BRCA1 p.His1673del is a pathogenic mutation associated with a predominant ovarian cancer phenotype

Abstract

We have investigated the clinical significance of the BRCA1 variant p.His1673del in 14 families from the Emilia-Romagna region of Italy, including 20 breast and 23 ovarian cancer cases; four families displayed site-specific ovarian cancer.The variant, absent in human variation databases, has been reported three times in BRCA1 specific databases; all probands shared the same rare haplotype at the BRCA1 locus, consistent with a common ancestor.The multifactorial likelihood method by Goldgar, used to estimate the probability of the variant being causative, gave a ratio of 2,263,474:1 in favor of causality. Moreover, in silico modeling suggested that His1673-lacking BRCA1 protein may have a decreased ability to bind BARD1 and other related proteins. All six ovarian carcinomas and two out of four breast carcinomas available showed a loss of the BRCA1 wild-type allele, which in three out of four ovarian carcinomas analyzed by FISH was associated with duplication of the chromosome 17 containing the variant. Although the pathogenicity of the allele is strongly supported by the multifactorial ratio,we cannot exclude that p.His1673del is not itself deleterious, but is linked to another undetected mutation on the same ancestral allele.

Keywords: BRCA1; VUS; breast cancer; hereditary cancer; ovarian cancer.

Figure 1. LOH analysis in patient 129-O-14;III-1

(A) Sanger sequence alignment with reference sequence; electropherograms shows wild-type sequence (upper), heterozygous sequence (middle) and homozygous deletion (lower). (B) MLPA analysis of normal tissue (upper plot) and tumor tissue (lower plot). (C) Microsatellite analysis of normal and tumor tissues, showing length of each marker for both tissues. Red circle indicates centromere position. (D) CGH array analysis, pattern of chromosome 17 in normal tissue (upper plot) and tumor tissue (lower). Arrows indicate chromosomal position of BRCA. (E) Fluorescence in situ hybridization (FISH) with centromeric probe for chromosome 17 (Spectrum Orange) showing nuclei with chromosome 17 gains in the ovarian carcinoma of the patient (DAPI, ×100).

Figure 2. Pedigree of the most representative family carrying the p.His1673del variant, showing the co-segregation of the variant with breast and ovarian cancer in three first-degree cousins (III-1, III-7, III-9)

Based on their relationship, the probability that three cousins share a genetic variant by chance would be 1/64. Incomplete penetrance and possible variable expressivity are demonstrated by the presence of the variant in a healthy 87 years old woman (II-2) and in a woman affected by endometrial cancer (II-3).

Figure 3. Putative interaction between the BRCT domains of BRCA1 and BARD1

The human BRCT domain of BRCA1 (PDB code: 4Y2G) and of BARD1 (PDB code: 2NTE) are represented in green and blue, respectively. Residues participating in the interaction surface are highlighted with a space-fill representation. Residue His1673 in the BRCT domain of BRCA1 is colored in red and interacts with residues Glu599 (in yellow) and Lys 693 (in magenta) of the BRCT domain of BARD1. The Abraxas phospho-peptide complexed with the BRCA1 BRCT domain is shown in red with a “balls and sticks” representation.