Breast Cancer Cytogenetics: Clues to Genetic Complexity of the
Marilyn L. Slovak, Ph.D.* and Sandra R. Wolrnant Department of Cytogenetics, City of Hope National Medical Center,
Duarte, California and tOncor, Inc., Gaithersburg, Maryland
enetic aberrations in cancer have become a pri- G mary focus for understanding the pathogenesis of neoplasia. In this respect, cancer cytogenetic observa- tions have been pivotal to studies of specific genetic alter- ations that contribute to tumor development and progres-
sion in neoplasia. Cytogenetic analysis has spearheaded the localization, identification, and cloning of genes crit- ical to the development of hematopoietic and small, round cell tumors of childhood. Today, cytogenetic in- vestigations are yielding insights into epithelial tumor initiation and progression as well. In fact, 6% of the solid tumor cytogenetic studies listed in the 1994 Cutu- log of Chromosome Aberrations in Cancer (1) (the ma- jor data bank for neoplastic cytogenetic abnormalities) are derived from breast tumors, a figure very different from the fewer than 70 tumors in the 1988 edition.
Characteristics inherent in breast cancers (e.g., heter- ogeneity, slow growth) have delayed identification of breast-cancer-specific cytogenetic associations. Other features that have hampered recognition of common cy- togenetic alterations include tumor necrosis, low mitotic activity, the outgrowth of stromal elements, relatively few good quality metaphases, and highiy complex rear- rangements in tumor cells. The published cytogenetic
Address correspondence and reprint requests to: Marilyn L. Slovak, Ph.D., Department of Cytogenetics, City of Hope National Medical Center, Northwest Building, Room 2255, 1500 E. Duarte Road, Duarte, CA 91010- 3000, U.S.A.
0 1996 Blackwell Science Inc., 107S-l22Xl96/$10.SOlO The Breast Journal, Volume 2, Number 2, 1996 124-140
studies of breast cancer before 1985 often examined tu- mor cells at advanced stages of disease or were based on data gleaned from cell lines established from pleural ef- fusions. Few clinicopathological correlations were avail- able. These studies described numerous and complex karyotypic aberrations defining multiple related clones and provided little evidence pointing toward the possi- ble primary (initiation) or secondary (progression) ge- netic events in breast cancer pathogenesis. The extraor- dinary diversity of chromosomal aberrations with high intra- and intertumor variability made interpretation of their clinical and biological significance in breast tumors difficult.
Additionally, diploid primary breast tumors that re- tained their invasive capacity after a week in culture have been reported, suggesting that at least some pri- mary breast tumors are characterized by apparently dip- loid karyotypes. Differences in the results of breast can- cer cytogenetic analyses (i.e., cytogenetically normal diploid tumors versus aneuploid tumors) were often at- tributed to problems of methodology. Caution had to be exercised in interpretation of chromosome changes de- rived from tumor tissue maintained in vitro. Cytogenetic studies of benign tumors and early breast cancers, which are needed to establish the primary karyotypic events re- lated to the disease, were essentially nonexistent prior to 1985 because of procedural limitations. Not only the technical limitations, but also difficulties in interpreta- tion of results, and the uncertainty of the role of the chromosome/genetic alterations in breast disease were major considerations. Technical improvements account
Breast Cancer Cytogenetics 125
for many of the recent data that have increased our un- derstanding of the genetics of breast cancer.
Even though the pre-1985 cytogenetic studies were of limited value, they raised some very important biologic questions: (a) Do the multiple clones observed in vitro also occur in vivo? (b) What are the short-term versus long-term effects of cell culture on the results of chro- mosome studies? (c) Will methodological advances re- flect the true genetic changes in breast cancer? (d) Do cy- togenetically normal diploid breast tumors exist? and (e) Do the nonrandom break points observed in advanced breast cancers localize to sites in the genome that iden- tify genes relevant to mechanisms of origin, progression, and clinical behavior of breast tumors? These questions acknowledge that breast cancer is a complex, polygenic disease (2) that will require merging of information from many disciplines to permit a broad overview of the tumor, incorporating data on the genetic makeup of in- dividual tumor cells, specific gene alterations, clonal evolution of disease, intratumor heterogeneity, and in- tertumor heterogeneity. Thus, the early breast cancer cy- togenetic studies essentially outlined the need and, thus, laid the foundation for a systematic, multiparameter ap- proach to our current studies of the disease.
At the cellular level, classic cytogenetic studies and DNA content by flow cytometry or image analysis are
Table 1. Comparisons of Genetic Techniques
the methods of choice to determine overall genetic changes, whereas individual gene mutations, deletions, and amplifications are best investigated by molecular strategies (Table 1). Each aspect of genetic analysis has its advantages and limitations. The chief advantage of classic cytogenetic analysis is that it is currently the only genetic method that provides an overview of the com- plexity of the genetic changes in individual tumor cells, and best illustrates intratumor heterogeneity and clonal evolution. Although cytogenetic alterations do not ex- plain what is happening at the genetic level, they focus attention on areas where critical genes may be found and thus lead to the development of molecular genetic assays.
Molecular testing is not the procedure of choice to describe events in individual tumor cells. Although mo- lecular tests are highly specific, the information gleaned is limited to the selected genetic aberration being tested and reveals only the composition of an idealized average tumor cell. After a recurring karyotypic aberration is de- fined within a tumor, restriction fragment length poly- morphism (RFLPs) analysis using a set of polymorphic markers for that targeted chromosomal region is per- formed. Tumor DNA is compared to the constitutional genotype and an allelotype describing the loss of het- erozygosity (LOH) is generated. This approach, how-
Techniaues Strenaths Weaknesses Resolution
Molecular Defines selected genetic aberrations in cell population
Sensitive/specific Evaluation of minimal residual disease for
individual gene mutations, deletions, and amplification
~l~~~~~~~~~~ in situ Defines complex rearrangements ID abnormalities in interphase Associate genetic alteration with morphology/or
May use archived tissues
Detects DNA amplified sequences within tumor
No need for specific probes No need of prior knowledge of aberrations
Comparative genomic hybridization (CGH) genome
Cytogenetics Overview of genetic changes in individual cells ID intratumor heterogeneity ID clonality Defines target regions
Estimate DNA content and proliferative (5- phase)
Aneuploid detection in paraffin-embedded, fresh,
Flow Cytometry fraction
frozen, and formalin-fixed
Limited to specific probe (gene) alterations 0.2-50 Kb No evaluation of intratumor cell heterogeneity or
May need polymorphic (informative) markers Tumor clone 325% Need consitutional DNA
Limited probe availability >2.5 Kb
?>2 Mb Little data regarding intratumor heterogeneity Requires >5 t o 7-fold amplification for detection Compromised by normal cell contamination No data regarding point mutations, transcriptional
activation, or chromosomal translocation
Limited genic level data 2-20 Mb Requires mitotic cells Selection due to in vitro culturing
No specific genetic alterations defined Low sensitivity Loss or gain of small chromosomes not detectable False aneuploidy due t o fixation, stain variations, or
Chromosome number 2 2