Somatic second hit mutation of RASA1 in vascular endothelial cells in capillary malformation-arteriovenous malformation

Arteriovenous cerebral high-flow shunts: genetic analysis of patients from a pediatric tertiary care center

Introduction

Arteriovenous cerebral high-flow shunts include the vein of Galen aneurysmal malformation (VGAM) and vein of Galen dilatation, which are considered secondary to arteriovenous malformations or arteriovenous fistulas. These entities are often sporadic but are found in association with variants of the RASA1 and EPHB4 genes (capillary malformation-arteriovenous malformation, CMAVM; OMIM #608354) or ACVRL1, ENG, and SMAD4 genes (hereditary hemorrhagic telangiectasia, HHT; OMIM #187300). The clinical phenotypes associated with these conditions are highly variable, with incomplete penetrance and mostly dependent on the hemodynamic consequences (including heart failure and cerebral hemorrhage) or management complications rather than anatomical vascular variations per se. The present study aimed to genetically characterize a cohort of 29 patients affected by arteriovenous cerebral high-flow shunts who were treated at a pediatric referral center.

Methods

The genetic techniques employed include next-generation sequencing, multiplex ligation-dependent probe amplification, and whole-exome sequencing.

Results

Of the 29 patients, 11 cases were found to have variants in genes associated with vascular functions, five cases received a genetic diagnosis, one case presented with a variant of uncertain significance in the EPHB4 gene, and five cases showed variants in novel genes possibly linked with cerebrovascular disorders.

Discussion

We provide extensive case descriptions and attempt to infer the genotype-phenotype correlations; variants in all of the known genes associated with arteriovenous cerebral shunts were reported in VGAM patients, while cutaneous angiomas were specific to RASA1 mutations. The genotypic and phenotypic descriptions of the affected individuals may thus have relevant implications in terms of better pathophysiological understanding, genotype-phenotype correlations, treatment strategies, and outcomes.

Second Report of the p.Leu874Pro Missense Variant in EPHB4 in a Family With Capillary Malformation-Arteriovenous Malformation Syndrome (CM-AVM) Syndrome

Capillary malformation-arteriovenous malformation (CM-AVM) syndrome is characterized by the presence of multiple small (1-2 cm in diameter) capillary malformations of the skin. This disorder has been described as two distinct entities: CM-AVM1 and CM-AVM2. The diagnosis of these disorders has been associated with pathogenic variants in the RASA1 gene for RASA1-CM-AVM, formerly known as CM-AVM1, and, more recently, the EPHB4 genes for EPHB4-CM-AVM, formerly known as CM-AVM2. Affected patients with either type may also have arteriovenous malformations and fistulas, which can cause life-threatening bleeding, congestive heart failure, or neurologic consequences such as stroke. These syndromes are typically either sporadic or inherited in an autosomal dominant manner with variable expressivity. We report a case series of a father and three daughters who have clinically diagnosed EPHB4-CM-AVM syndrome who were found to have a variant of uncertain significance (VUS) in EPHB4 that has only been reported once prior.

The C2 domain augments Ras GTPase-activating protein catalytic activity

Regulation of Ras GTPases by GTPase-activating proteins (GAPs) is essential for their normal signaling. Nine of the ten GAPs for Ras contain a C2 domain immediately proximal to their canonical GAP domain, and in RasGAP (p120GAP, p120RasGAP; RASA1) mutation of this domain is associated with vascular malformations in humans. Here, we show that the C2 domain of RasGAP is required for full catalytic activity toward Ras. Analyses of the RasGAP C2-GAP crystal structure, AlphaFold models, and sequence conservation reveal direct C2 domain interaction with the Ras allosteric lobe. This is achieved by an evolutionarily conserved surface centered around RasGAP residue R707, point mutation of which impairs the catalytic advantage conferred by the C2 domain in vitro. In mice, R707C mutation phenocopies the vascular and signaling defects resulting from constitutive disruption of the RASA1 gene. In SynGAP, mutation of the equivalent conserved C2 domain surface impairs catalytic activity. Our results indicate that the C2 domain is required to achieve full catalytic activity of GAPs for Ras.

Diagnosis and management of superficial arteriovenous malformations: French healthcare network's recommendations

Superficial arteriovenous malformations are rare fast-flow lesions. They consist of arteriovenous shunts, without cellular hyperplasia or proliferation, which develop in the surrounding tissues (cutaneous, subcutaneous, muscular, bone). Although benign, they are among the most severe of superficial malformations. Their evolution can be life-threatening in exceptional cases. With the aim of optimizing diagnosis and management worldwide, this  protocol offers a state of the art for the diagnosis and management of these diseases. To this end, the French healthcare network specialized in these diseases have drawn on literature data and experience. Developed from the French National Diagnosis and Care Protocol, it presents the patient journeys for initial and differential diagnoses, and personalized therapeutic strategies. This requires a multidisciplinary team, with specialized professionals in handling genetic, treatment and psychosocial issues.

Inhibition of miR-92a normalizes vascular gene expression and prevents diastolic dysfunction in heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) remains a major public health burden with increasing prevalence but only few effective therapies. Endothelial dysfunction and inflammation are identified as pathophysiological drivers of HFpEF disease progression. MicroRNAs are increasingly recognized as key regulators of these pathological processes, while antimiR-based therapies have been emerged as promising therapeutics in mice and humans. Therefore, we tested whether miR-92a-3p inhibition is a promising therapeutic intervention to target HFpEF in vivo. By injection of locked nucleic acid (LNA)-based antimiR (LNA-92a) weekly, we demonstrate that inhibition of miR-92a-3p attenuates the development of diastolic dysfunction and left atrial dilation following experimental induction of HFpEF in mice. Indeed, LNA-92a depleted miR-92a-3p expression in the myocardium and peripheral blood, and derepressed predicted target genes in a cell type-specific manner. Furthermore, cell-type specific efficacy of LNA-92a treatment was assessed by single-nuclear RNA sequencing of HFpEF hearts either treated with LNA-92a or LNA-Control. Endothelial cells of LNA-92a treated mice showed normalized vascular gene expression and reduced gene signatures associated with endothelial-mesenchymal transition. CONCLUSION: This study demonstrates that LNA-based antimiR-92a is an effective therapeutic strategy to target diastolic dysfunction and left atrial dilation in HFpEF.

Another face of RASA1: Report of familial germline variant in RASA1 with dysmorphic features

RASopathies encompass a diverse set of disorders affecting genes that encode proteins within the RAS-MAPK pathway. RASA1 mutations are the cause of an autosomal dominant disorder called capillary malformation-arteriovenous malformation type 1 (CM-AVM1). Unlike other RASopathies, facial dysmorphism has not been described in these patients. We phenotypically delineated a large family of individuals with multifocal fast-flow capillary malformations, severe lymphatic anomalies of perinatal onset, and dysmorphic features not previously described. Sequencing studies were performed on probands and related family members, confirming the segregation of dysmorphic features in affected members of a novel heterozygous variant in RASA1 (NM_002890.3:c.2366G>A, p.(Arg789Gln)). In this work, we broaden the phenotypic spectrum of CM-AVM type 1 and propose a new RASA1 variant as likely pathogenic.

