Search for loss of heterozygosity and mutation analysis of KRIT1 gene in CCM patients

Cerebral Cavernous Malformations: An Update on Prevalence, Molecular Genetic Analyses, and Genetic Counselling

Based on the latest gnomAD dataset, the prevalence of symptomatic hereditary cerebral cavernous malformations (CCMs) prone to cause epileptic seizures and stroke-like symptoms was re-evaluated in this review and calculated to be 1:5,400-1:6,200. Furthermore, state-of-the-art molecular genetic analyses of the known CCM loci are described which reach an almost 100% mutation detection rate for familial CCMs if whole genome sequencing is performed for seemingly mutation-negative families. An update on the spectrum of CCM1, CCM2, and CCM3 mutations demonstrates that deep-intronic mutations and submicroscopic copy-number neutral genomic rearrangements are rare. Finally, this review points to current guidelines on genetic counselling, neuroimaging, medical as well as neurosurgical treatment and highlights the formation of active patient organizations in various countries.

Mutation analysis of CCM1, CCM2 and CCM3 genes in a cohort of Italian patients with cerebral cavernous malformation

Cerebral cavernous malformations (CCMs) are vascular lesions of the CNS characterized by abnormally enlarged capillary cavities. CCMs can occur as sporadic or familial autosomal dominant form. Familial cases are associated with mutations in CCM1[K-Rev interaction trapped 1 (KRIT1)], CCM2 (MGC4607) and CCM3 (PDCD10) genes. In this study, a three-gene mutation screening was performed by direct exon sequencing, in a cohort of 95 Italian patients either sporadic or familial, as well as on their at-risk relatives. Sixteen mutations in 16 unrelated CCM patients were identified,nine mutations are novel: c.413T > C; c.601C > T; c.846 + 2T > G; c.1254delA; c.1255-4delGTA; c.1682-1683 delTA in CCM1; c.48A > G; c.82-83dupAG in CCM2; and c.395 + 1G > A in CCM3 genes [corrected].The samples, negative to direct exon sequencing, were investigated by MLPA to search for intragenic deletions or duplications. One deletion in CCM1 exon 18 was detected in a sporadic patient. Among familial cases 67% had a mutation in CCM1, 5.5% in CCM2, and 5.5% in CCM3, whereas in the remaining 22% no mutations were detected, suggesting the existence of either undetectable mutations or other CCM genes. This study represents the first extensive research program for a comprehensive molecular screening of the three known genes in an Italian cohort of CCM patients and their at-risk relatives.

Cerebral cavernous malformations: somatic mutations in vascular endothelial cells

OBJECTIVE

Germline mutations in 3 genes have been found in familial cases of cerebral cavernous malformations (CCMs). We previously discovered somatic and germline truncating mutations in the KRIT1 gene, supporting the "2-hit" mechanism of CCM lesion formation in a single lesion. The purpose of this study was to screen for somatic, nonheritable mutations in 3 more lesions from different patients and identify the cell type(s) in which somatic mutations occur.

METHODS

Somatic mutations were sought in DNA from 3 surgically excised, fresh-frozen CCM lesions by cloning and screening polymerase chain reaction products generated from KRIT1 or PDCD10 coding regions. Laser capture microdissection was used on isolated endothelial and nonendothelial cells to determine whether somatic mutations were found in endothelial cells.

RESULTS

CCM lesions harbor somatic and germline KRIT1 mutations on different chromosomes and are therefore biallelic. Both mutations are predicted to truncate the protein. The KRIT1 somatic mutations (novel c.1800delG mutation and previously identified 34 nucleotide deletion) in CCMs from 2 different patients were found only in the vascular endothelial cells lining caverns. No obvious somatic mutations were identified in the 2 other lesions; however, the results were inconclusive, possibly owing to the technical limitations or the fact that these specimens had a small proportion of vascular endothelial cells lining pristine caverns.

