O06 Clinical phenotype of the Klippel-Trenaunay syndrome with mosaic PIK3R1 mutations

Abstract Segmental overgrowth syndromes with cutaneous vascular malformations (Klippel-Trenaunay syndrome (KTS), CLOVES syndrome) are mainly due to mosaic PIK3CA mutations, but the GNAQ, GNA11, HRAS, KRAS, MAP2K1, AKT1, AKT3, and PIK3R1 genes may also be involved. We sought to determine the frequency and phenotype of patients with PIK3R1-related segmental overgrowth. Patients were ascertained nationwide in the French MUSTARD cohort of skin mosaic phenotypes. We searched for PIK3R1 mutations by next-generation sequencing on affected tissue in 297 patients with a clinical presentation suggestive of PIK3CA related overgrowth (PROS), but without PIK3CA variants. We retrospectively studied the clinical phenotype of PIK3R1-positive patients. We identified three missense variants and six in-phase indels of PIK3R1 in 15 patients (5.0%), exclusively in affected tissue. Variant allele fraction was 2.0% to 12.0%. We found extensive pale or dark capillary malformations in 14 patients, segmental overgrowth in 11, venous truncal anomalies (varicose veins or persistent lateral marginal vein) in 10, lymphatic malformation in six and a café-­au-lait macule or naevus spilus in three, thus defining phacomatosis pigmento-vascularis (PPV). One patient had limb hypoplasia. None had MCAP (Megalencephaly-Capillary malformation). PIKR1 mutations account for a minority of patients with KTS/CLOVES (5%). They are about 20 times less common than PIK3CA mutations. Truncal venous malformations, limb hypoplasia and PPV (spilorosea or melanorosea type) may be more common in PIK3R1-related KTS. No patient had severe or life-threatening involvement. Whether Sirolimus or Alpelisib are treatment options remains to be determined.