What the Interventional Radiologist Needs to Know about the Genetics of Vascular Anomalies

The purpose of this article is to familiarize the reader with the basic genetics and vascular biology behind the array of vascular anomalies they may encounter in their practice. Individuals with vascular malformations are often referred to multidisciplinary clinics composed of diverse specialists all with the same goal: how to provide the best care possible. The team is typically composed of physicians, nurses, social workers, and technical staff from multiple specialties including diagnostic and interventional radiology, dermatology, hematology/oncology, otolaryngology, plastic surgery, and several additional subspecialties. Imaging plays a crucial role in diagnosis and treatment planning, but increasingly biopsies are needed for more accurate histopathological and genetic information to inform the plan of treatment, as well as for counseling patients and their families on the natural history, heritability, and long-term prognosis of the condition. Understanding the molecular mechanism that gives rise to vascular anomalies is crucial for arriving at the proper diagnosis and choosing among treatment options. As oncological medications are being increasingly repurposed to treat vascular malformations, it is vital for those caring for patients with vascular anomalies to understand how these anomalies develop, and which drug may be appropriate to repurpose for this benign disease.

Pathogenic variants in HGF give rise to childhood-to-late onset primary lymphoedema by loss of function

Developmental and functional defects in the lymphatic system are responsible for primary lymphoedema (PL). PL is a chronic debilitating disease caused by increased accumulation of interstitial fluid, predisposing to inflammation, infections and fibrosis. There is no cure, only symptomatic treatment is available. Thirty-two genes or loci have been linked to PL, and another 22 are suggested, including Hepatocyte Growth Factor (HGF). We searched for HGF variants in 770 index patients from the Brussels PL cohort. We identified ten variants predicted to cause HGF loss-of-function (six nonsense, two frameshifts, and two splice-site changes; 1.3% of our cohort), and 14 missense variants predicted to be pathogenic in 17 families (2.21%). We studied co-segregation within families, mRNA stability for non-sense variants, and in vitro functional effects of the missense variants. Analyses of the mRNA of patient cells revealed degradation of the nonsense mutant allele. Reduced protein secretion was detected for nine of the 14 missense variants expressed in COS-7 cells. Stimulation of lymphatic endothelial cells with these 14 HGF variant proteins resulted in decreased activation of the downstream targets AKT and ERK1/2 for three of them. Clinically, HGF-associated PL was diverse, but predominantly bilateral in the lower limbs with onset varying from early childhood to adulthood. Finally, aggregation study in a second independent cohort underscored that rare likely pathogenic variants in HGF explain about 2% of PL. Therefore, HGF signalling seems crucial for lymphatic development and/or maintenance in human beings and HGF should be included in diagnostic genetic screens for PL.

Loss-of-function mutations of the TIE1 receptor tyrosine kinase cause late-onset primary lymphedema

Primary lymphedema (PL), characterized by tissue swelling, fat accumulation, and fibrosis, results from defects in lymphatic vessels or valves caused by mutations in genes involved in development, maturation, and function of the lymphatic vascular system. Pathogenic variants in various genes have been identified in about 30% of PL cases. By screening of a cohort of 755 individuals with PL, we identified two TIE1 (tyrosine kinase with immunoglobulin- and epidermal growth factor-like domains 1) missense variants and one truncating variant, all predicted to be pathogenic by bioinformatic algorithms. The TIE1 receptor, in complex with TIE2, binds angiopoietins to regulate the formation and remodeling of blood and lymphatic vessels. The premature stop codon mutant encoded an inactive truncated extracellular TIE1 fragment with decreased mRNA stability, and the amino acid substitutions led to decreased TIE1 signaling activity. By reproducing the two missense variants in mouse Tie1 via CRISPR/Cas9, we showed that both cause edema and are lethal in homozygous mice. Thus, our results indicate that TIE1 loss-of-function variants can cause lymphatic dysfunction in patients. Together with our earlier demonstration that ANGPT2 loss-of-function mutations can also cause PL, our results emphasize the important role of the ANGPT2/TIE1 pathway in lymphatic function.

Central conducting lymphatic anomaly: from bench to bedside

Central conducting lymphatic anomaly (CCLA) is a complex lymphatic anomaly characterized by abnormalities of the central lymphatics and may present with nonimmune fetal hydrops, chylothorax, chylous ascites, or lymphedema. CCLA has historically been difficult to diagnose and treat; however, recent advances in imaging, such as dynamic contrast magnetic resonance lymphangiography, and in genomics, such as deep sequencing and utilization of cell-free DNA, have improved diagnosis and refined both genotype and phenotype. Furthermore, in vitro and in vivo models have confirmed genetic causes of CCLA, defined the underlying pathogenesis, and facilitated personalized medicine to improve outcomes. Basic, translational, and clinical science are essential for a bedside-to-bench and back approach for CCLA.

Defining the Functional Influence of Endothelial Cell-Expressed Oncogenic Activating Mutations on Vascular Morphogenesis and Capillary Assembly

This study sought to define key molecules and signals controlling major steps in vascular morphogenesis, and how these signals regulate pericyte recruitment and pericyte-induced basement membrane deposition. The morphogenic impact of endothelial cell (EC) expression of activating mutants of Kirsten rat sarcoma virus (kRas), mitogen-activated protein kinase 1 (Mek1), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), Akt serine/threonine kinase 1 (Akt1), Ras homolog enriched in brain (Rheb) Janus kinase 2 (Jak2), or signal transducer and activator of transcription 3 (Stat3) expression versus controls was evaluated, along with EC signaling events, pharmacologic inhibitor assays, and siRNA suppression experiments. Primary stimulators of EC lumen formation included kRas, Akt1, and Mek1, whereas PIK3CA and Akt1 stimulated a specialized type of cystic lumen formation. In contrast, the key drivers of EC sprouting behavior were Jak2, Stat3, Mek1, PIK3CA, and mammalian target of rapamycin (mTor). These conclusions are further supported by pharmacologic inhibitor and siRNA suppression experiments. EC expression of active Akt1, kRas, and PIK3CA led to markedly dysregulated lumen formation coupled to strongly inhibited pericyte recruitment and basement membrane deposition. For example, activated Akt1 expression in ECs excessively stimulated lumen formation, decreased EC sprouting behavior, and showed minimal pericyte recruitment with reduced mRNA expression of platelet-derived growth factor-BB, platelet-derived growth factor-DD, and endothelin-1, critical EC-derived factors known to stimulate pericyte invasion. The study identified key signals controlling fundamental steps in capillary morphogenesis and maturation and provided mechanistic details on why EC activating mutations induced a capillary deficiency state with abnormal lumens, impaired pericyte recruitment, and basement deposition: predisposing stimuli for the development of vascular malformations.

Updates in Genetic Testing for Head and Neck Vascular Anomalies

Vascular anomalies include benign or malignant tumors or benign malformations of the arteries, veins, capillaries, or lymphatic vasculature. The genetic etiology of the lesion is essential to define the lesion and can help navigate choice of therapy. . In the United States, about 1.2% of the population has a vascular anomaly, which may be underestimating the true prevalence as genetic testing for these conditions continues to evolve.