CONCLUSION

The "2-hit" mechanism occurs in vascular endothelial cells lining CCM caverns from 2 patients with somatic and Hispanic-American KRIT1 germline mutations. Methods for somatic mutation detection should focus on vascular endothelial cells lining pristine caverns.

Biallelic somatic and germline mutations in cerebral cavernous malformations (CCMs): evidence for a two-hit mechanism of CCM pathogenesis

Cerebral cavernous malformations (CCMs) are vascular anomalies of the central nervous system, comprising dilated blood-filled capillaries lacking structural support. The lesions are prone to rupture, resulting in seizures or hemorrhagic stroke. CCM can occur sporadically, manifesting as solitary lesions, but also in families, where multiple lesions generally occur. Familial cases follow autosomal-dominant inheritance due to mutations in one of three genes, CCM1/KRIT1, CCM2/malcavernin or CCM3/PDCD10. The difference in lesion burden between familial and sporadic CCM, combined with limited molecular data, suggests that CCM pathogenesis may follow a two-hit molecular mechanism, similar to that seen for tumor suppressor genes. In this study, we investigate the two-hit hypothesis for CCM pathogenesis. Through repeated cycles of amplification, subcloning and sequencing of multiple clones per amplicon, we identify somatic mutations that are otherwise invisible by direct sequencing of the bulk amplicon. Biallelic germline and somatic mutations were identified in CCM lesions from all three forms of inherited CCMs. The somatic mutations are found only in a subset of the endothelial cells lining the cavernous vessels and not in interstitial lesion cells. These data suggest that CCM lesion genesis requires complete loss of function for one of the CCM genes. Although widely expressed in the different cell types of the brain, these data also suggest a unique role for the CCM proteins in endothelial cell biology.

A two-hit mechanism causes cerebral cavernous malformations: complete inactivation of CCM1, CCM2 or CCM3 in affected endothelial cells

Cavernous vascular malformations occur with a frequency of 1:200 and can cause recurrent headaches, seizures and hemorrhagic stroke if located in the brain. Familial cerebral cavernous malformations (CCMs) have been associated with germline mutations in CCM1/KRIT1, CCM2 or CCM3/PDCD10. For each of the three CCM genes, we here show complete localized loss of either CCM1, CCM2 or CCM3 protein expression depending on the inherited mutation. Cavernous but not adjacent normal or reactive endothelial cells of known germline mutation carriers displayed immunohistochemical negativity only for the corresponding CCM protein but not for the two others. In addition to proving loss of function at the protein level, our data are the first to demonstrate endothelial cell mosaicism within cavernous tissues and provide clear pathogenetic evidence that the endothelial cell is the cell of disease origin.

Genetics of cavernous angiomas

Cerebral cavernous malformations (CCM) are vascular malformations that can occur as a sporadic or a familial autosomal dominant disorder. Clinical and cerebral MRI data on large series of patients with a genetic form of the disease are now available. In addition, three CCM genes have been identified: CCM1/KRIT1, CCM2/MGC4607, and CCM3/PDCD10. These recent developments in clinical and molecular genetics have given us useful information about clinical care and genetic counselling and have broadened our understanding of the mechanisms of this disorder.

Spectrum of genotype and clinical manifestations in cerebral cavernous malformations

OBJECTIVE

Cerebral cavernous malformations (CCMs) are focal dysmorphic blood vessel anomalies predisposing individuals to hemorrhagic stroke and epilepsy. CCMs are sporadic or inherited as autosomal dominant disease with three known genes. The hypothesis that genetic heterogeneity would account for the remarkable variability in CCM manifestations was tested.

METHODS

CCM cases were prospectively enrolled. Germline CCM1 gene mutations were sought in 89 CCM samples. Associations with clinical manifestations and lesion characteristics were made among 41 symptomatic familial cases, including one cohort of 26 cases with CCM1 mutations and a second cohort of 15 cases without identifiable CCM1 mutations. The 15 cases were screened for CCM2 and CCM3 mutations.

