The c.156_157insAlu BRCA2 rearrangement accounts for more than one-fourth of deleterious BRCA mutations in northern/central Portugal

Abstract

We evaluated the contribution of an Alu insertion in BRCA2 exon 3 (c.156_157insAlu) to inherited predisposition to breast/ovarian cancer in 208 families originated mostly from northern/central Portugal. We identified the c.156_157insAlu BRCA2 mutation in 14 families and showed that it accounts for more that one-fourth of deleterious BRCA1/BRCA2 mutations in breast/ovarian cancer families originated from this part of the country. This mutation originates BRCA2 exon 3 skipping and we demonstrated its pathogenic effect by showing that the BRCA2 full length transcript is derived only from the wild type allele in carriers, that it is absent in 262 chromosomes from healthy blood donors, and that it co-segregates with the disease. Polymorphic microsatellite markers were used for haplotype analysis in three informative families. In two of the three families one haplotype was shared for all but two markers, whereas in the third family all markers telomeric to BRCA2 differed from that observed in the other two. Although the c.156_157insAlu BRCA2 mutation has so far only been identified in Portuguese breast/ovarian cancer families, screening of this rearrangement in other populations will allow evaluation of whether or not it is a population-specific founder mutation and a more accurate estimation of its distribution and age.

International distribution and age estimation of the Portuguese BRCA2 c.156_157insAlu founder mutation

“…From Portugal, 149 new suspected HBOC families were selected for BRCA1 and BRCA2 mutation screening using previously described criteria (Peixoto et al, 2006;Peixoto et al, 2009a) after written informed consent. Molecular testing at the Department of Genetics of the Portuguese Oncology Institute, Porto, Portugal (IPO-Porto) started by looking for the c.156_157insAlu BRCA2 mutation, followed by full BRCA1 and BRCA2 mutation screening with the previously reported methodology (Peixoto et al, 2006;Peixoto et al, 2009a;Peixoto et al, 2009b).…”

“…From Portugal, 149 new suspected HBOC families were selected for BRCA1 and BRCA2 mutation screening using previously described criteria (Peixoto et al, 2006;Peixoto et al, 2009a) after written informed consent. Molecular testing at the Department of Genetics of the Portuguese Oncology Institute, Porto, Portugal (IPO-Porto) started by looking for the c.156_157insAlu BRCA2 mutation, followed by full BRCA1 and BRCA2 mutation screening with the previously reported methodology (Peixoto et al, 2006;Peixoto et al, 2009a;Peixoto et al, 2009b). The c.156_157insAlu BRCA2 mutation was additionally screened in 5,291 suspected HBOC families living in countries other than Portugal in whom no deleterious BRCA1/BRCA2 mutations had previously been found, with the following Additionally, predictive testing was performed in four individuals from two additional families (two relatives from each family living in Rhode Island, USA, and in Villejuif, France, respectively) with the c.156_157insAlu BRCA2 mutation identified elsewhere.…”

“…For the purpose of haplotype studies and age estimation of the c.156_157insAlu BRCA2 mutation, the 14 HBOC families we previously reported (Peixoto et al, 2009a) and the family (four c.156_157insAlu carriers) initially identified by Teugels et al (2005) were also included. The geographic origin of the c.156_157insAlu BRCA2 positive families was inferred from the birthplace of the oldest carrier or of the oldest family member most likely to be a carrier.…”

“…Machado et al (2007) later described a regional founder effect for this rearrangement in HBOC families mostly originated from central/southern Portugal. We recently evaluated the contribution of the c.156_157insAlu BRCA2 mutation to inherited predisposition to breast/ovarian cancer in families originated mostly from northern/central Portugal (Peixoto et al, 2009a) and found that this rearrangement is responsible for more than half of all deleterious BRCA2 mutations and about one-fourth of all deleterious mutations in HBOC families. Additionally, in light of some doubts raised about the pathogenic effect of BRCA2 exon 3 skipping (Diez et al, 2007), we demonstrated its pathogenic effect by showing that the BRCA2 full length transcript is produced almost exclusively from the wildtype allele in patients carrying the c.156_157insAlu BRCA2 rearrangement and that this mutation co-segregates with the disease in HBOC families and is absent in healthy blood donors, although residual exon 3 skipping in BRCA2 mRNA can be found in negative controls (Peixoto et al, 2009a;Peixoto et al, 2009b).…”

“…Although all reported c.156_157insAlu BRCA2 mutations have so far been identified in Portuguese HBOC families (Teugels et al, 2005;Machado et al, 2007;Peixoto et al, 2009a), this mutation is not detected using the commonly used screening methodologies and must be specifically sought (Teugels et al, 2005;Peixoto et al, 2009b), so one can not currently rule out its presence in other populations. To gain insight into the ancestral origin and population spread of the c.156_157insAlu BRCA2 mutation, we screened for this rearrangement in 5,440 suspected HBOC patients from several countries and performed an extensive haplotype study using closely linked microsatellite markers and single nucleotide polymorphisms (SNPs) in carrier families.…”