Mutation of key signaling regulators of cerebrovascular development in vein of Galen malformations

To elucidate the pathogenesis of vein of Galen malformations (VOGMs), the most common and most severe of congenital brain arteriovenous malformations, we performed an integrated analysis of 310 VOGM proband-family exomes and 336,326 human cerebrovasculature single-cell transcriptomes. We found the Ras suppressor p120 RasGAP (RASA1) harbored a genome-wide significant burden of loss-of-function de novo variants (2042.5-fold, p = 4.79 x 10-7). Rare, damaging transmitted variants were enriched in Ephrin receptor-B4 (EPHB4) (17.5-fold, p = 1.22 x 10-5), which cooperates with p120 RasGAP to regulate vascular development. Additional probands had damaging variants in ACVRL1, NOTCH1, ITGB1, and PTPN11. ACVRL1 variants were also identified in a multi-generational VOGM pedigree. Integrative genomic analysis defined developing endothelial cells as a likely spatio-temporal locus of VOGM pathophysiology. Mice expressing a VOGM-specific EPHB4 kinase-domain missense variant (Phe867Leu) exhibited disrupted developmental angiogenesis and impaired hierarchical development of arterial-capillary-venous networks, but only in the presence of a "second-hit" allele. These results illuminate human arterio-venous development and VOGM pathobiology and have implications for patients and their families.

rasa1-related arteriovenous malformation is driven by aberrant venous signalling

Arteriovenous malformations (AVMs) develop where abnormal endothelial signalling allows direct connections between arteries and veins. Mutations in RASA1, a Ras GTPase activating protein, lead to AVMs in humans and, as we show, in zebrafish rasa1 mutants. rasa1 mutants develop cavernous AVMs that subsume part of the dorsal aorta and multiple veins in the caudal venous plexus (CVP) - a venous vascular bed. The AVMs progressively enlarge and fill with slow-flowing blood. We show that the AVM results in both higher minimum and maximum flow velocities, resulting in increased pulsatility in the aorta and decreased pulsatility in the vein. These hemodynamic changes correlate with reduced expression of the flow-responsive transcription factor klf2a. Remodelling of the CVP is impaired with an excess of intraluminal pillars, which is a sign of incomplete intussusceptive angiogenesis. Mechanistically, we show that the AVM arises from ectopic activation of MEK/ERK in the vein of rasa1 mutants, and that cell size is also increased in the vein. Blocking MEK/ERK signalling prevents AVM initiation in mutants. Alterations in venous MEK/ERK therefore drive the initiation of rasa1 AVMs.

Pathophysiology of Slow-Flow Vascular Malformations: Current Understanding and Unanswered Questions

Background

Slow-flow vascular malformations include venous, lymphatic, and lymphaticovenous malformations. Recent studies have linked genetic variants hyperactivating either the PI3K/AKT/mTOR and/or RAS/RAF/MAPK signaling pathways with slow-flow vascular malformation development, leading to the use of pharmacotherapies such as sirolimus and alpelisib. It is important that clinicians understand basic and translational research advances in slow-flow vascular malformations.

Methods

A literature review of basic science publications in slow-flow vascular malformations was performed on Pubmed, using search terms "venous malformation," "lymphatic malformation," "lymphaticovenous malformation," "genetic variant," "genetic mutation," "endothelial cells," and "animal model." Relevant publications were reviewed and summarized.

Results

The study of patient tissues and the use of primary pathogenic endothelial cells from vascular malformations shed light on their pathological behaviors, such as endothelial cell hyperproliferation and disruptions in vessel architecture. The use of xenograft and transgenic animal models confirmed the pathogenicity of genetic variants and allowed for preclinical testing of potential therapies. These discoveries underscore the importance of basic and translational research in understanding the pathophysiology of vascular malformations, which will allow for the development of improved biologically targeted treatments.

Conclusion

Despite basic and translation advances, a cure for slow-flow vascular malformations remains elusive. Many questions remain unanswered, including how genotype variants result in phenotypes, and genotype-phenotype heterogeneity. Continued research into venous and lymphatic malformation pathobiology is critical in understanding the mechanisms by which genetic variants contribute to vascular malformation phenotypic features.

Large saccular intracranial aneurysm in a child with RASA1-associated capillary malformation-arteriovenous malformation syndrome: illustrative case

BACKGROUND

Large cerebral aneurysms are much less common in children than in adults. Thus, when present, these lesions require careful surgical evaluation and comprehensive genetic testing. RASA1-associated capillary malformation-arteriovenous malformation (RASA1-CM-AVM) syndrome is a rare disorder of angiogenic remodeling known to cause port-wine stains and arteriovenous fistulas but not previously associated with pediatric aneurysms.

OBSERVATIONS

The authors report the case of a previously healthy 6-year-old boy who presented with seizure-like activity. Imaging demonstrated a lesion in the right ambient cistern with compression of the temporal lobe. Imaging characteristics were suggestive of a thrombosed aneurysm versus an epidermoid cyst. The patient underwent craniotomy, revealing a large saccular aneurysm, and clip ligation and excision were performed. Postoperative genetic analysis revealed a RASA1-CM-AVM syndrome.

LESSONS

This is a rare case of a RASA1-associated pediatric cerebral aneurysm in the neurosurgical literature. This unique case highlights the need for maintaining a broad differential diagnosis as well as the utility of genetic testing for detecting underlying genetic syndromes in young children presenting with cerebral aneurysms.

Importance of Arachnoid Dissection in Arteriovenous Malformation Microsurgery: A Technical Note

Intracranial arteriovenous malformations (AVMs) are congenital anomalies where arteries and veins connect without a capillary bed. AVMs are the leading cause of nontraumatic intracerebral hemorrhages in people younger than 35 years old.1 The leptomeninges (arachnoid and pia) form from the meninx primitiva.2,3 Endothelial channels produce a vascular plexus in the meninx connected by primitive arachnoid. Remodeling of the plexus in response to changing metabolic demands results in a recognizable pattern of arteries and veins.2,3 Defects at the level of capillaries during arteriovenous specification are most likely responsible for arteriovenous fistula formation.4-6 Interplay between the congenital dysfunction and flow-related maturation in adulthood, when vasculogenesis has stopped, produces the AVM.6,7 The relationship between the primitive arachnoid and aberrant AVM vessels is preserved and forms the basis of microsurgical disconnection discussed in Video 1. Several authors have described dissecting these natural planes to delineate the abnormal AVM vessels, relax the brain, and avoid morbidity during AVM surgery.8-10 We recommend sharp arachnoid dissection with a scalpel or microscissors, occasionally helped by blunt dissection with patties or bipolar forceps. We present a 2-dimensional video of the microsurgical resection of a right parietal AVM. The patient, a healthy 30-year-old female, presented with intermittent headaches and mild impairment of arithmetic and visuospatial ability. Magnetic resonance imaging and digital subtraction angiography showed a compact 3.5-cm supramarginal gyrus AVM supplied by the middle cerebral artery, with superficial drainage. Complete microsurgical resection was performed without morbidity. We demonstrate the principles of arachnoid dissection requisite to disentanglement of the nidus and safe resection of the AVM.

