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Human Cytogenetics Laboratory, Imperial Cancer Research Fund, London WC2A 3PX [R. R., P. G., D. S.], and Hedley Atkins/ICRF Breast Pathology Laboratory, Guys Hospital, London SE1 9RT [W. H., R. L., D. B., A. H.], United Kingdom
| ABSTRACT |
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| Introduction |
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Up to 70% of breast carcinomas are classified as ductal carcinoma of no special type. In a system originally devised by Scarff, Bloom, and Richardson, histological assessment of nuclear pleomorphism, mitotic activity, and tubule formation allows further subdivision by grade (6) . Using such a classification, grade I tumors have well differentiated attributes and grade III tumors have poorly differentiated attributes, whereas grade II tumors fall into an intermediate category. Although these phenotypic appearances provide a guide to tumor behavior (6) , they do not necessarily relate directly to biological progression. Indeed, there is considerable controversy regarding biological progression. One hypothesis suggests that as tumors grow they become dedifferentiated in terms of grade (i.e., grade I tumors become grade III tumors over time; Ref. 7 ). We set out to test the pathological hypothesis of dedifferentiation by taking a novel genetic approach. CGH2 was used to screen the entire genomes of 90 invasive ductal breast carcinomas, which were stratified by grade (40 grade I and 50 grade III tumors). If dedifferentiation occurs, we expected to find genetic changes in grade I tumors that were a subset of those found in grade III tumors.
| Materials and Methods |
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CGH.
CGH was performed with minor modifications of a standard protocol (8)
. Briefly, tumor and reference DNA were labeled by nick translation. Approximately 1 µg each of fluorescein-labeled tumor and Texas red-labeled reference DNA were coprecipitated in the presence of 50 µg of human Cot I DNA and resuspended in hybridization mix [50% formamide, 10% dextran sulfate, and 2 x SSC (pH 7.0)] before hybridization to denatured human metaphase chromosome spreads, prepared from phytohaemaglutinin-stimulated lymphocytes from normal individuals. After hybridization for 72 h, the slides were washed and counterstained with 4,6-diamidino-2-phenylindole, which gives a banding pattern resembling G-bands. Images were captured with a cooled charge-coupled device camera attached to a Zeiss axioskop microscope and then analyzed using Quips software (Vysis, Inc., Downers Grove, IL). Between 510 metaphases were analyzed for each tumor. A chromosome region was considered to be lost if the mean hybridization ratio between tumor and normal was <0.85:1 or gained if the ratio was >1.15:1. These cutoff values were determined based on negative control hybridizations (with normal male versus normal female DNA), which showed the mean hybridization ratio, and its SD remained within these limits for all of the autosomes. For the X chromosome, the mean hybridization ratio was below 0.6. Negative control hybridizations were included in each batch of experiments.
| Results and Discussion |
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Loss of 16q clearly plays an important role in tumorigenesis, although the specific function played by deletion of this region is uncertain. Loss of heterozygosity studies of primary breast tumors have identified three minimal regions of deletion (16q22.1, 16q23.224.1, and 16q24.3), suggesting the presence of possible tumor suppressor genes (11) . A candidate gene for the region 16q24.3 is the cadherin gene, CDH15 (12) , although its expression in breast tissue is not yet confirmed. Within the 16q2122.1 region there are an additional five cadherin genes (12) . Because disturbance of cell-cell interactions, in which the cadherins are involved, is necessary for invasion and metastasis of tumor cells, all of the cadherins are potential tumor suppressor genes. Of particular interest is the cadherin gene CDH1, which maps to 16q22.1. Although it lies outside the minimal area of deletion (13) , it is known to play an important role in lobular breast carcinomas (14) , whereas its role in ductal carcinomas remains to be established. Another candidate tumor suppressor gene for the region 16q22.1 is CTCF (13) , which has recently been identified and shown to encode a transcription factor with binding sites in a number of genes including MYC and the POLO-like kinase genes. It is envisaged that abnormalities of CTCF could lead to dysregulated expression of these target genes, resulting in a positive effect on cell proliferation.
Previous studies have led to the belief that loss of 16q is an early change in breast tumorigenesis. Some evidence for this is from CGH analyses, which have shown 16q loss occurs either as a sole change or in association with only a few chromosomal changes (9) . Our findings in grade I tumors confirm these earlier results by showing that loss of 16q occurs predominantly in association with gain of 1q, and in association with only a few other changes (mean number of changes, 3.9), although it was never a sole abnormality in our study. The finding of 16q loss in association with 1q gain is interesting because this is consistent with cytogenetic data that have found der(1;16)(q10;p10) unbalanced translocations in breast cancer (15) . However, the grade III tumors that showed 16q loss had a completely different pattern with many other chromosomal abnormalities present (mean number of changes, 15.5), and none with just gain of 1q, thus, suggesting that 16q loss may be an early change only in grade I tumors and that other genetic changes are important for most grade III tumors.
Gains of 8q, 17q, and 20q are changes that have been reported previously as frequent findings in breast cancer (16)
. However, we found these to be frequent changes only in grade III tumors (see Fig. 2
). In grade I tumors, gains of 17q and 20q were seen in only four tumors each. Interestingly, gains of 8q and 20q occurred predominantly in grade III tumors, which are known to have a poorer prognosis (6)
, supporting previous findings correlating these changes with a shorter overall survival and more aggressive phenotype (17)
. The findings in grade I tumors of predominant loss of 16q without high frequency gain of 8q and 20q are very similar to changes found in lobular breast carcinomas (18)
. This leads us to speculate that at the molecular level the origins of these apparently morphologically different breast tumors may be very similar.
Our findings have important implications for the biological study of breast tumors. Most genetic studies to date have analyzed breast tumors as a homogeneous group, without specifically considering grade. As the present study shows, a prerequisite for a meaningful description of grade-specific changes is the selection of a large number of tumors for study. Grade I tumors comprise only 12% of all invasive ductal breast carcinomas (6) . Changes that are very prevalent within a subgroup may, therefore, be missed, or their significance diluted, when the whole group is considered. This may explain why previous CGH studies have found loss of 16q ranging from 2438% in breast tumors (5 , 9 , 17) and why the importance of gains of 17q and 20q have been overestimated.
In conclusion, this study shows the use of taking a genetic approach to make inferences about the biological progression of breast cancer. By studying a large sample stratified by histological grade, our findings show that the majority of grade I tumors do not progress to grade III tumors. Breast cancer seems to show a more complex biological behavior than simple progression through histological grades. Therefore, different grades may present discrete therapeutic targets. We suggest that a complete understanding of the genetic events that underlie breast tumorigenesis requires a full appreciation of the relationship between genetic events and histological grade.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at Human Cytogenetics Laboratory, Imperial Cancer Research Fund, P.O Box 123, Lincolns Inn Fields, London WC2A 3PX, United Kingdom. Phone: 44-171-269-3220; Fax: 44-171-269-3655; E-mail: sheer{at}icrf.icnet.uk ![]()
2 The abbreviation used is: CGH, comparative genomic hybridization. ![]()
Received 12/11/98. Accepted 2/15/99.
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