Single-nucleus DNA sequencing reveals hidden somatic loss-of-heterozygosity in Cerebral Cavernous Malformations.

Cerebral Cavernous Malformations (CCMs) are vascular malformations of the central nervous system which can lead to moderate to severe neurological phenotypes in patients. A majority of CCM lesions are driven by a cancer-like three-hit mutational mechanism, including a somatic, activating mutation in the oncogene PIK3CA, as well as biallelic loss-of-function mutations in a CCM gene. However, standard sequencing approaches often fail to yield a full complement of pathogenic mutations in many CCMs. We suggest this reality reflects the limited sensitivity to identify low-frequency variants and the presence of mutations undetectable with bulk short-read sequencing. Here we report a single-nucleus DNA-sequencing approach that leverages the underlying biology of CCMs to identify lesions with somatic loss-of-heterozygosity, a class of such hidden mutations. We identify an alternative genetic mechanism for CCM pathogenesis and establish a method that can be repurposed to investigate the genetic underpinning of other disorders with multiple somatic mutations.

Circulating Plasma miRNA Homologs in Mice and Humans Reflect Familial Cerebral Cavernous Malformation Disease.

Patients with familial cerebral cavernous malformation (CCM) inherit germline loss of function mutations and are susceptible to progressive development of brain lesions and neurological sequelae during their lifetime. To date, no homologous circulating molecules have been identified that can reflect the presence of germ line pathogenetic CCM mutations, either in animal models or patients. We hypothesize that homologous differentially expressed (DE) plasma miRNAs can reflect the CCM germline mutation in preclinical murine models and patients. Herein, homologous DE plasma miRNAs with mechanistic putative gene targets within the transcriptome of preclinical and human CCM lesions were identified. Several of these gene targets were additionally found to be associated with CCM-enriched pathways identified using the Kyoto Encyclopedia of Genes and Genomes. DE miRNAs were also identified in familial-CCM patients who developed new brain lesions within the year following blood sample collection. The miRNome results were then validated in an independent cohort of human subjects with real-time-qPCR quantification, a technique facilitating plasma assays. Finally, a Bayesian-informed machine learning approach showed that a combination of plasma levels of miRNAs and circulating proteins improves the association with familial-CCM disease in human subjects to 95% accuracy. These findings act as an important proof of concept for the future development of translatable circulating biomarkers to be tested in preclinical studies and human trials aimed at monitoring and restoring gene function in CCM and other diseases.

Cerebral Cavernous Malformation: From Mechanism to Therapy.

Cerebral cavernous malformations are acquired vascular anomalies that constitute a common cause of central nervous system hemorrhage and stroke. The past 2 decades have seen a remarkable increase in our understanding of the pathogenesis of this vascular disease. This new knowledge spans genetic causes of sporadic and familial forms of the disease, molecular signaling changes in vascular endothelial cells that underlie the disease, unexpectedly strong environmental effects on disease pathogenesis, and drivers of disease end points such as hemorrhage. These novel insights are the integrated product of human clinical studies, human genetic studies, studies in mouse and zebrafish genetic models, and basic molecular and cellular studies. This review addresses the genetic and molecular underpinnings of cerebral cavernous malformation disease, the mechanisms that lead to lesion hemorrhage, and emerging biomarkers and therapies for clinical treatment of cerebral cavernous malformation disease. It may also serve as an example for how focused basic and clinical investigation and emerging technologies can rapidly unravel a complex disease mechanism.

PIK3CA and CCM mutations fuel cavernomas through a cancer-like mechanism.

Vascular malformations are thought to be monogenic disorders that result in dysregulated growth of blood vessels. In the brain, cerebral cavernous malformations (CCMs) arise owing to inactivation of the endothelial CCM protein complex, which is required to dampen the activity of the kinase MEKK31-4. Environmental factors can explain differences in the natural history of CCMs between individuals5, but why single CCMs often exhibit sudden, rapid growth, culminating in strokes or seizures, is unknown. Here we show that growth of CCMs requires increased signalling through the phosphatidylinositol-3-kinase (PI3K)-mTOR pathway as well as loss of function of the CCM complex. We identify somatic gain-of-function mutations in PIK3CA and loss-of-function mutations in the CCM complex in the same cells in a majority of human CCMs. Using mouse models, we show that growth of CCMs requires both PI3K gain of function and CCM loss of function in endothelial cells, and that both CCM loss of function and increased expression of the transcription factor KLF4 (a downstream effector of MEKK3) augment mTOR signalling in endothelial cells. Consistent with these findings, the mTORC1 inhibitor rapamycin effectively blocks the formation of CCMs in mouse models. We establish a three-hit mechanism analogous to cancer, in which aggressive vascular malformations arise through the loss of vascular 'suppressor genes' that constrain vessel growth and gain of a vascular 'oncogene' that stimulates excess vessel growth. These findings suggest that aggressive CCMs could be treated using clinically approved mTORC1 inhibitors.