Co-existence of 2 clinically significant variants causing disorders of somatic mosaicism

Purpose

Disorders of somatic mosaicism (DoSM) are a heterogeneous group of conditions caused by postzygotic variants in genes within the PI3K/AKT/mTOR and RAS/MAPK signaling pathway. The co-existence of 2 activating variants in this disease group is extremely rare.

Methods

A deep sequencing next-generation sequencing assay for the molecular diagnosis of DoSM was run on 936 individuals with DoSM.

Results

A single pathogenic or likely pathogenic (P/LP) variant was identified in 584 of 617 (94.8%) positive cases; 33 of 617 (5.2%) cases carried 2 P/LP variants. Of these 33 cases, 22 carried 2 P/LP variants in the same gene, including 8 associated with a loss-of-function disease mechanism and 14 associated with a gain-of-function disease mechanism. Eleven cases had P/LP variants in 2 different genes, including PIKC3A variants in 7 cases and 4 cases with 2 P/LP variants in non-PIK3CA genes.

Conclusion

To our knowledge, this is the largest cohort with the co-existence of 2 P/LP somatic variants causing DoSM. The study of the co-existence of 2 clinically significant variants in DoSM requires unique considerations regarding variant allelic fractions, the combination of variants, affected tissue types, and the severity of the disease. Investigations into this unique cohort may further our understanding of the disease mechanism and potential therapeutic options.

Genetic dysregulation of an endothelial Ras signaling network in vein of Galen malformations

To elucidate the pathogenesis of vein of Galen malformations (VOGMs), the most common and severe congenital brain arteriovenous malformation, we performed an integrated analysis of 310 VOGM proband-family exomes and 336,326 human cerebrovasculature single-cell transcriptomes. We found the Ras suppressor p120 RasGAP ( RASA1 ) harbored a genome-wide significant burden of loss-of-function de novo variants (p=4.79×10 ^-7 ). Rare, damaging transmitted variants were enriched in Ephrin receptor-B4 ( EPHB4 ) (p=1.22×10 ^-5 ), which cooperates with p120 RasGAP to limit Ras activation. Other probands had pathogenic variants in ACVRL1 , NOTCH1 , ITGB1 , and PTPN11 . ACVRL1 variants were also identified in a multi-generational VOGM pedigree. Integrative genomics defined developing endothelial cells as a key spatio-temporal locus of VOGM pathophysiology. Mice expressing a VOGM-specific EPHB4 kinase-domain missense variant exhibited constitutive endothelial Ras/ERK/MAPK activation and impaired hierarchical development of angiogenesis-regulated arterial-capillary-venous networks, but only when carrying a "second-hit" allele. These results illuminate human arterio-venous development and VOGM pathobiology and have clinical implications.

Genetics of brain arteriovenous malformations and cerebral cavernous malformations

Cerebrovascular malformations comprise abnormal development of cerebral vasculature. They can result in hemorrhagic stroke due to rupture of lesions as well as seizures and neurological defects. The most common forms of cerebrovascular malformations are brain arteriovenous malformations (bAVMs) and cerebral cavernous malformations (CCMs). They occur in both sporadic and inherited forms. Rapidly evolving molecular genetic methodologies have helped to identify causative or associated genes involved in genesis of bAVMs and CCMs. In this review, we highlight the current knowledge regarding the genetic basis of these malformations.

EPHB4-RASA1-Mediated Negative Regulation of Ras-MAPK Signaling in the Vasculature: Implications for the Treatment of EPHB4- and RASA1-Related Vascular Anomalies in Humans

Ephrin receptors constitute a large family of receptor tyrosine kinases in mammals that through interaction with cell surface-anchored ephrin ligands regulate multiple different cellular responses in numerous cell types and tissues. In the cardiovascular system, studies performed in vitro and in vivo have pointed to a critical role for Ephrin receptor B4 (EPHB4) as a regulator of blood and lymphatic vascular development and function. However, in this role, EPHB4 appears to act not as a classical growth factor receptor but instead functions to dampen the activation of the Ras-mitogen activated protein signaling (MAPK) pathway induced by other growth factor receptors in endothelial cells (EC). To inhibit the Ras-MAPK pathway, EPHB4 interacts functionally with Ras p21 protein activator 1 (RASA1) also known as p120 Ras GTPase-activating protein. Here, we review the evidence for an inhibitory role for an EPHB4-RASA1 interface in EC. We further discuss the mechanisms by which loss of EPHB4-RASA1 signaling in EC leads to blood and lymphatic vascular abnormalities in mice and the implications of these findings for an understanding of the pathogenesis of vascular anomalies in humans caused by mutations in EPHB4 and RASA1 genes. Last, we provide insights into possible means of drug therapy for EPHB4- and RASA1-related vascular anomalies.

Somatic GJA4 gain-of-function mutation in orbital cavernous venous malformations

Orbital cavernous venous malformation (OCVM) is a sporadic vascular anomaly of uncertain etiology characterized by abnormally dilated vascular channels. Here, we identify a somatic missense mutation, c.121G > T (p.Gly41Cys) in GJA4, which encodes a transmembrane protein that is a component of gap junctions and hemichannels in the vascular system, in OCVM tissues from 25/26 (96.2%) individuals with OCVM. GJA4 expression was detected in OCVM tissue including endothelial cells and the stroma, through immunohistochemistry. Within OCVM tissue, the mutation allele frequency was higher in endothelial cell-enriched fractions obtained using magnetic-activated cell sorting. Whole-cell voltage clamp analysis in Xenopus oocytes revealed that GJA4 c.121G > T (p.Gly41Cys) is a gain-of-function mutation that leads to the formation of a hyperactive hemichannel. Overexpression of the mutant protein in human umbilical vein endothelial cells led to a loss of cellular integrity, which was rescued by carbenoxolone, a non-specific gap junction/hemichannel inhibitor. Our data suggest that GJA4 c.121G > T (p.Gly41Cys) is a potential driver gene mutation for OCVM. We propose that hyperactive hemichannel plays a role in the development of this vascular phenotype.

The Genetic Architecture of Vascular Anomalies: Current Data and Future Therapeutic Perspectives Correlated with Molecular Mechanisms

Vascular anomalies (VAs) are morphogenesis defects of the vascular system (arteries, capillaries, veins, lymphatic vessels) singularly or in complex combinations, sometimes with a severe impact on the quality of life. The progress made in recent years with the identification of the key molecular pathways (PI3K/AKT/mTOR and RAS/BRAF/MAPK/ERK) and the gene mutations that lead to the appearance of VAs has allowed the deciphering of their complex genetic architecture. Understanding these mechanisms is critical both for the correct definition of the phenotype and classification of VAs, as well as for the initiation of an optimal therapy and the development of new targeted therapies. The purpose of this review is to present in synthesis the current data related to the genetic factors involved in the etiology of VAs, as well as the possible directions for future research. We analyzed the data from the literature related to VAs, using databases (Google Scholar, PubMed, MEDLINE, OMIM, MedGen, Orphanet) and ClinicalTrials.gov. The obtained results revealed that the phenotypic variability of VAs is correlated with genetic heterogeneity. The identification of new genetic factors and the molecular mechanisms in which they intervene, will allow the development of modern therapies that act targeted as a personalized therapy. We emphasize the importance of the geneticist in the diagnosis and treatment of VAs, as part of a multidisciplinary team involved in the management of VAs.

