CCM molecular screening in a diagnosis context: novel unclassified variants leading to abnormal splicing and importance of large deletions

Florence Riant, Michaelle Cecillon, Pascale Saugier-Veber, Elisabeth Tournier-Lasserve

Loss of function mutations in CCM1/KRIT1, CCM2/MGC4607, and CCM3/PDCD10 gene are identified in about 95 % of familial cases of cerebral cavernous malformations and 2/3 of sporadic cases with multiple lesions. In this study, 279 consecutive index patients referred for either genetic counseling or for diagnosis of cerebral hemorrhage of unknown etiology were analyzed for the three cerebral cavernous malformations (CCM) genes by direct sequencing and quantitative studies, to characterize in more detail the mutation spectrum associated with cerebral cavernous malformations and to optimize CCM gene screening. Analysis of the cDNA was performed when possible to detect the consequences of the genomic variations. A pathogenic mutation was identified in 122 patients. CCM1 was mutated in 80 patients (65 %), CCM2 in 23 (19 %), and CCM3 in 19 (16 %). One hundred patients harbored a loss of function point mutation (82 %) and 22 had a large deletion (18 %). Novel unclassified variants were detected in the patients among whom six led to a splicing defect. The causality of three missense variants that did not modify the splicing could not be established. These findings expand the CCM mutation spectrum and highlight the importance of screening the three CCM genes with both direct sequencing and a quantitative method. In addition, six new unclassified variants were shown to be deleterious because they led to a splicing defect. This underlines the necessity of the cDNA analysis when an unknown variant is detected.