Transcriptome clarifies mechanisms of lesion genesis versus progression in models of Ccm3 cerebral cavernous malformations.

Cerebral cavernous malformations (CCMs) are dilated capillaries causing epilepsy and stroke. Inheritance of a heterozygous mutation in CCM3/PDCD10 is responsible for the most aggressive familial form of the disease. Here we studied the differences and commonalities between the transcriptomes of microdissected lesional neurovascular units (NVUs) from acute and chronic in vivo Ccm3/Pdcd10ECKO mice, and cultured brain microvascular endothelial cells (BMECs) Ccm3/Pdcd10ECKO.We identified 2409 differentially expressed genes (DEGs) in acute and 2962 in chronic in vivo NVUs compared to microdissected brain capillaries, as well as 121 in in vitro BMECs with and without Ccm3/Pdcd10 loss (fold change ≥ |2.0|; p < 0.05, false discovery rate corrected). A functional clustered dendrogram generated using the Euclidean distance showed that the DEGs identified only in acute in vivo NVUs were clustered in cellular proliferation gene ontology functions. The DEGs only identified in chronic in vivo NVUs were clustered in inflammation and immune response, permeability, and adhesion functions. In addition, 1225 DEGs were only identified in the in vivo NVUs but not in vitro BMECs, and these clustered within neuronal and glial functions. One miRNA mmu-miR-3472a was differentially expressed (FC = - 5.98; p = 0.07, FDR corrected) in the serum of Ccm3/Pdcd10+/- when compared to wild type mice, and this was functionally related as a putative target to Cand2 (cullin associated and neddylation dissociated 2), a DEG in acute and chronic lesional NVUs and in vitro BMECs. Our results suggest that the acute model is characterized by cell proliferation, while the chronic model showed inflammatory, adhesion and permeability processes. In addition, we highlight the importance of extra-endothelial structures in CCM disease, and potential role of circulating miRNAs as biomarkers of disease, interacting with DEGs. The extensive DEGs library of each model will serve as a validation tool for potential mechanistic, biomarker, and therapeutic targets.

Cerebral Cavernous Malformations Develop Through Clonal Expansion of Mutant Endothelial Cells.

RATIONALE: Vascular malformations arise in vessels throughout the entire body. Causative genetic mutations have been identified for many of these diseases; however, little is known about the mutant cell lineage within these malformations. OBJECTIVE: We utilize an inducible mouse model of cerebral cavernous malformations (CCMs) coupled with a multicolor fluorescent reporter to visualize the contribution of mutant endothelial cells (ECs) to the malformation. METHODS AND RESULTS: We combined a Ccm3 mouse model with the confetti fluorescent reporter to simultaneously delete Ccm3 and label the mutant EC with 1 of 4 possible colors. We acquired Z-series confocal images from serial brain sections and created 3-dimensional reconstructions of entire CCMs to visualize mutant ECs during CCM development. We observed a pronounced pattern of CCMs lined with mutant ECs labeled with a single confetti color (n=42). The close 3-dimensional distribution, as determined by the nearest neighbor analysis, of the clonally dominant ECs within the CCM was statistically different than the background confetti labeling of ECs in non-CCM control brain slices as well as a computer simulation ( P<0.001). Many of the small (<100 µm diameter) CCMs consisted, almost exclusively, of the clonally dominant mutant ECs labeled with the same confetti color, whereas the large (>100 µm diameter) CCMs contained both the clonally dominant mutant cells and wild-type ECs. We propose of model of CCM development in which an EC acquires a second somatic mutation, undergoes clonal expansion to initiate CCM formation, and then incorporates neighboring wild-type ECs to increase the size of the malformation. CONCLUSIONS: This is the first study to visualize, with single-cell resolution, the clonal expansion of mutant ECs within CCMs. The incorporation of wild-type ECs into the growing malformation presents another series of cellular events whose elucidation would enhance our understanding of CCMs and may provide novel therapeutic opportunities.