A human model of arteriovenous malformation (AVM)-on-a-chip reproduces key disease hallmarks and enables drug testing in perfused human vessel networks

Brain arteriovenous malformations (AVMs) are a disorder wherein abnormal, enlarged blood vessels connect arteries directly to veins, without an intervening capillary bed. AVMs are one of the leading causes of hemorrhagic stroke in children and young adults. Most human sporadic brain AVMs are associated with genetic activating mutations in the KRAS gene. Our goal was to develop an in vitro model that would allow for simultaneous morphological and functional phenotypic data capture in real time during AVM disease progression. By generating human endothelial cells harboring a clinically relevant mutation found in most human patients (activating mutations within the small GTPase KRAS) and seeding them in a dynamic microfluidic cell culture system that enables vessel formation and perfusion, we demonstrate that vessels formed by KRAS4A^G12V mutant endothelial cells (ECs) were significantly wider and more leaky than vascular beds formed by wild-type ECs, recapitulating key structural and functional hallmarks of human AVM pathogenesis. Immunofluorescence staining revealed a breakdown of adherens junctions in mutant KRAS vessels, leading to increased vascular permeability, a hallmark of hemorrhagic stroke. Finally, pharmacological blockade of MEK kinase activity, but not PI3K inhibition, improved endothelial barrier function (decreased permeability) without affecting vessel diameter. Collectively, our studies describe the creation of human KRAS-dependent AVM-like vessels in vitro in a self-assembling microvessel platform that is amenable to phenotypic observation and drug delivery.

Molecular Mechanisms Driving Lymphedema and Other Lymphatic Anomalies

Lymphatic vasculature regulates fluid homeostasis by absorbing interstitial fluid and returning it to blood. Lymphatic vasculature is also critical for lipid absorption and inflammatory response. Lymphatic vasculature is composed of lymphatic capillaries, collecting lymphatic vessels, lymphatic valves, and lymphovenous valves. Defects in any of these structures could lead to lymphatic anomalies such as lymphedema, cystic lymphatic malformation, and Gorham-Stout disease. Basic research has led to a deeper understanding of the stepwise development of the lymphatic vasculature. VEGF-C and shear stress signaling pathways have evolved as critical regulators of lymphatic vascular development. Loss-of-function and gain-of-function mutations in genes that are involved in these signaling pathways are associated with lymphatic anomalies. Importantly, drugs that target these molecules are showing outstanding efficacy in treating certain lymphatic anomalies. In this article, we summarize these exciting developments and highlight the future challenges.

Capillary malformation-arteriovenous malformation syndrome associated with basilar artery aneurysm

A 23-day-old boy with prenatal diagnosis of basilar artery aneurysm presented with multiple congenital red patches consistent with capillary malformations. Genetic testing confirmed the presence of a heterozygous pathogenic variant of the RASA1 gene, confirming the diagnosis of capillary malformation-arteriovenous malformation (CM-AVM) syndrome. This case illustrates an atypical presentation of the RASA1 associated CM-AVM syndrome, with the intracranial vascular malformation diagnosis preceding the identification of the skin lesions. Arterial aneurysms have been associated with CM-AVM syndrome in rare instances but to our knowledge this is the first reported case of an aneurysm of the basilar artery.

Capillary Malformation-arteriovenous Malformation Type 2: A Case Report and Review

Capillary malformation-arteriovenous malformation syndrome is a rare genodermatosis with cutaneous capillary malformations and a risk of associated fast-flow malformations. We describe here a four-generation family with a novel heterozygous pathogenic variant in the EPHB4 gene (NM_004444.5 (EPHB4): c.2224G>C, p.(Ala742Pro)). A review of the literature retrieved 127 patients with capillary malformation-arteriovenous malformation syndrome and confirmed variants in EPHB4. Multiple capillary malformations were present in 114 (89.76%) patients, and 12 (9.44%) patients had a solitary capillary malformation. Arteriovenous malformations/fistulas were present in 23 (18.1%) patients, and were located within the central nervous system in 5 (3.9%) patients. Not all papers included description of epistaxis. Telangiectasias were reported in 28 (22%) patients, and Bier spots were described in 20 (15.7%) patients. The clinical characteristics of capillary malformation-arteriovenous malformation syndrome are diverse and often discrete, which can make it difficult to distinguish capillary malformationarteriovenous malformation syndrome from hereditary haemorrhagic telangiectasia.

Exome-wide Analysis of De Novo and Rare Genetic Variants in Patients With Brain Arteriovenous Malformation

BACKGROUND AND OBJECTIVES

Brain arteriovenous malformation (bAVM) is a congenital disorder and a leading cause of hemorrhagic stroke. Germline genetic variants play an essential role in the pathogenesis of brain arteriovenous malformation. However, the biological relevance of the disease-associated genes identified in previous studies is elusive. In this study, we aim to systematically investigate the contribution of germline variants to bAVM and explore the critical molecular pathways underlying the pathogenesis of bAVM.

METHODS

Probands with sporadic bAVM were consecutively recruited into this study from November 2015 to November 2018 and underwent exome sequencing. The controls were aggregated from individuals who were not known to have vascular malformation and underwent exome sequencing for clinical or research purposes. The retained control dataset included 4609 individuals, including 251 individuals with parental samples sequenced. We firstly compared de novo variants in cases and controls and performed a pathway enrichment analysis. A gene-based rare variant association analysis was then performed to identify genes whose variants were significantly enriched in cases.

RESULTS

We collected an exome-sequenced bAVM cohort consisting of 152 trios and 40 singletons. By firstly focusing on de novo variants, we observed a significant mutational burden of de novo likely gene-disrupting variants in cases versus controls. By performing a pathway enrichment analysis of all nonsynonymous de novo variants identified in cases, we found the angiopoietin-like protein 8 (ANGPTL8) regulatory pathway to be significantly enriched in patients with bAVM. Through an exome-wide rare variant association analysis utilizing 4394 in-house exome data as controls, we identified SLC19A3 as a disease-associated gene for bAVM. In addition, we found that the SLC19A3 variants in cases are preferably located at the N' side of the SLC19A3 protein. These findings implicate a phenotypic extension of SLC19A3-related disorders with a domain-specific effect.

DISCUSSION

This study provides insights into the biological basis of bAVM by identifying novel molecular pathways and candidate genes.

Cerebral neurovascular embryology, anatomic variations, and congenital brain arteriovenous lesions

Cerebral neurovascular development is a complex and coordinated process driven by the changing spatial and temporal metabolic demands of the developing brain. Familiarity with the process is helpful in understanding neurovascular anatomic variants and congenital arteriovenous shunting lesions encountered in endovascular neuroradiological practice. Herein, the processes of vasculogenesis and angiogenesis are reviewed, followed by examination of the morphogenesis of the cerebral arterial and venous systems. Common arterial anatomic variants are reviewed with an emphasis on their development. Finally, endothelial genetic mutations affecting angiogenesis are examined to consider their probable role in the development of three types of congenital brain arteriovenous fistulas: vein of Galen malformations, pial arteriovenous fistulas, and dural sinus malformations.