RESULTS

CCM1 mutations were found in 34 out of 50 subjects with familial disease and in none of 39 sporadic CCM cases. CCM2 and CCM3 mutations were found in three out of 10 families screened without CCM1 mutations. Clinical manifestations in 22 Hispanic-American cases with identical CCM1 mutations were highly variable. Fewer CCM1 patients experienced hemorrhage than others with familial disease (P = 0.0139 for all cases and P = 0.0442 for symptomatic cases). Adjusting for sex and age improved the logistic regression model, suggesting decreased numbers of patients with hemorrhage in CCM1 familial disease (P = 0.003 for all cases and P = 0.014 for symptomatic cases). Hemorrhage differences were not related to size or number of lesions.

CONCLUSION

Factors in addition to CCM1 germline mutation contribute to CCM clinical manifestations. However, this evidence suggests that familial cases with CCM1 mutations may have less severe clinical manifestations than other familial cases.

Cerebral cavernous malformation: new molecular and clinical insights

Cerebral cavernous malformation (CCM) is a vascular malformation causing neurological problems, such as headaches, seizures, focal neurological deficits, and cerebral haemorrhages. CCMs can occur sporadically or as an autosomal dominant condition with variable expression and incomplete penetrance. Familial forms have been linked to three chromosomal loci, and loss of function mutations have been identified in the KRIT1/CCM1, MGC4607/CCM2, and PDCD10/CCM3 genes. Recently, many new pieces of data have been added to the CCM puzzle. It has been shown that the three CCM genes are expressed in neurones rather than in blood vessels. The interaction between CCM1 and CCM2, which was expected on the basis of their structure, has also been proven, suggesting a common functional pathway. Finally, in a large series of KRIT1 mutation carriers, clinical and neuroradiological features have been characterised. These data should lead to more appropriate follow up, treatment, and genetic counselling. The recent developments will also help to elucidate the precise pathogenic mechanisms leading to CCM, contributing to a better understanding of normal and pathological angiogenesis and to the development of targeted treatment.

Genetics of cerebral cavernous malformations

The past few years have seen rapid advances in our understanding of the genetics and molecular biology of cerebral cavernous malformations (CCM). This article summarizes the recent cloning of the CCM1, CCM2, and CCM3 genes, which are responsible for autosomal dominant CCM, and also describes current hypotheses for their roles in integrin and p38 mitogen-activated protein kinase- mediated regulation of angiogenesis. A mouse model of CCM has been generated by mutation of the Ccm1 gene, and it indicates a role for that protein in arterial development. Future studies will probably focus on integration of data from each of the three CCM genes into a single model of the pathogenesis of cavernous malformation.

RASA1: variable phenotype with capillary and arteriovenous malformations

Capillary malformation-arteriovenous malformation (CM-AVM) is a newly discovered hereditary disorder. Its defining features are atypical cutaneous multifocal capillary malformations often in association with high-flow lesions: cutaneous, subcutaneous, intramuscular, intraosseous and cerebral arteriovenous malformations and arteriovenous fistulas. Some patients have Parkes Weber syndrome - a large congenital cutaneous vascular stain in an extremity, with bony and soft tissue hypertrophy and microscopic arteriovenous shunting. In the past, arteriovenous malformations and arteriovenous fistulas had been considered non-hereditary. A classical genetic approach was used to identify the locus. Candidate gene screening pinpointed mutations in RASA1 (p120-RASGAP) - a RasGTPase. RASA1 reverts active GTP-bound Ras into inactive GDP-bound form. Murine Rasa1 knockout and tetraploid-aggregated embryos with RNA interference exhibited abnormal vascular development. Lack of RASA1 activity caused inhibition of cell motility, possibly through p190-RhoGAP. Thus, RASA1 defects probably cause abnormal angiogenic remodeling of the primary capillary plexus that cannot be compensated for by other RasGAPs: RASA2, RASAL and NF1. Signaling pathways involving RASA1 might offer novel targets for treatment of high-flow vascular anomalies.