Is second-trimester genetic amniocentesis for trisomy 18 ever indicated in the presence of a normal genetic sonogram?

Single-Cell Transcriptomics of Cultured Amniotic Fluid Cells Reveals Complex Gene Expression Alterations in Human Fetuses With Trisomy 18

Trisomy 18, commonly known as Edwards syndrome, is the second most common autosomal trisomy among live born neonates. Multiple tissues including cardiac, abdominal, and nervous systems are affected by an extra chromosome 18. To delineate the complexity of anomalies of trisomy 18, we analyzed cultured amniotic fluid cells from two euploid and three trisomy 18 samples using single-cell transcriptomics. We identified 6 cell groups, which function in development of major tissues such as kidney, vasculature and smooth muscle, and display significant alterations in gene expression as detected by single-cell RNA-sequencing. Moreover, we demonstrated significant gene expression changes in previously proposed trisomy 18 critical regions, and identified three new regions such as 18p11.32, 18q11 and 18q21.32, which are likely associated with trisomy 18 phenotypes. Our results indicate complexity of trisomy 18 at the gene expression level and reveal genetic reasoning of diverse phenotypes in trisomy 18 patients.

Prenatal diagnosis of i(18q) and dup(18q) cases by quantitative fluorescent PCR

Particular sonographic fetal malformations are common in chromosome 18 aberrations, requiring invasive prenatal tests to confirm the diagnosis. Karyotyping is the gold standard assay in these cases, although it is a high complexity, expensive and approximately 2 weeks turnaround time test. On the contrary, quantitative fluorescent PCR is considered an accurate, simple, low cost and rapid assay, particularly useful for the diagnosis of aneuploidies of chromosomes 13, 18 and 21 and for the detection of maternal cell contamination of the sample. Clinical presentation of two cases of rare chromosome 18 defects, diagnosed using both techniques. One case was an isochromosome and the other was a partial duplication. Quantitative fluorescent PCR was an invaluable tool for the cytogenetics laboratory.

Sonographic findings of trisomy 18 in the second trimester of pregnancy

OBJECTIVE

The purpose of this study was to examine the sonographic findings in fetuses with trisomy 18 in the second trimester of pregnancy.

METHODS

A retrospective review of the cytogenetic laboratory databases at 6 tertiary referral centers identified all cases of trisomy 18. The prenatal sonographic studies in fetuses at 15 to 21 weeks' gestation, done before invasive testing for the karyotype, were reviewed for anatomic and biometric findings. We defined abnormal fetal biometric findings as a biometric measurement (biparietal diameter, abdominal circumference, or femur length) below the fifth percentile in the second trimester.

RESULTS

Of 98 fetuses with trisomy 18, 95 (97%) were detected sonographically; an anomaly was found in 92 (94%). A biometric measurement below the fifth percentile was noted in 50 (51%). Cardiac (63%) and central nervous system (34%) anomalies were most frequently detected. Although choroid plexus cysts were commonly seen, no fetuses with trisomy 18 and isolated choroid plexus cysts were found.

CONCLUSIONS

Targeted sonography identified abnormal fetal anatomy or abnormal biometric findings in 97% of fetuses with trisomy 18 in the second trimester. A biometric measurement below the fifth percentile was noted in half of the cases in the second trimester.