Endothelial k-RasV12 Expression Induces Capillary Deficiency Attributable to Marked Tube Network Expansion Coupled to Reduced Pericytes and Basement Membranes

OBJECTIVE

We sought to determine how endothelial cell (EC) expression of the activating k-Ras (kirsten rat sarcoma 2 viral oncogene homolog) mutation, k-RasV12, affects their ability to form lumens and tubes and interact with pericytes during capillary assembly Approach and Results: Using defined bioassays where human ECs undergo observable tubulogenesis, sprouting behavior, pericyte recruitment to EC-lined tubes, and pericyte-induced EC basement membrane deposition, we assessed the impact of EC k-RasV12 expression on these critical processes that are necessary for proper capillary network formation. This mutation, which is frequently seen in human ECs within brain arteriovenous malformations, was found to markedly accentuate EC lumen formation mechanisms, with strongly accelerated intracellular vacuole formation, vacuole fusion, and lumen expansion and with reduced sprouting behavior, leading to excessively widened tube networks compared with control ECs. These abnormal tubes demonstrate strong reductions in pericyte recruitment and pericyte-induced EC basement membranes compared with controls, with deficiencies in fibronectin, collagen type IV, and perlecan deposition. Analyses of signaling during tube formation from these k-RasV12 ECs reveals strong enhancement of Src (Src proto-oncogene, non-receptor tyrosine kinase), Pak2 (P21 [RAC1 (Rac family small GTPase 1)] activated kinase 2), b-Raf (v-raf murine sarcoma viral oncogene homolog B1), Erk (extracellular signal-related kinase), and Akt (AK strain transforming) activation and increased expression of PKCε (protein kinase C epsilon), MT1-MMP (membrane-type 1 matrix metalloproteinase), acetylated tubulin and CDCP1 (CUB domain-containing protein 1; most are known EC lumen regulators). Pharmacological blockade of MT1-MMP, Src, Pak, Raf, Mek (mitogen-activated protein kinase) kinases, Cdc42 (cell division cycle 42)/Rac1, and Notch markedly interferes with lumen and tube formation from these ECs.

CONCLUSIONS

Overall, this novel work demonstrates that EC expression of k-RasV12 disrupts capillary assembly due to markedly excessive lumen formation coupled with strongly reduced pericyte recruitment and basement membrane deposition, which are critical pathogenic features predisposing the vasculature to develop arteriovenous malformations.

Angiogenesis depends upon EPHB4-mediated export of collagen IV from vascular endothelial cells

Capillary malformation-arteriovenous malformation (CM-AVM) is a blood vascular anomaly caused by inherited loss of function mutations in RASA1 or EPHB4 genes that encode p120 Ras GTPase-activating protein (p120 RasGAP/RASA1) and Ephrin receptor B4 (EPHB4) respectively. However, whether RASA1 and EPHB4 function in the same molecular signaling pathway to regulate the blood vasculature is uncertain. Here, we show that induced endothelial cell (EC)-specific disruption of Ephb4 in mice results in accumulation of collagen IV in the EC endoplasmic reticulum leading to EC apoptotic death and defective developmental, neonatal and pathological angiogenesis, as reported previously in induced EC-specific RASA1-deficient mice. Moreover, defects in angiogenic responses in EPHB4-deficient mice can be rescued by drugs that inhibit signaling through the Ras pathway and drugs that promote collagen IV export from the ER. However, EPHB4 mutant mice that express a form of EPHB4 that is unable to physically engage RASA1 but retains protein tyrosine kinase activity show normal angiogenic responses. These findings provide strong evidence that RASA1 and EPHB4 function in the same signaling pathway to protect against the development of CM-AVM independent of physical interaction and have important implications with regards possible means of treatment of this disease.

Cutaneous epithelioid hemangiomas show somatic mutations in the MAPK pathway

BACKGROUND

Epithelioid hemangioma (EH) arising from the skin is a benign vascular tumor with marked inflammatory cell infiltration, which exhibits a high tendency to persist and frequently recurs after resection. So far, the underlying pathogenesis is largely elusive.

OBJECTIVES

To identify genetic alterations by next-generation-sequencing and/or droplet digital PCR (ddPCR) in cutaneous EH.

METHODS

DNA and RNA from an EH lesion of an index patient were subjected to whole genome and RNA sequencing. Multiplex PCR-based panel sequencing of genomic DNA isolated from archival formalin-fixed paraffin-embedded (FFPE) tissue of 18 cutaneous EH patients was performed. ddPCR was used to confirm mutations.

RESULTS

We identified somatic mutations in genes of the MAPK pathway (MAP2K1 and KRAS) in cutaneous EH biopsies. By ddPCR we could confirm the recurrent presence of activating, low-frequency mutations affecting MAP2K1. In total, 9 out of 18 analyzed patients showed activating MAPK pathway mutations, which were mutually exclusive. Comparative analysis of tissue areas enriched for lymphatic infiltrate or aberrant endothelial cells, respectively, revealed an association of these mutations with the presence of endothelial cells.

CONCLUSIONS

Taken together, our data suggest that EH shows somatic mutations in genes of the MAPK pathway which might contribute to the formation of this benign tumor.

The RASopathies: Biology, genetics and therapeutic options

The RASopathies are a group of genetic diseases in which the Ras/MAPK signaling pathway is inappropriately activated as a result of mutations in genes encoding proteins within this pathway. As their causative mutations have been identified, this group of diseases has expanded to include neurofibromatosis type 1 (NF1), Legius syndrome, Noonan syndrome, CBL syndrome, Noonan syndrome-like disorder with loose anagen hair, Noonan syndrome with multiple lentigines, Costello syndrome, cardiofaciocutaneous syndrome, gingival fibromatosis and capillary malformation-arteriovenous malformation syndrome. Many of these genetic disorders share clinical features in common such as abnormal facies, short stature, varying degrees of cognitive impairment, cardiovascular abnormalities, skeletal abnormalities and a predisposition to develop benign and malignant neoplasms. Others are more dissimilar, even though their mutations are in the same gene that is mutated in a different RASopathy. Here, we describe the clinical features of each RASopathy and contrast them with the other RASopathies. We discuss the genetics of these disorders, including the causative mutations for each RASopathy, the impact that these mutations have on the function of an individual protein and how this dysregulates the Ras/MAPK signaling pathway. As several of these individual disorders are genetically heterogeneous, we also consider the different genes that can be mutated to produce disease with the same phenotype. We also discuss how our growing understanding of dysregulated Ras/MAPK signaling had led to the development of new therapeutic agents and what work will be critically important in the future to improve the lives of patients with RASopathies.

CM-AVM syndrome - A prospective observational study of unrelated paediatric cases

OBJECTIVE

The main objective of this study is to describe the clinical spectrum of CM-AVM syndrome as well as radiological and genetic findings.

