Cerebral Cavernous Malformations Develop Through Clonal Expansion of Mutant Endothelial Cells

Matthew R Detter , Daniel A Snellings , Douglas A Marchuk

Circulation Research 2018;0. (18 Sep 2018): 1-14.

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 multi-color 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 one of four possible colors. We acquired Z-series confocal images from serial brain sections and created 3D 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 3D 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m diameter) CCMs consisted, almost exclusively, of the clonally dominant mutant ECs labeled with the same confetti color whereas the large (>100m 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.