We report the results of next-generation sequencing of 29 candidate genes in affected tissues from 59 patients with SFVMs. Consistent with the results of previous studies,4–6 we identified three somatic PIK3CA variants (c.1624G>A; p.Glu542Lys, c.1633G>A; p.Glu545Lys, and c.3140A>G; p.His1047Arg) in patients with VM, LM, LVM, and KTS, and eight somatic TEK variants (c.2690A>G; p.Tyr897Cys, c.2689T>C; p.Tyr897His, c.2740C>T; p.Leu914Phe, c.2743C>T; p.Arg915C, c.2752C>T; p.Arg918Cys, c.2753G>T; p.Arg918Leu, and c.3295C>T; p.Arg1099*) in patients with VM.
VMs are painful and deforming lesions caused by dilated vascular channels.27 Somatic activating variants in TEK have previously been identified in about 60% of VMs.27 More recently, somatic activating variants in PIK3CA have been identified in about 25% of VM cases.24 In patients with wild-type TEK and PIK3CA genes, the VMs are likely caused by infrequent variants in other genes connected to the PI3K/AKT/mTOR and RAS/MAPK pathways, as suggested by Castel et al.8 TEK and PIK3CA variants drive constitutive activation of the PI3K/AKT/mTOR pathway, resulting in increased proliferation and survival of endothelial cells, which could account for the increased accumulation of endothelial cells observed in VMs.8 In two patients with VMs, we detected multiple pathogenic variants in TEK. The ratio of single/multiple TEK variants in VMs in our study (80%/20%) was similar to that (85.7%/14.3%) reported by Limaye et al.28 These data indicated that the somatic “second hit” might play a determining role in a few of VMs.28 TEK and PIK3CA variants are typically mutually exclusive but both occur in some patients with VMs,8,29,30 as was the case for one patient with hotspot variants of both TEK and PIK3CA in the present study. It is likely that this can be explained by the presence of two related events in the same cells, because the VAFs of both variants were similar in our case. Another possibility is that variants in TEK and PIK3CA do not co-exist in the same cell. Further studies are needed to clarify the roles of simultaneous pathogenic variants in TEK and PIK3CA in the development of VM.
The TEK nonsense mutation c.921C>G: p.Tyr307*, identified in one VM patient in the present study, has previously been associated with primary congenital glaucoma.31 This variant is located in the ectodomain of TEK and is reportedly loss of function.31 This differs strikingly from TEK variants linked to hereditary and sporadic VMs,27 which are located solely in the intracellular domain and result in enhanced kinase activity.32 Interestingly, Limaye et al. identified a somatic second hit in TEK, a loss-of-function deletion, in a VM lesion from a patient carrying the TEK R849W variant in the germline.28 This would suggest that loss-of-function variants in TEK might potentiate the development of VMs33 and thus also implicates the nonsense variant detected in our study might be associated with development of VMs. Further investigation will be needed to clarify the pathogenicity of this variant.
LMs are characterized by the presence of abnormal lymphatic vessels with progressive cystic dilation.34 Upregulation of the PI3K/AKT/mTOR pathway may be a causal factor in the development of the abnormal lymphatic vessels.35 Previous studies performed on LM specimens have identified somatic activating variants in the PIK3CA gene.35 For example, Blesinger et al. showed that activating PIK3CA variants in patients with LM were specifically localized in lymphatic endothelial cells.36 Using deep targeted sequencing methods with a cohort of 64 patients, Zenner et al. reported PIK3CA variants in LM tissues from 68.8% of the patients with a VAF of <5%.37 In our study, 60% (9/15) of the individuals with VMs had a maximum VAF of <5%. These data highlight the need for optimization of DNA sequencing methods to enable detection of very low VAFs in LM tissues.
One of our patients harbored a variant in RASA1, which encodes p120-RasGAP protein that inhibits activity of RAS protein.38 Variants in this gene have been reported to be associated with capillary malformation-arteriovenous malformation and Parkes Weber syndrome, a congenital vascular malformation consisting of capillary malformation, VM, LM, and arteriovenous malformation.38,39 Most of the RASA1 mutations responsible for capillary malformation-arteriovenous malformation and Parkes Weber syndrome are loss of function mutations and may lead to activation of RAS and increase downstream signaling via MAPK and PI3K/AKT/mTOR pathways.38 The RASA1 missense variant (c.1772G>A; p.Arg591His) identified in our LM patient is potentially pathogenic according to two prediction tools, but further genetic and functional studies are needed to determine whether the variant is indeed pathogenic.
LVMs composed of combined lymphatic and venous elements are present at birth and develop due to errors in venolymphatic development.26 Two hypotheses have been proposed for the pathogenesis of LVMs. One suggests that the condition results from malformation of lymphatic vascular pathways, while the second considers that it represents a tumor that grows by cellular (mainly endothelial) hyperplasia.26 A somatic variant of PIK3CA was identified in affected tissues from a LVM patient in the present study; however, the cell type harboring the variant is unknown. Further research is needed to elucidate the cellular and molecular pathways driving LVM pathogenesis.
KTS is a syndrome involving capillary and venous malformations as well as limb overgrowth with or without LM.40 KTS is caused by a mutation in primitive limb-forming cells that are destined to become blood and lymphatic vessels, fat, and bones.35 In most cases of KTS analyzed to date, the cause is mosaic activating variants of PIK3CA.35 In the present study, we identified pathogenic PIK3CA variants in 8 of the 10 patients with KTS.
In recent years, sirolimus has emerged as a new medical treatment option for SFVMs through inhibiting the PI3K/AKT/mTOR signaling pathway.3 Notably, sirolimus has demonstrated substantial clinical benefit, as reflected by a decrease in the size of most lesions and an improvement in quality of life.41 However, long-term sirolimus treatment may cause significant side effects due to immunosuppression, and the clinical studies performed thus far suggest that it does not always reduce the volume of existing SFVM lesions.42 Direct targeting of chronically activated TIE2 and/or PIK3CA kinases using specific inhibitors may provide the best clinical response for patients with SFVMs.42
There are several possible reasons why genetic alterations were not detected in 21 of the 59 patients in the present study. For example, the pathogenic alterations could be located in genes other than the 29 investigated here, or they could be located in other gene regions, such as deep intronic regions, not included in our targeted next-generation sequencing approach. Alternatively, the VAFs may have been below the detection limit, the changes may have been epigenetic alterations or large deletions that were undetected using the current sequencing methodology, or sampling errors may have occurred.
In conclusion, pathogenic variants in genes involved in the PI3K signaling pathway were predominant among the 29 genes and 59 samples examined here. Inhibitors of this pathway may therefore have utility as molecular targeted treatments for SFVMs.