METHODS

This is a single-centre prospective observational study performed at Sydney Children's Hospital. Patients under the age of 18 years that presented to our paediatric dermatology clinic or vascular birthmark clinic between January 2015 and September 2020 with one or more geometric shaped pink/ red/ brown macule with a peripheral pallor characteristic of a high-flow vascular stain were included. Children subsequently diagnosed with other diagnosis or family members with CM-AVM syndrome were excluded.

RESULTS

Sixty children were included, with two subsequently excluded. A third of patients (n = 22, 38%) presented with a single characteristic HFVS, whereas the remaining two thirds (n = 36; 62%) had multiple HFVS. In children with multiple HFVS, one notably larger HFVS was detected in the majority of children (n = 32, 88%). In 33 patients, a brain and spine MRI was performed, which detected a spine AVM in one symptomatic patient with sensorimotor deficits. No cerebral AVM or AVF was picked up in the cohort. A RASA 1 result was available for evaluation in 24, of which 16 (67%) were positive. An EPHB4 result was available in eight, two (25%) of which were positive.

CONCLUSIONS

One large HFVS often accompanied by multiple small HFVS can be seen in most patients. Despite of the lack of genetic confirmation of diagnosis in single lesions, this phenotype might be of interest and warrants further investigation.

Mouse models of vascular development and disease

PURPOSE OF REVIEW

The use of genetic models has facilitated the study of the origins and mechanisms of vascular disease. Mouse models have been developed to specifically target endothelial cell populations, with the goal of pinpointing when and where causative mutations wreck their devastating effects. Together, these approaches have propelled the development of therapies by providing an in-vivo platform to evaluate diagnoses and treatment options. This review summarizes the most widely used mouse models that have facilitated the study of vascular disease, with a focus on mouse models of vascular malformations and the road ahead.

RECENT FINDINGS

Over the past 3 decades, the vascular biology scientific community has been steadily generating a powerful toolkit of useful mouse lines that can be used to tightly regulate gene ablation, or to express transgenic genes, in the murine endothelium. Some of these models inducibly (constitutively) alter gene expression across all endothelial cells, or within distinct subsets, by expressing either Cre recombinase (or inducible versions such as CreERT), or the tetracycline controlled transactivator protein tTA (or rtTA). This now relatively standard technology has been used to gain cutting edge insights into vascular disorders, by allowing in-vivo modeling of key molecular pathways identified as dysregulated across the vast spectrum of vascular anomalies, malformations and dysplasias. However, as sequencing of human patient samples expands, the number of interesting candidate molecular culprits keeps increasing. Consequently, there is now a pressing need to create new genetic mouse models to test hypotheses and to query mechanisms underlying vascular disease.

SUMMARY

The current review assesses the collection of mouse driver lines that have been instrumental is identifying genes required for blood vessel formation, remodeling, maintenance/quiescence and disease. In addition, the usefulness of these driver lines is underscored here by cataloguing mouse lines developed to experimentally assess the role of key candidate genes in vascular malformations. Despite this solid and steady progress, numerous new candidate vascular malformation genes have recently been identified for which no mouse model yet exists.

Janus-faced EPHB4-associated disorders: novel pathogenic variants and unreported intrafamilial overlapping phenotypes

Purpose

Several clinical phenotypes including fetal hydrops, central conducting lymphatic anomaly or capillary malformations with arteriovenous malformations 2 (CM-AVM2) have been associated with EPHB4 (Ephrin type B receptor 4) variants, demanding new approaches for deciphering pathogenesis of novel variants of uncertain significance (VUS) identified in EPHB4, and for the identification of differentiated disease mechanisms at the molecular level.

Methods

Ten index cases with various phenotypes, either fetal hydrops, CM-AVM2, or peripheral lower limb lymphedema, whose distinct clinical phenotypes are described in detail in this study, presented with a variant in EPHB4. In vitro functional studies were performed to confirm pathogenicity.

Results

Pathogenicity was demonstrated for six of the seven novel EPHB4 VUS investigated. A heterogeneity of molecular disease mechanisms was identified, from loss of protein production or aberrant subcellular localization to total reduction of the phosphorylation capability of the receptor. There was some phenotype–genotype correlation; however, previously unreported intrafamilial overlapping phenotypes such as lymphatic-related fetal hydrops (LRFH) and CM-AVM2 in the same family were observed.

Conclusion

This study highlights the usefulness of protein expression and subcellular localization studies to predict EPHB4 variant pathogenesis. Our accurate clinical phenotyping expands our interpretation of the Janus-faced spectrum of EPHB4-related disorders, introducing the discovery of cases with overlapping phenotypes.

Somatic Mosaicism in the Pathogenesis of de novo Cerebral Arteriovenous Malformations: A Paradigm Shift Implicating the RAS-MAPK Signaling Cascade

Cerebral arteriovenous malformations (AVMs) are leading causes of lesional hemorrhagic stroke in both the pediatric and young adult population, with sporadic AVMs accounting for the majority of cases. Recent evidence has identified somatic mosaicism in key proximal components of the RAS-MAPK signaling cascade within endothelial cells collected from human sporadic cerebral AVMs, with early preclinical models supporting a potential causal role for these mutations in the pathogenesis of these malformations. Germline mutations that predispose to deregulation of the RAS-MAPK signaling axis have also been identified in hereditary vascular malformation syndromes, highlighting the key role of this signaling axis in global AVM development. Herein, we review the most recent genomic and preclinical evidence implicating somatic mosaicism in the RAS-MAPK signaling pathway in the pathogenesis of sporadic cerebral AVMs. Also, we review evidence for RAS-MAPK dysregulation in hereditary vascular malformation syndromes and present a hypothesis suggesting that this pathway is central for the development of both sporadic and syndrome-associated AVMs. Finally, we examine the clinical implications of these recent discoveries and highlight potential therapeutic targets within this signaling pathway.

Brain vascular biology

The complex development of the brain vascular system can be broken down by embryonic stages and anatomic locations, which are tightly regulated by different factors and pathways in time and spatially. The adult brain is relatively quiescent in angiogenesis. However, under disease conditions, such as trauma, stroke, or tumor, angiogenesis can be activated in the adult brain. Disruption of any of the factors or pathways may lead to malformed vessel development. In this chapter, we will discuss factors and pathways involved in normal brain vasculogenesis and vascular maturation, and the pathogenesis of several brain vascular malformations.

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.

RASA1-driven cellular export of collagen IV is required for the development of lymphovenous and venous valves in mice

RASA1, a negative regulator of Ras-MAPK signaling, is essential for the development and maintenance of lymphatic vessel valves. However, whether RASA1 is required for the development and maintenance of lymphovenous valves (LVV) and venous valves (VV) is unknown. In this study, we show that induced disruption of Rasa1 in mouse embryos did not affect initial specification of LVV or central VV, but did affect their continued development. Similarly, a switch to expression of a catalytically inactive form of RASA1 resulted in impaired LVV and VV development. Blocked development of LVV was associated with accumulation of the basement membrane protein, collagen IV, in LVV-forming endothelial cells (EC), and could be partially or completely rescued by MAPK inhibitors and drugs that promote collagen IV folding. Disruption of Rasa1 in adult mice resulted in venous hypertension and impaired VV function that was associated with loss of EC from VV leaflets. In conclusion, RASA1 functions as a negative regulator of Ras signaling in EC that is necessary for EC export of collagen IV, thus permitting the development of LVV and the development and maintenance of VV.

