Background: The molecular genetic analysis of invasive breast cancer has identified breast cancer as a genetically complex disease. Ductal carcinoma in situ (DCIS) is thought to represent a preinvasive step in breast cancer progression, yet we know little about its biologic behavior or the genetic alterations present. Because of the increasing diagnosis of DCIS by mammography screening and the debate over how DCIS should be managed, there is a clear need to define the molecular events underlying the development of DCIS.
Purpose: Our purpose was to identify patterns of genetic alterations in DCIS.
Methods: A group of 30 formalin-fixed, paraffin-embedded blocks of tissue collected from 1987 through 1989 from 21 patients with DCIS was studied. Chromosomal imbalances were determined by interphase cytogenetic analysis using the fluorescence in situ hybridization (FISH) technique. DNA probes were used that recognize chromosome-specific repetitive sequence loci at the centromeres of chromosomes 1, 3, 4, 6, 7, 8, 9, 10, 11, 16, 17, and 18. FISH was also used to detect ERBB2 gene amplification in DCIS. To complement the FISH studies, microsatellite analysis of markers near the BRCA1 region of chromosome 17 was done on tissue microdissected from multiple areas of DCIS. Chromosomal imbalances were determined by comparisons of chromosomal indices (total number of hybridization spots per total number of nuclei counted) of normal and DCIS tissue, using the two-sided Mann-Whitney test.
Results: Using FISH, we have identified patterns of DNA loss and gain of certain chromosome-specific centromeric markers in DCIS. We observed frequent gains of markers on chromosomes 3, 10, and 17 as well as loss of chromosome 18-specific centromeric sequences. ERBB2 gene amplification was detected in tumors from four of 15 patients studied and was clearly limited to the tumor cells within the ducts. Because of the availability of topologically distinct regions of tumors from individuals, we were able to show that paired tumor specimens from individuals share genetic alterations and also have unique ones, suggesting clonal diversity within tumors. The combination of FISH and microsatellite analyses suggested that alterations in chromosome 17 may be quite complex; three of five patients whose samples were analyzed had allelic imbalance at markers on the long arm of chromosome 17.
Conclusions: FISH and microsatellite analyses are useful in detecting extensive genetic alterations in DCIS. Examinations of DCIS tissue using these techniques have identified chromosomes 1, 3, 10, 16, 17, and 18 as candidate sites worthy of immediate study.
Implications: This approach may give direction to future research aimed at precisely mapping loci altered in DCIS and help in understanding the biologic events associated with tumor progression or recurrence.