Increased nuchal translucency: From etiology to clinical consequences

Nuchal translucency (NT) refers to the translucent area in the neck region of the developing fetus and can be visualized by ultrasound scan between 11 and 13+6 weeks of gestation, after which it normally resolves. An increased NT represents nuchal edema and is defined at equal or greater than 3.5 mm (99th percentile) and is prevalent in 1% of all pregnancies. The etiology of an increased NT has not been elucidated thus far. An increased NT is associated with structural chromosomal abnormalities in the fetus and indicates further invasive diagnostic testing such as chorionic villi sampling or amniocentesis. However, when karyotype is normal after follow-up testing, the fetus is still at risk for structural anomalies and/or genetic syndromes. The studies in this thesis were performed to: 1) gain more insights into the etiology of the increased NT and 2) collect more clinical data on the association of an increased NT and postnatal outcome. The most suggested theory on the development of nuchal edema has been abnormal lymphatic development. Lymphatic development starts with forming jugular lymph sacs (JLS) from the internal jugular veins. A disturbed differentiation of lymphatic endothelial cells (LEC) leads to distension of JLS and nuchal edema. We showed that neural crest cells (NCCs) were observed in the JLS and LEC, therefore suggesting involvement of neural crest cells in the formation of JLS and LEC and accordingly in nuchal edema development. Additionally, it was shown that blocking retinoic acid signaling, which is an active metabolite of vitamin A and important in nervous and lymphatic vascular development, caused aberrant lymphatic development (abnormal JLS) and nuchal edema in mouse embryos. When karyotype is normal after invasive diagnostic testing, prenatal microarray is performed to rule out submicroscopic deletions and/or duplications. This thesis describes the diagnostic yield of prenatal microarray to detect submicroscopic deletions and duplications. A 5% additional diagnostic yield was shown, which is in agreement with other studies (overall 4-7% diagnostic yield). However, no specific recurrent deletions or duplications were discovered in fetuses with an isolated increased NT. The list of associated syndromes in fetuses with h an increased NT is endless, but thus far only Noonan syndrome and its associated RASopathies have been evaluated systematically and proven to be of clinical significance in relation to increased NT. We published two of the largest studies on prenatal RASopathies and showed that the percentage of RASopathies in fetuses with an increased NT (isolated and non-isolated) is approximately 9%. The detection chance is however, highly dependent on the thickness of the NT; pathogenic variant positive fetuses showed a (normally distributed) mean NT of 8.46 mm and pathogenic variant negative samples showed a (non-normal distributed) median NT of 4.7 mm. Based on these findings we formulated new recommendations when to test for RASopathies. In our last study we described a consanguineous family with recurrent pregnancies with severely increased NT, fetal akinesia and joint contractures is described. Chromosomal microarray showed a large homozygous region on chromosome 19 on which the RYR1 gene is located. Sequencing this gene revealed a homozygous nonsense variant in all seven fetuses and parents were heterozygous carriers. Histopathological studies also showed structural abnormalities in the RYR1-mutant skeletal muscle. In conclusion, the expanding importance of an increased NT in associated structural anomalies over the years and its reduction in use and renewed interest in this, due to NIPT implementation and 13-week ultrasound scan respectively, show the complexity of (dealing with) this ultrasound anomaly. The diversity in which congenital structural anomalies and genetic syndromes occur, suggests that a single explanation for the development of nuchal edema is unlikely.