Capillary malformation-arteriovenous malformation syndrome: a multicentre study

BACKGROUND

Capillary malformation-arteriovenous malformation (CM-AVM) syndrome is a rare syndrome with characteristic skin lesions that are associated with fast-flow vascular malformations (FFVMs) in one-third of patients. Few case series have been described, and none in Spain.

AIM

To identify the prevalence of dermatological parameters, FFVMs and associated features in a large series of patients with CM-AVM.

METHODS

We conducted an observational study of patients with CM-AVM syndrome diagnosed in 15 Spanish hospitals over 3 years. The main clinical, radiological, genetic findings and associated diseases were analysed.

RESULTS

In total, 64 patients were assessed. In 26.5% of cases, the diagnosis was incidental. In 75% of patients, there was one significantly larger macule, which we termed the 'herald patch'. FFVMs were detected in 34% of the patients, with 30% located on the skin, 7.8% in the brain and in 1.5% in the spine. There was a positive family history in 65% of the 64 patients. Genetic analysis was performed for RASA1 mutations in 57 patients, of whom 42 (73%) had a positive result. All 4 patients tested for EPHB4 mutations had a positive result. No tumour lesions were detected in the series, except for five infantile haemangiomas.

CONCLUSIONS

Our data on clinical lesions, associated FFVM, family history and genetics are similar to those previously published in the literature. An extensive data analysis failed to demonstrate any statistically significant association between the presence of an FFVM and any clinical, familial or genetic parameter that could predict its onset, although a link between the presence of a herald patch on the midline face and the presence of a brain FFVM was observed. We did not detect any genotype-phenotype correlation.

Parkes Weber syndrome associated with two somatic pathogenic variants in RASA1

Parkes Weber syndrome is associated with autosomal dominant inheritance, caused by germline heterozygous inactivating changes in the RASA1 gene, characterized by multiple micro arteriovenous fistulas and segmental overgrowth of soft tissue and skeletal components. The focal nature and variable expressivity associated with this disease has led to the hypothesis that somatic “second hit” inactivating changes in RASA1 are necessary for disease development. We report a 2-yr-old male with extensive capillary malformation and segmental overgrowth of his lower left extremity. Ultrasound showed subcutaneous phlebectasia draining the capillary malformation; magnetic resonance imaging showed overgrowth of the extremity with prominence of fatty tissues, fatty infiltration, and enlargement of all the major muscle groups. Germline RASA1 testing was normal. Later somatic testing from affected tissue showed two pathogenic variants in RASA1 consistent with the c.934_938del, p.(Glu312Argfs*14) and the c.2925del, p.(Asn976Metfs*20) with variant allele fractions of 3.6% and 4.2%, respectively. The intrafamilial variability of Parkes Weber syndrome involving segmental overgrowth of soft tissue, endothelium, and bone is strongly suggestive of a somatic second-hit model. There are at least two reports of confirmed second somatic hits in RASA1. To our knowledge, this is the first report of an individual with two somatic pathogenic variants in the RASA1 gene in DNA from a vascular lesion.

Somatic Gain of KRAS Function in the Endothelium Is Sufficient to Cause Vascular Malformations That Require MEK but Not PI3K Signaling

Supplemental Digital Content is available in the text.

Rationale:

We previously identified somatic activating mutations in the KRAS (Kirsten rat sarcoma viral oncogene homologue) gene in the endothelium of the majority of human sporadic brain arteriovenous malformations; a disorder characterized by direct connections between arteries and veins. However, whether this genetic abnormality alone is sufficient for lesion formation, as well as how active KRAS signaling contributes to arteriovenous malformations, remains unknown.

Objective:

To establish the first in vivo models of somatic KRAS gain of function in the endothelium in both mice and zebrafish to directly observe the phenotypic consequences of constitutive KRAS activity at a cellular level in vivo, and to test potential therapeutic interventions for arteriovenous malformations.

Methods and Results:

Using both postnatal and adult mice, as well as embryonic zebrafish, we demonstrate that endothelial-specific gain of function mutations in Kras (G12D or G12V) are sufficient to induce brain arteriovenous malformations. Active KRAS signaling leads to altered endothelial cell morphogenesis and increased cell size, ectopic sprouting, expanded vessel lumen diameter, and direct connections between arteries and veins. Furthermore, we show that these lesions are not associated with altered endothelial growth dynamics or a lack of proper arteriovenous identity but instead seem to feature exuberant angiogenic signaling. Finally, we demonstrate that KRAS-dependent arteriovenous malformations in zebrafish are refractory to inhibition of the downstream effector PI3K but instead require active MEK (mitogen-activated protein kinase kinase 1) signaling.

Conclusions:

We demonstrate that active KRAS expression in the endothelium is sufficient for brain arteriovenous malformations, even in the setting of uninjured adult vasculature. Furthermore, the finding that KRAS-dependent lesions are reversible in zebrafish suggests that MEK inhibition may represent a promising therapeutic treatment for arteriovenous malformation patients.

Histopathological hallmarks of cutaneous lesions of capillary malformation-arteriovenous malformation syndrome

IMPORTANCE

Capillary malformation-arteriovenous malformation (CM-AVM) syndrome is a recently described syndrome with distinctive cutaneous lesions. Very little is known about the histopathology of these lesions.

OBJECTIVE

The purpose of the study was to evaluate the histopathological characteristics of the pink macules of the CM-AVM syndrome and to investigate if these pink macules could be classified as capillary malformations or arteriovenous malformations based on their histopathological features.

DESIGN-SETTINGS-PARTICIPANTS

We conducted a retrospective multicenter study involving eight hospitals in Spain. Fifteen biopsies from pink macules of the CM-AVM syndrome were analysed and compared with five biopsies of diverse capillary malformations and three stage I arteriovenous malformations.

RESULTS

Pink macules' biopsies of the CM-AVM syndrome showed similar features including a high vascular density encompassing capillaries and numerous thick-walled arterioles mainly located in the superficial dermis, a predominance of elongated over round vessels, scarce or absent erythrocytes within the lumina and discrete perivascular inflammation. CMs were characterized by an increased number of capillary-type vessels mostly rounded and located in the upper dermis. AVMs were composed by highly increased numbers of vessels with a branching pattern involving the full thickness of the dermis, without erythrocytes within the lumina. Wilms tumour 1 protein was positive in the endothelial cells both in pink macules of the CM-AVM and in arteriovenous malformations.

CONCLUSIONS AND RELEVANCE

Pink macules of the CM-AVM syndrome seem to be different from capillary malformations. Our results suggest that histologically and immunohistochemically they are closer to incipient arteriovenous malformations than to capillary malformations. A deepened knowledge about the nature of these skin lesions will contribute to the better understanding of capillary malformation-arteriovenous malformation syndrome, and will open the possibility of new and more specific treatments in the future.