Variable expression of cerebral cavernous malformations in carriers of a premature termination codon in exon 17 of the Krit1 gene

Abstract

Background: Cerebral cavernous malformations (CCM) present as either sporadic or autosomal dominant conditions with incomplete penetrance of symptoms. Differences in genetic and environmental factors might be minimized among first-degree relatives. We therefore studied clinical expression in a family with several affected members.

Methods: We studied a three-generation family with the onset of CCM as a cerebral haemorrhage in the younger (four-year-old) sibling. Identification and enumeration of CCMs were performed in T2-weighted or gradient-echo MRIs of the whole brains. Genetic analysis comprised SCCP, sequencing and restriction polymorphism of the Krit1 gene in the proband and at risk relatives.

Results: The phenotypes of cerebral cavernous malformations (CCMs) in carriers of Krit1 mutations were very variable. We identified a novel frameshift mutation caused by a 1902A insertion in exon 17 of the Krit1 gene, which leads to a premature TAA triplet and predicts the truncating phenotype Y634X. A very striking finding was the absence of both clinical symptoms and CCMs in the eldest sibling harbouring the 1902insA.

Conclusions: Patients in this family, harbouring the same mutation, illustrate the very variable clinical and radiological expression of a Krit1 mutation. The early and critical onset in the proband contrasts with minor clinical findings in affected relatives. This consideration is important in genetic counselling.

Figure 1

MRIs of CCMs. The echo gradient sequences of the proband (left), grandmother (middle) and the older brother (right) harbouring the Y634X mutation (III-2 in figure 3). Note the typical images of cavernous malformation and the absence of lesions in III-2, the asymptomatic carrier of the 1902insA.

Figure 2

Left panel, SCCP of the proband and parents of CVE10 PCR was performed with the reaction mixture supplemented with [α³²P]-dCTP. Aliquots of the product were run in acrylamide gels (see Methods) and revealed by auto-radiography. Right panel, chromatogram of the 1902InsA. Traces of the sense (upper) and antisense (lower) DNA strands of the proband. The wild type and mutated nucleotide sequences and the aminoacid change are indicated over the sense strand. The sequence shows the site of the 1902insA mutation (see arrow) and the frameshift. The ambiguities are caused by the overlapping bands of the wild type and mutated strands due to the nucleotide insertion. The stop TAA triplet predicts a truncating protein with the changed phenotype Y634X.

Figure 3

Haplotypes and restriction fragment polymorphism (RFLP). Haplotypes were analysed using the markers surrounding CCM1 as described in Methods. The markers from top to bottom were: D7S2409, D7S1813, D7S1789, D7S646, D7S558, D7S689, D7S652 and D7S492. Exon 17 was amplified with the primers and aliquots of the PCR were digested overnight at 37°C with MseI. The fragments were separated in an 8% polyacrylamide gel containing urea and, after staining with ethidium bromide, the gel was photographed under UV. The undigested product is 297 pb long and the fragments separated from the TTAA restriction site are 158 and 139 pb. Therefore, individuals harbouring the 1902insA mutation have three fragments of 297, 158 and 139 bp. Filled symbols in the pedigree refer to patients with CCMs in the echo-gradient MRIs. Arrow indicates the proband. Stairs refer to asymptomatic carriers of the mutation. The double stair denotes the asymptomatic carrier with MRIs free of CCMs