICE? How much is the aneuploidy risk altered by ICE?

Temporal persistence of residual fetal cell-free DNA from a deceased cotwin after selective fetal reduction in dichorionic diamniotic twin pregnancies

OBJECTIVES

To determine the temporal persistence of the residual cell-free DNA (cfDNA) of the deceased cotwin in maternal circulation after selective fetal reduction and evaluate its long persistence in noninvasive prenatal testing (NIPT).

METHODS

Dichorionic diamniotic twins (N = 5) undergoing selective fetal reduction because of a trisomy were recruited. After informed consent, maternal blood was collected immediately before reduction and periodically after reduction until birth. The plasma cfDNA of each sample was sequenced and analyzed for fetal aneuploidy and fetal fractions.

RESULTS

In all pregnancies, the fetal fraction of the cfDNA of the deceased fetus increased to peak at 7-9 weeks after fetal reduction, and subsequently decreased gradually to almost undetectable during the late third trimester. The NIPT T-scores persistently reflected the detection of fetal trisomy up to 16 (median 9.5) weeks after fetal reduction.

CONCLUSIONS

Residual cfDNA from the deceased cotwin after selective reduction at 14-17 gestational weeks led to the persistent generation of false-positive NIPT results for up to 16 weeks postdemise. Thus, providing NIPT for pregnancies with a cotwin demise in early second trimester is prone to misleading results and not recommended.

Should vanishing twin pregnancies be systematically excluded from cell-free fetal DNA testing?

OBJECTIVE

To demonstrate the feasibility of cell-free DNA (cfDNA) testing in vanishing twin (VT) pregnancies in routine clinical practice.

METHODS

Our study included 24 874 singleton and 206 VT consecutive pregnancies. Cell-free DNA was analyzed by massively parallel sequencing. Both aneuploidy analysis (chromosomes 13,18, 21, X, and Y) and fetal fraction estimation were performed according to an Illumina algorithm. Contaminant DNA contribution from the demised co-twin was studied in detail.

RESULTS

VT pregnancies exhibited a higher prevalence of screen-positive cases (5.8% vs 2.5%), sex discrepancies (10.2% vs 0.05%), and false positive rates (FPR) (2.6% vs 0.3%) than singleton pregnancies. However, their incidence was significantly lower in tests performed after the 14th week (screen-positive cases: 3.1%; sex discrepancies: 7.8%; and FPR: 0.8%). Among the 12 cases in which cfDNA was performed at two time points, fading of contaminating cfDNA was observed in four cases with a sex discrepancy and in one false positive for trisomy 18, resulting in a final correct result.

CONCLUSIONS

Our data suggest VT pregnancies could be included in cfDNA testing as long as it is applied after the 14th week of pregnancy. However, future studies to validate our findings are needed before including VT cases in routine clinical practice. Once established, unnecessary invasive procedures could be avoided, mitigating negative emotional impact on future mothers.

Implementation of cell-free DNA-based non-invasive prenatal testing in a National Health Service Regional Genetics Laboratory

Background

Non-invasive prenatal testing (NIPT) for the detection of foetal aneuploidy through analysis of cell-free DNA (cfDNA) in maternal blood is offered routinely by many healthcare providers across the developed world. This testing has recently been recommended for evaluative implementation in the UK National Health Service (NHS) foetal anomaly screening pathway as a contingent screen following an increased risk of trisomy 21, 18 or 13. In preparation for delivering a national service, we have implemented cfDNA-based NIPT in our Regional Genetics Laboratory. Here, we describe our validation and verification processes and initial experiences of the technology prior to rollout of a national screening service.

Methods

Data are presented from more than 1000 patients (215 retrospective and 840 prospective) from ‘high- and low-risk pregnancies’ with outcome data following birth or confirmatory invasive prenatal sampling. NIPT was by the Illumina Verifi® test.

Results

Our data confirm a high-fidelity service with a failure rate of ~0.24% and a high sensitivity and specificity for the detection of foetal trisomy 13, 18 and 21. Secondly, the data show that a significant proportion of patients continue their pregnancies without prenatal invasive testing or intervention after receiving a high-risk cfDNA-based result. A total of 46.5% of patients referred to date were referred for reasons other than high screen risk. Ten percent (76/840 clinical service referrals) of patients were referred with ultrasonographic finding of a foetal structural anomaly, and data analysis indicates high- and low-risk scan indications for NIPT.

Conclusions

NIPT can be successfully implemented into NHS regional genetics laboratories to provide high-quality services. NHS provision of NIPT in patients with high-risk screen results will allow for a reduction of invasive testing and partially improve equality of access to cfDNA-based NIPT in the pregnant population. Patients at low risk for a classic trisomy or with other clinical indications are likely to continue to access cfDNA-based NIPT as a private test.