Details of the studies
In a prospective cohort study by Petrovski and colleagues, DNA samples from 234 fetuses with a structural anomaly (identified on ultrasonography) and both parents (parent-fetus "trios") were used for analysis. WES identified diagnostic genetic variants in 24 trios (10%). An additional 46 (20%) had variants that indicated pathogenicity but without sufficient evidence to be considered diagnostic.
The anomalies with the highest frequency of a genetic diagnosis were lymphatic, 24%; skeletal, 24%; central nervous system, 22%; and renal, 16%; while cardiac anomalies had the lowest yield at 5%.
In another prospective cohort study, known as the Prenatal Assessment of Genomes and Exomes (PAGE), Lord and colleagues sequenced DNA samples from 610 parent-fetus trios, but they restricted sequencing to a predefined list of 1,628 genes. Diagnostic genetic variants were identified in 52 fetuses (8.5%), while 24 (3.9%) had a variant of uncertain significance that was thought to be of potential clinical usefulness.
Fetuses with multiple anomalies had the highest genetic yield (15.4%), followed by skeletal (15.4%) and cardiac anomalies (11.1%), with the lowest yield in fetuses with isolated increased nuchal translucency (3.2%).
Diagnostic yield is high, but prenatal utility is limited
Both studies showed a clinically significant diagnostic yield of 8% to 10% for prenatal exome sequencing in cases of fetal structural anomalies with normal karyotype and microarray testing. While this yield demonstrates the utility of prenatal exome sequencing, it is significantly lower than what has been reported in postnatal studies. One of the reasons for this is the inherent limitation of prenatal phenotyping (discussed below).
The cohort studies by both Petrovski and Lord and their colleagues show the feasibility and potential diagnostic utility of exome sequencing in cases of fetal structural anomalies where karyotype and microarray are not diagnostic. However, the lower yield found in these studies compared with those in postnatal studies highlights in part the limitations of prenatal phenotyping.
The importance of prenatal phenotyping
Aarabi M, Sniezek O, Jiang H, et al. Importance of complete phenotyping in prenatal whole exome sequencing. Hum Genet. 2018;137:175-181.
In postnatal exome sequencing, the physical exam, imaging findings, and laboratory results are components of the phenotype that are used to interpret the sequencing data. Prenatal phenotyping, however, is limited to the use of fetal ultrasonography and, occasionally, the addition of magnetic resonance imaging. Prenatal phenotyping is without the benefit of an exam to detect more subtle anomalies or functional status, such as developmental delay, seizures, or failure to thrive.
When a structural anomaly is identified on prenatal ultrasonography, it is especially important that detailed imaging be undertaken to detect other anomalies, including more subtle facial features and dysmorphology.
Value of reanalyzing exome sequencing data
Aarabi and colleagues conducted a retrospective study of 20 fetuses with structural anomalies and normal karyotype and microarray. They performed trio exome sequencing first using information available only prenatally and then conducted a reanalysis using information available after delivery.
With prenatal phenotyping only, the investigators identified no pathogenic, or likely pathogenic, variants. On reanalysis of combined prenatal and postnatal findings, however, they identified pathogenic variants in 20% of cases.
Significance of the findings
This study highlights both the importance of a careful, detailed fetal ultrasonography study and the possible additional benefit of a postnatal examination (such as an autopsy) in order to yield improved results. In addition, the authors noted that the development of a prenatal phenotype-genotype database would significantly help exome sequencing interpretation in the prenatal setting.
Careful prenatal ultrasonography is crucial to help in the interpretation of prenatal exome sequencing. Patients who have undergone prenatal clinical exome sequencing may benefit from reanalysis of the genetic data based on detailed postnatal findings.
Social impact of WES: Parent and provider perspectives
Wou K, Weitz T, McCormack C, et al. Parental perceptions of prenatal whole exome sequencing (PPPWES) study. Prenat Diagn. 2018;38:801-811.
Horn R, Parker M. Health professionals' and researchers' perspectives on prenatal whole genome and exome sequencing: 'We can't shut the door now, the genie's out, we need to refine it.' PLoS One. 2018;13:e0204158.
As health care providers enter a new era of prenatal genetic testing with exome sequencing, it is crucial to the path forward that we obtain perspectives from the parents and providers who participated in these studies. Notably, in both of the previously discussed Lancet reports, the authors interviewed the participants to discuss the challenges involved and identify strategies for improving future testing.
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