There are a number of distinct cellular and molecular characteristics of the SUM-44PE cell line that emerge from the work from our lab and others and that come from an overview analysis of the screen hit data. What I’ve described below is based on published data and the papers can be found in the SUM-44 bibliography. The key papers from the Ethier lab in which the data for the findings discussed below are listed in the SUM-44 Bibliography page.
First, as noted previously, SUM-44 cells were obtained from a patient with lobular breast cancer, and indeed, these cells possess the pathognomonic CDH1 (E-Cadherin) mutation. Other key molecular alterations present in these cells include the amplification of genes in the 8p11-p12 region of the genome, amplification of the 11q14 region of the genome containing the CCND1 gene (Cyclin D1), and amplification of the NMYC gene (really!). In addition, these cells are TP53 wild-type and ESR1 wild-type, although recent published work from the Dowsett lab showed that there is a minority of SUM-44 cells (approximately 1 per million) that harbor an ESR1 point mutation, and these cells emerge when cultured for long times in estrogen-deprived conditions. Finally, SUM-44 cells express very high levels of ESR1 mRNA and ERa protein levels and the estrogen receptor is functional, but independent of exogenous estrogen. These cells exhibit the expression profile of a luminal B breast cancer.
We and others have worked extensively to characterize the transforming oncogenes present in the 8p11-p12 amplicon in these cells and this work has elucidated the following:
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NSD3 (WHSC1L1) is a transforming oncogene from the region in these cells based both on in vitro results and the recent development of an NSD3 transgenic mouse which develops mammary gland dysplasias and mammary carcinoma.
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KAT6A is a driving oncogene from the region in these cells and is part of the functional oncogene signature for these cells
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EIF4EBP1 is a driving oncogene from the region in these cells as this gene is amplified, overexpressed, and was a hit in the screen.
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FGFR1 while being amplified and overexpressed at the mRNA level, appears to play no role in the proliferative potential of these cells. While FGFR1 is present in the amplicon in SUM-44 cells, we were not able to detect FGFR1 protein expression, and a targeted inhibitor of FGFR kinase activity had no effect on the growth of the cells, nor was FGFR1 a hit in the functional screen. For more on this topic, check out my blog titled, “The curious case of FGFR1“.
Based on all of these results and those from other labs we conclude that the 8p11-p12 amplicon acts as functional transforming unit in these breast cancer cells, and that genes in the amplicon play a direct role in the high level expression of the estrogen receptor in these cells. Amplification and overexpression of NSD3 in breast cancer is often associated with overexpression of ERa, and recent studies have confirmed the association between the 8p11-p12 amplicon in aggressive ERa positive breast cancer in patients who fail aromatase inhibitor therapy. Thus, the SUM-44PE cell line is a good model for this type of luminal B breast cancer.
Analysis of the screen hit data in the KEGG pathways also provides interesting insights into this cell line. As indicated above, there is little or no evidence that these cells are regulated in proliferation by classical growth factor signaling pathways, and as a result, there were very few hits in the MAPK signaling pathway. That said, it is interesting that FGF13 was a hit in the screen, though this hit has not yet been validated.
Likewise, the PI3’K/AKT pathway has relatively few hits compared to other cell lines (compart to, for example, SUM-185 which has many). Furthermore the PI3’K activity that is present appears to be driven by isoforms that we don’t often consider. In this case, PIK3R2 and PIK3CD were screen hits. Also in this pathway, one can see that BCL2 was a screen hit. This result is not surprising in that BCL2 has been shown by others to be the major cell survival gene in ER-positive luminal breast cancer.
The screen-hit genes that map to the WNT pathway also strongly point to a role of canonical WNT signaling in these cells. In this case, WNT8A and SFRP5 were screen hits as were other genes in the pathway leading to FRA1 as a screen hit. It is interesting that none of the genes in either of the non-canonical WNT pathways were screen hits.
Finally, a few words about the genes that were hits in the screen that map to the cytokine-cytokine receptor interaction map and the ECM receptor interaction map are worth mentioning as once again, a similar pattern was observed for many of the SUM lines. As can be seen, eleven different cytokines or cytokine receptors, such as CCL4, CXCL4, CCL1 and others, were hits in this functional screen. In addition, two different collagens and two different integrins were also screen hits. These results show that SUM-44 cells synthesize a number of cytokines, cytokine receptors and matrix molecules that play a role in their proliferation or survival. These results suggest a complexity with respect to autocrine mediators of cancer cell growth and survival in vitro that has not been appreciated. For more on this topic, watch for the Blog post on why some cancers yield cell lines while most do not.
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