Somatic RIT1 delins in arteriovenous malformations hyperactivate RAS-MAPK signaling amenable to MEK inhibition.

Arteriovenous malformations (AVM) are benign vascular anomalies prone to pain, bleeding, and progressive growth. AVM are mainly caused by mosaic pathogenic variants of the RAS-MAPK pathway. However, a causative variant is not identified in all patients. Using ultra-deep sequencing, we identified novel somatic RIT1 delins variants in lesional tissue of three AVM patients. RIT1 encodes a RAS-like protein that can modulate RAS-MAPK signaling. We expressed RIT1 variants in HEK293T cells, which led to a strong increase in ERK1/2 phosphorylation. Endothelial-specific mosaic overexpression of RIT1 delins in zebrafish embryos induced AVM formation, highlighting their functional importance in vascular development. Both ERK1/2 hyperactivation in vitro and AVM formation in vivo could be suppressed by pharmacological MEK inhibition. Treatment with the MEK inhibitor trametinib led to a significant decrease in bleeding episodes and AVM size in one patient. Our findings implicate RIT1 in AVM formation and provide a rationale for clinical trials with targeted treatments.

Infantile hemangioma: the common and enigmatic vascular tumor.

Infantile hemangioma (IH) is a benign vascular tumor that occurs in 5% of newborns. The tumor follows a life cycle of rapid proliferation in infancy, followed by slow involution in childhood. This unique life cycle has attracted the interest of basic and clinical scientists alike as a paradigm for vasculogenesis, angiogenesis, and vascular regression. Unanswered questions persist about the genetic and molecular drivers of the proliferating and involuting phases. The beta blocker propranolol usually accelerates regression of problematic IHs, yet its mechanism of action on vascular proliferation and differentiation is unclear. Some IHs fail to respond to beta blockers and regrow after discontinuation. Side effects occur and long-term sequelae of propranolol treatment are unknown. This poses clinical challenges and raises novel questions about the mechanisms of vascular overgrowth in IH.

R(+) Propranolol decreases lipid accumulation in hemangioma-derived stem cells.

Background: Infantile hemangioma (IH) is a benign vascular tumor that undergoes an initial rapid growth phase followed by spontaneous involution. A fibrofatty residuum remains in many tumors and often necessitates resection. We recently discovered that R(+) propranolol, the non-ß blocker enantiomer, inhibits blood vessel formation of IH patient-derived hemangioma stem cells (HemSC) xenografted in mice. HemSC are multipotent cells with the ability to differentiate into endothelial cells, pericytes, and adipocytes. Objectives: We investigated how R(+) propranolol affects HemSC adipogenic differentiation and lipid accumulation, in vitro and in a preclinical murine model for IH. Methods: We conducted a 10-day adipogenesis assay on 4 IH patient-derived HemSCs. Oil Red O (ORO) staining was used to identify the onset and level of lipid accumulation in HemSC while quantitative real-time polymerase chain reaction was conducted to determine the temporal expression of key factors implicated in adipogenesis. 5-20µM R(+) propranolol treatment was added to HemSC induced to undergo adiogenesis for 4 and 8 days, followed by quantification of lipid-stained areas and transcript levels of key adipogenic factors. We immunostained for lipid droplet-associated protein Perilipin 1 (PLIN1) in HemSC-xenograft sections from mice treated with R(+) propranolol and quantified the area using ImageJ. Results: We found that different patient-derived HemSC exhibit a robust and heterogenous adipogenic capacity when induced for adipogenic differentiation in vitro. Consistently across four IH patient-derived HemSC isolates, R(+) propranolol reduced ORO-stained areas and lipoprotein lipase (LPL) transcript levels in HemSC after 4 and 8 days of adipogenic induction. In contrast, R(+) propranolol had no significant inhibitory effect on transcript levels encoding adipogenic transcription factors. In a pre-clinical HemSC xenograft model, PLIN1-positive area was significantly reduced in xenograft sections from mice treated with R(+) propranolol, signifying reduced lipid accumulation. Conclusions: Our findings suggest a novel regulatory role for the R(+) enantiomer of propranolol in modulating lipid accumulation in HemSC. This highlights a novel role of R(+) propranolol in the involuting phase of IH and a strategy to reduce fibrofatty residua in IH. What is already known about this topic?: Propranolol is the mainstay treatment for infantile hemangioma (IH), the most common tumor of infancy, but its use can be associated with concerning ß-blocker side effects.R(+) propranolol, the enantiomer largely devoid of ß-blocker activity, was recently shown to inhibit endothelial differentiation of hemangioma-derived stem cells (HemSC) in vitro and reduce blood vessel formation in a HemSC-derived xenograft murine model of IH. What does this study add?: R(+) propranolol inhibits lipid accumulation in HemSC in vitro.R(+) propranolol does not affect mRNA transcript levels of key adipogenic transcription factors in differentiating HemSC in vitro.R(+) propranolol reduces lipid accumulation in a pre-clinical xenograft murine model of IH. What is the translational message?: The R(+) enantiomer of propranolol could be advantageous in terms of reduction in ß-adrenergic side effects and fibrofatty tissue formation in the involuting phase of IH.Less fibrofatty residua might reduce the need for surgical resection.Disfigurement and associated psychosocial impacts might be improved in this young patient cohort.

