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Finetti et al. Vessel Plus 2021;5:29 https://dx.doi.org/10.20517/2574-1209.2021.49 Page 3 of 9

disease depends on the position and size of the lesions and its association with hemorrhages or intractable
seizures. When possible, the microsurgical resection of lesions is the eligible route [11,24] . Therefore, the
development of new pharmacological strategies is necessary to prevent lesion formation and for treating
patients with severe inoperable disease or with multiple lesions. The knowledge of molecular mechanisms at
the basis of disease onset appears in this context fundamental to identify new drugs able to inhibit the
dysregulation of specific signaling pathways and aberrant immune responses. Recent studies focus on
different molecules that are under investigation due to their ability to revert the CCM gene loss effects, such
as Vitamin D3 and Tempol (reactive oxygen species inhibitors), atorvastatin and fasudil (Rho/Rock
signaling inhibitors), propranolol and bevacizumab [angiogenesis and vascular endothelial growth factor,
vascular endothelial growth factor (VEGF), inhibitors], sulindac, LY-364947, and SB-431542 (endothelial-
mesenchymal transition inhibitors), TM, and endothelial protein C receptor (EPCR) inhibitors. However, at
the present there are no therapies that have demonstrated an effective activity on reducing CCM lesion
formation [3,14,24] .

CEREBRAL CAVERNOUS MALFORMATIONS ONSET AND DEVELOPMENT
As mentioned, CCM lesions are a consequence of a complete loss-of-function of any of the three CCM
genes. In familial cases, mutation is inherited as a constitutional germline mutation with incomplete
penetrance, and is associated with a biallelic somatic mutation that occurs in the same cell. In particular,
CCM genesis may follow a “two-hit” mechanism in which loss of one of the two alleles (first hit) would be
the result of the germline mutation and loss of the second allele (second hit) will occur somatically [25-28] .

CCM onset might be explained with the Knudson’s two-hit model of tumor suppressor genes. According to
this model, tumors derive from two mutations, one for each of the two alleles of the gene of interest. At the
present, several cancer genes exhibit biallelic disruption and truncating point mutations, revealing the
[29]
validity of the model . According to Knudson hypothesis, vascular lesion formation derives from two
mutational events in both hereditary (familial) and non-hereditary (sporadic) cases. In familial CCM the
first mutation occurs before conception (germline mutation) and the second after conception (somatic
mutation). In sporadic CCM only somatic mutations are present.

However, the mechanisms at the basis of these mutations are unknown and some questions still remain
open. It is not completely understood if a single second somatic mutation for familial CCM is sufficient to
induce vascular lesion formation, where the mutant cells reside within the cellular architecture of the
vascular wall and how this second mutation can promote vascular malformation.

In familial CCM, the identification of the somatic mutation remains technically challenging also by using
very sensitive approaches as next-generation sequencing techniques. The expected allele frequency of
somatic mutation within vascular lesions is low due to the tissue nature, characterized by a vascular
structure with a single monolayer of endothelial cells. However, some reports describe a genetic two-step
inactivation of all the three genes involved in CCM onset [25-27,30-32] .

In the last three years new experimental evidences, supported by mouse models and sophisticated cellular
and molecular biology techniques, sustained the idea that the loss of a CCM gene in a single cell may be
responsible for CCM development [30,33,34] .

[33]
Detter et al. , by using a CCM3 KO mouse model, showed for the first time that vascular lesions are
composed of both mutated and non-mutated endothelial cells. The authors propose a model in which
endothelial cells acquire a somatic mutation promoted by a second hit, and undergo clonal expansion