Robin A. Pilz, Dariush Skowronek, Tamara Ehresmann, Ute Felbor, Matthias Rath

• Key Clinical MessageNot only germline but also postzygotic mutations in the RASA1 or EPHB4 genes can lead to capillary malformation‐arteriovenous malformation (CM‐AVM) syndrome. As it is not always possible to clinically distinguish between constitutional variants and postzygotic mosaicism, a sufficiently high sequencing depth must be used in genetic diagnostics to detect both.AbstractCapillary malformation‐arteriovenous malformation (CM‐AVM) syndrome, with or without Parkes Weber syndrome, is a rare autosomal dominant disease caused by pathogenic RASA1 or EPHB4 variants. Up to 80% of CM‐AVM cases have an affected parent. Gene panel sequencing was performed for a 4‐year‐old girl with multiple CMs, two capillary stains on the left leg, and associated overgrowth of the second toe. We also reviewed published cases with mosaic RASA1 and EPHB4 mutations. A mosaic RASA1 loss‐of‐function mutation was detected with a variant allele frequency (VAF) of 20% in the blood and oral epithelial cells of the index patient. The literature review illustrates that the severity of the clinical phenotype does not correlate with the VAF. We also identified a germline nonsense variant in the patient's TEK gene. However, inactivating TEK variants do not cause a vascular phenotype but can confer an increased risk for primary congenital glaucoma with variable expressivity. The case presented here illustrates that the choice of the sequencing depth of a diagnostic next‐generation sequencing test for CM‐AVM patients should always take mosaicism into account and that a good knowledge of the sequenced genes and associated disease mechanisms is necessary for adequate genetic counseling.