MRC1 and LYVE1 expressing macrophages in vascular beds of GNAQ p.R183Q driven capillary malformations in Sturge Weber syndrome.

Sturge-Weber syndrome (SWS), a neurocutaneous disorder, is characterized by capillary malformations (CM) in the skin, brain, and eyes. Patients may suffer from seizures, strokes, and glaucoma, and only symptomatic treatment is available. CM are comprised of enlarged vessels with endothelial cells (ECs) and disorganized mural cells. Our recent finding indicated that the R183Q mutation in ECs leads to heightened signaling through phospholipase Cß3 and protein kinase C, leading to increased angiopoietin-2 (ANGPT2). Furthermore, knockdown of ANGPT2, a crucial mediator of pro-angiogenic signaling, inflammation, and vascular remodeling, in EC-R183Q rescued the enlarged vessel phenotype in vivo. This prompted us to look closer at the microenvironment in CM-affected vascular beds. We analyzed multiple brain histological sections from patients with GNAQ-R183Q CM and found enlarged vessels devoid of mural cells along with increased macrophage-like cells co-expressing MRC1 (CD206, a mannose receptor), CD163 (a scavenger receptor and marker of the monocyte/macrophage lineage), CD68 (a pan macrophage marker), and LYVE1 (a lymphatic marker expressed by some macrophages). These macrophages were not found in non-SWS control brain sections. To investigate the mechanism of increased macrophages in the perivascular environment, we examined THP1 (monocytic/macrophage cell line) cell adhesion to EC-R183Q versus EC-WT under static and laminar flow conditions. First, we observed increased THP1 cell adhesion to EC-R183Q compared to EC-WT under static conditions. Next, using live cell imaging, we found THP1 cell adhesion to EC-R183Q was dramatically increased under laminar flow conditions and could be inhibited by anti-ICAM1. ICAM1, an endothelial cell adhesion molecule required for leukocyte adhesion, was strongly expressed in the endothelium in SWS brain histological sections, suggesting a mechanism for recruitment of macrophages. In conclusion, our findings demonstrate that macrophages are an important component of the perivascular environment in CM suggesting they may contribute to the CM formation and SWS disease progression.

An endothelial SOX18-mevalonate pathway axis enables repurposing of statins for infantile hemangioma.

Infantile hemangioma (IH) is the most common tumor in children and a paradigm for pathological vasculogenesis, angiogenesis and regression. Propranolol is the mainstay of treatment for IH. It inhibits hemangioma vessel formation via a ß-adrenergic receptor independent off-target effect of its R(+) enantiomer on the endothelial specific transcription factor sex-determining region Y (SRY) box transcription factor 18 (SOX18). Transcriptomic profiling of patient-derived hemangioma stem cells uncovered the mevalonate pathway (MVP) as a target of R(+) propranolol. Loss of SOX18 function confirmed R(+) propranolol mode of action on the MVP. Functional validation in preclinical IH models revealed that statins - targeting the MVP - are potent inhibitors of hemangioma vessel formation. We propose a novel SOX18-MVP-axis as a central regulator of IH pathogenesis and suggest statin repurposing to treat IH. Our findings reveal novel pleiotropic effects of beta-blockers and statins acting on the SOX18-MVP axis to disable an endothelial specific program in IH, which may impact other scenarios involving pathological vasculogenesis and angiogenesis.

Extracranial Vascular Anomalies Driven by RAS/MAPK Variants: Spectrum and Genotype-Phenotype Correlations.

BACKGROUND: We aimed to correlate alterations in the rat sarcoma virus (RAS)/mitogen-activated protein kinase pathway in vascular anomalies to the clinical phenotype for improved patient and treatment stratification. METHODS AND RESULTS: This retrospective multicenter cohort study included 29 patients with extracranial vascular anomalies containing mosaic pathogenic variants (PVs) in genes of the RAS/mitogen-activated protein kinase pathway. Tissue samples were collected during invasive treatment or clinically indicated biopsies. PVs were detected by the targeted sequencing of panels of genes known to be associated with vascular anomalies, performed using DNA from affected tissue. Subgroup analyses were performed according to the affected genes with regard to phenotypic characteristics in a descriptive manner. Twenty-five vascular malformations, 3 vascular tumors, and 1 patient with both a vascular malformation and vascular tumor presented the following distribution of PVs in genes: Kirsten rat sarcoma viral oncogene (n=10), neuroblastoma ras viral oncogene homolog (n=1), Harvey rat sarcoma viral oncogene homolog (n=5), V-Raf murine sarcoma viral oncogene homolog B (n=8), and mitogen-activated protein kinase kinase 1 (n=5). Patients with RAS PVs had advanced disease stages according to the Schobinger classification (stage 3-4: RAS, 9/13 versus non-RAS, 3/11) and more frequent progression after treatment (RAS, 10/13 versus non-RAS, 2/11). Lesions with Kirsten rat sarcoma viral oncogene PVs infiltrated more tissue layers compared with the other PVs including other RAS PVs (multiple tissue layers: Kirsten rat sarcoma viral oncogene, 8/10 versus other PVs, 6/19). CONCLUSIONS: This comparison of patients with various PVs in genes of the RAS/MAPK pathway provides potential associations with certain morphological and clinical phenotypes. RAS variants were associated with more aggressive phenotypes, generating preliminary data and hypothesis for future larger studies.