The blended phenotype of a germline and a mosaic variant.

We report a patient with a germline and a mosaic variant. The diagnosis of the RASopathy was confirmed by targeted sequencing following the identification of transient cardiomyopathy in a patient with -related overgrowth spectrum (PROS). Our observation confirms that the gain-of-function (GoF) variant effects dominate those of the RASopathy, and the resulting blended phenotype mostly resembles megalencephaly-capillary malformation syndrome (MCAP PROS). There appears to be interaction between RIT1 and PI3K-AKT because the latter pathway is needed for the growth-promoting activity of the first, at least in adenocarcinomas, but the details of this interaction are not known. If so, the somatic variant may not be just a chance event. It could also be of etiological relevance that Rit activation mediates resistance to cellular stress-that is, promotes cell survival. This anti-apoptotic effect could also make it more likely that a cell that spontaneously acquires a GoF variant will survive and proliferate. We aim to encourage clinicians to investigate atypical findings in individuals with PROS. If further similar cases are reported, this would suggest that the establishment of PROS mosaicism is facilitated by the background of a RASopathy.