Mapping loss of heterozygosity in normal human breast cells from BRCA1/2 carriers

Abstract

We have studied loss of heterozygosity at the BRCA1 and BRCA2 loci in 992 normal cell clones derived from topographically defined areas of normal tissue in four samples from BRCA1/BRCA2 mutation carriers. The frequency of loss of heterozygosity in the clones was low (1.01%), but it was found in all four samples, whether or not a tumour was present. Topographical mapping revealed that the genetic changes were clustered in some breast samples. Our study confirms the previous finding that a field of genetic instability can exist around a tumour, suggesting that sufficient tissue must be removed at surgery to avoid local recurrence. We also demonstrate that such a field of genetic change can exist in morphologically normal tissue before a tumour develops and, for the first time, we demonstrate that the field is of a size greater than one terminal duct-lobular unit. The genetic changes are not identical, however, which suggests that genetic instability in these regions may play an early role in tumour development. We also confirm and extend our original observation of loss of the wild-type BRCA1 allele in some clones, and loss of the mutant allele in others, demonstrating that loss of either allele is a stochastic event.

Figure 1

Distribution of LOH detected in areas of histologically normal tissue of a mastectomy specimen (sample 3). Areas of tissue from which cells were cloned are shown in white, tumour is shown in black (T). Wt=loss from wild type allele, mut=loss from mutant allele.

Figure 2

Loss of heterozygosity in normal cell clones demonstrated by amplification of microsatellites D17s1323 (A) and D13s267 (B). The normal tissue control for each sample shows two allele peaks. The ‘normal’ clones from samples 3 and 4 each demonstrate loss of one allele (arrow).