Genes and phenotypes in vascular malformations.

Vascular malformations (VMs) are caused by localized defects of vascular development. Most VMs are due to sporadic, postzygotic mutations, while some are the result of autosomal dominant germline mutations. Genotype-phenotype correlation is influenced by many factors. Individual genes can induce different phenotypes (pleiotropy), and similar phenotypes can be due to different genes/mutations (redundancy). The phenotypic spectrum of somatic mutations is wide, and depends on variant allele frequency, timing during embryogenesis, cell type(s) involved and type of mutation. The phenotype of germline mutations is determined by penetrance and expressivity, and is influenced by epigenetic factors (DNA methylation, histone modification) or 'second-hit' somatic mutations. Except for disorders with pathognomonic phenotypes such as Proteus syndrome or a characteristic constellation of symptoms such as CLOVES [congenital lipomatous (fatty) overgrowth, vascular malformations, epidermal naevi and scoliosis/skeletal/spinal anomalies] or PIK3CA-related overgrowth spectrum syndrome, differential diagnosis of VM is therefore difficult. It will be greatly facilitated with increasing analytic sensitivity of sequencing techniques such as next-generation sequencing. High-sensitivity molecular techniques are a prerequisite for targeted pharmacotherapy, i.e. selective therapeutic inhibition of activating mutations underlying VM, which has shown promising results in preliminary studies.