Study strengths and limitations
This study presents a large cohort of prenatal and neonatal patients tested for RASopathies at 2 international centers with very granular and clinically useful data about ultrasonography findings and yield of panel testing. Prenatal care providers, geneticists, and computational biologists may find this study of great interest and take away useful information and ideas due to the authors’ presentation and details.
The number of genes tested changed over the inclusion time period, but this is an inescapable reality of retrospective clinical research in an advancing field. The authors presented the prenatal and postnatal diagnoses ultrasonography findings separately and together. Given the different nature of cohort ascertainment, we prefer to consider these groups separately and have presented the data for the prenatal group.
WHAT THIS EVIDENCE MEANS FOR PRACTICE
Prenatal sequencing panels and exome sequencing are detecting disorders with important implications for prenatal care. If your practice is not testing for RASopathies in prenatal patients with concerning ultrasonography features, you are missing cases. In this study, the most concerning ultrasonography features (more than 20% diagnosis) were HCM, thoracic effusions and ascites, persistent hydrops, cystic hygroma combined with another suggestive ultrasonography finding, CHD, and persistent cystic hygroma. Isolated ultrasonography findings or findings that resolved had a lower diagnostic yield, and an isolated enlarged NT had a 1% diagnostic yield, with most cases having an NT larger than 6 mm.
For pretest counseling, in this study 20% of patients had a variant of uncertain significance, and preparing patients for this possibility is crucial. Most variants of uncertain significance are reclassified to benign when more information is available. Providers can consider sending parental samples concurrently with the fetal sample to help obtain useful information quickly, although the possibility of an inherited pathogenic variant still exists (12% in this study).
Prenatal diagnosis gives your patients the opportunity to learn about the disorder, plan for treatment and delivery location, and establish their care team before birth or consider pregnancy termination.
Sequencing provides insights into twin pregnancies
Jonsson H, Magnusdottir E, Eggertsson HP, et al. Differences between germline genomes of monozygotic twins. Nat Genet. 2021;53:27-34. doi:10.1038/s41588 -020-00755-1.
You have a monozygotic twin pair with an anomaly and intend to do diagnostic testing for prenatal diagnosis. The question always arises: Do you sample both twins or just one? Surely, they are genetically identical? A wise mentor once instilled a valuable lesson: Monozygotic twins are more likely to have an anomaly. Their existence is already out of the realm of normal. Finally, we now have an engaging and interesting answer to this and other fascinating embryology questions through the work of Jonsson and colleagues.
Study eligibility criteria and treatment protocol
The authors enrolled 381 twin pairs and 2 monozygotic triplets and compared genome sequencing of different tissues (cheek cells and blood). They went further to assess what other tissues might share the genetic change. To do this, they sequenced the children and the partners of 181 of the pairs. Presumably, if a twin and their offspring shared a genetic change that was not present in the spouse or twin, this genetic change must be present in the oocytes or sperm of the parent twin. The goal of sequencing multiple tissue sources in each twin was to help determine when the genetic change occurred in embryonic development.
Study outcomes
The authors found that 15% of twins had mutations that were absent in the other twin. Because of the extent of tissues that had the genetic change, the authors asserted that these changes must have occurred very early in embryonic development (even from one cell after twinning) for the changes to be near-constitutional (among sampled tissues).
An average of 14 genetic differences were found between twin pairs that developed after twinning. However, the number of differences varied. For example, 39 pairs of twins differed by more than 100 changes, and 38 did not differ at all. Differences between twins were more likely in blood samples than in cheek swabs, suggesting that some differences were due to acquired genetic changes in hematologic cell lines, or clonal hematopoiesis.
The authors also looked at what percentage of sequenced DNA contained the variants (or mutations) and found that many of these DNA differences were present at high amounts in sequencing reads. This suggests that the DNA changes happened very early after twinning in about one-third of pairs. Additionally, if one twin had a near-constitutional change, in 42% of pairs the other twin had a different near-constitutional change. Among the triplets, 2 of a triplet pair shared more genetic similarity and were likely descendent from a single split cell and the third likely was formed from a different set of cells.
By examining the offspring of twins, Jonsson and colleagues found that there were 2.6 early embryonic mutations, and this did not differ when blood or buccal DNA was compared. The rate of transmission of a variant to offspring was proportional to the variant allele frequency (proportion of alternate alleles) in the blood or buccal cells. This is an important counseling point when considering patients with mosaic genetic disorders and counseling about the likelihood of inheritance or transmission to future offspring. If the rate of mosaicism was higher in blood or buccal cells, the likelihood of transmission was higher. Additionally, the mutations did not differ by sex, and there was no relationship to whether the chromosome was maternally or paternally inherited.
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