CMA, also known as molecular karyotyping, has gradually replaced conventional G-banded karyotyping as the first-tier diagnostic test for the individual with developmental delay, intellectual disability, autism spectrum disorder, and/or multiple congenital anomalies, as well as for prenatal evaluation of fetuses with structural anomalies observed by ultrasound [12]. Compared with karyotyping, CMA is capable of detecting clinically significant submicroscopic aberrations up to a few kb. In this study, we used CMA (SNP array platform) and karyotyping for prenatal diagnosis of pregnant women with abnormal DS screening results. CMA is equivalent to traditional karyotyping for the prenatal diagnosis of aneuploidies. CMA provided additional clinically relevant information in 32 of pregnancies. In NT group, although CMA identified more abnormal cases than karyotyping, the difference was not statistically significant. However, in the serum screening group, there was statistically significant difference between CMA and karyotyping (P < 0.05). CMA could detect 1.8% more P/LP CNVs than karyotyping in NT group. The positive rate was lower than previous reports because the NT cut-off for invasive testing in our department is 2.5 mm vs. 3.0–3.5 mm in most previous studies. Consist with these studies, aneuploidy and gross deletion/duplication accounted for more than 80% chromosomal abnormalities (NT cut-off 2.5 ~ 3.0 mm: 80% ~ 90%, NT cut-off 3.5 mm: > 90%) [16,17,18,19,20,21]. CMA could detect 1.9% more CNVs than karyotyping in serum screening group, which is consistent with previous reports [22, 23]. Besides, we identified 2 mendelian monogenetic disease involving DMD gene and HBA1 + HBA2 gene respectively. Inheritance studies revealed the abnormalities was inherited from their mother.
Among the P/LP CNVs identified by CMA, 34.4% (11/32) were recurrent pathogenic CNVs associated with risk of neurodevelopmental disorders. In NT group no recurrent pathogenic CNVs was detected, while in serum group, 55.0% (11/20) were recurrent pathogenic CNVs. Whether there is an association between abnormal serum screening results and recurrent pathogenic CNVs requires further investigation. The penetrance for these recurrent pathogenic CNVs varies from race to race, [24, 25] and there was no large penetrance data available in Chinese population. So it was difficult to determine the clinical significance of these recurrent pathogenic CNVs, which would cause significant stress to pregnant women and their families, in some cases even resulted in the unnecessary abortion. According to previous reports, [25, 26], higher penetrance is seen with CNVs that have higher de novo frequencies. It was also reported that a strong association between IQ and the probability at which CNV deletions occur de novo [27]. Therefore inheritance studies of parents would be helpful to help determining source and counseling. Inheritance studies can bring some solace when the variant is inherited, or escalating of anxiety when it is de novo. However, inheritance of a variant from a healthy parent is no guarantee of it being benign—and the other way around. CMA identified 13 VUS, which is a difficult problem to genetic counseling. Inheritance studies of parents should be performed to help determining source and counseling. Pregnant women and their families should be fully informed of the possible outcomes and provide consent before CMA is performed.
The American College of Obstetrics and Gynecology (ACOG) and the American Maternal-Fetal Medicine Association’s 2016 guidelines clearly suggest CMA as a first-line prenatal diagnostic method in pregnant women with ultrasounds structural abnormalities [12, 28]. However, only a few reports had mentioned the effectiveness of CMA in pregnant women with DS screening abnormities [17,18,19,20,21,22]. In this study, CMA idenitified 1.9% more P/LP CNVs which is the first cause of congenital abnormities than karyotyping. However, the prevalence of P/LP CNVs in both groups (1.8 and 1.9%) is just a bit higher than that in common population (1.2%) [11]. What’s more, 34.4% of the P/LP CNVs detected were recurrent CNVs with uncertain outcome. So the necessity to perform CMA in pregnant women with DS screening abnormities remains indefinite. Multiple factors such as family history, pregnancy history, religious beliefs, ethical orientations and economic state should be considered.
When compared the incidence of different abnormities between NT and serum screening group, NT group had an obviously higher positive rate of aneuploidy. That was because a large part of aneuploidy was previously screened out by NT scan. In NT group, aneuploidy and gross deletion/duplication which could be identified by karyotyping accounted for 80.9% abnormal results (Table 2, (34 aneuploidy + 3 gross deletion/duplication + 1 microdeltion) / (34 aneuploidy + 3 gross deletion/duplication + 9 microdeltion/duplication + 1 UPD). Although the microdeletion/duplication accounted for a large part of abnormal results in serum screening group, the incidence between NT and serum screening group was almost the same, which was just a bit higher than that in common population [11]. This non-significant difference between women with abnormal DS screening results and common population would partly decrease the necessity of CMA in patients with abnormal DS screening results.
Despite the advantages of superior sensitivity and faster turn-around time, there are also some disadvantages compared to conventional karyotyping. CMA is unable to detect balanced chromosomal aberrations and mosaic chromosome abnormalities in low proportion. In this study, karyotyping identified 5 mosaic sex chromosome aneuploidy (Table 4, case 33, 43–46). CMA failed to identify three of them (Table 4, case 44–46). Although the proportions of abnormal cells were low, they might result in some symptoms of Turner syndrome or hermaphroditism according to previous reports [29, 30] and our experience in adults with such karyotype. In certain cases, karyotyping would give additional information for prognosis, such as case 43 and 50 in Table 4 and Fig. 2. In case 43 the karyotyping showed mos 45,X [61]/47,XXX[15] while CMA result was arr(X) × 1, and in case 50 the karyotyping showed mos 47,--,+psu idic (9)(q21.11)[13]/46,--[54] while CMA result was arr[GRCh38]9p24.3q21.11(208455_68398884)× 3. CMA can detect trisomy 13/21 but cannot discern whether it resulted from a non-disjunction event or due to a translocation. In such cases, karyotyping of the fetus and the parents is essential for determining reproductive risk for future offspring. Besides, karyotyping provided additional clinically relevant information in 5 pregnancies, 4 balanced translocation and 1 mosaic balanced translocation, which would be helpful for future pregnancies.
CMA and karyotyping have both advantages and disadvantages in prenatal diagnosis of pregnant women with abnormal DS screening results. So the genetic conseling before amniocentesis is very important, and both advantaged and disadvantages, charges of these tests should be carefully interpreted to parents so that they could make a choice whether CMA was performed.
