Stem cell-like transcriptional reprogramming mediates metastatic resistance to mTOR inhibition

Inhibitors of the mechanistic target of rapamycin (mTOR) are currently used to treat advanced metastatic breast cancer. However, whether an aggressive phenotype is sustained through adaptation or resistance to mTOR inhibition remains unknown. Here, complementary studies in human tumors, cancer models and cell lines reveal transcriptional reprogramming that supports metastasis in response to mTOR inhibition. This cancer feature is driven by EVI1 and SOX9. EVI1 functionally cooperates with and positively regulates SOX9, and promotes the transcriptional upregulation of key mTOR pathway components (REHB and RAPTOR) and of lung metastasis mediators (FSCN1 and SPARC). The expression of EVI1 and SOX9 is associated with stem cell-like and metastasis signatures, and their depletion impairs the metastatic potential of breast cancer cells. These results establish the mechanistic link between resistance to mTOR inhibition and cancer metastatic potential, thus enhancing our understanding of mTOR targeting failure.

Abstract P2-10-01: The BRCA-1 polymorphism (major homozygous rs5820483) is associated with higher expression of phosphorylated -IGF-1 receptor

Background: Data from the Consortium of Investigators of Modifiers of BRCA1/2 (CIMBA) indicates that common variation in the BRCA1 locus modifies the penetrance of mutations. The rs5820483 variation is one such polymorphism in which patients homozygous for the major allele have a greater risk of breast cancer. The BRCA1 gene contains 24 exons that generate a full-length (FL) protein. Major alternatively spliced isoforms include skipping of exon 11 (Δ11) and a truncated form (IRIS), each of which appear to have distinct roles in cellular processes. rs5820483 is the only variant that maps to intron 10, flanking exon 11. Experimental models suggest that deregulation of the FL/D11 isoform ratio can have profound functional consequences. We have shown that cells from patients with the major rs5820483 allele express more FL relative to Δ11 compared to cells from patients with the minor allele. Breast cancer exhibit increased IGF-1 and IGF-1 activity in patients with BRCA1 mutations relative to breast cancer in women without BRCA1 mutations. IGF-1 produced in the mammary stroma mediates estrogen dependent proliferation. Here we test whether the rs5820483 allele affects IGF-1 activity in breast tissue of BRCA1 mutation carriers.

Design: We assessed the zygosity of the rs5820483 alleles in 28 BRCA1 mutation carriers. Phosphorylated IGF-1 receptor (p-IGF-1R rabbit polyclonal Ab39398, Abcam, MA) was measured by immunofluorescence and Ki-67 was measured by immunohistochemistry in sections from histologically normal breast tissue from these cases were compared to heterozygous controls. Image analysis was used to assess the intensity of the p-IGF-1R immunofluorescence at the epithelial cell membrane with appropriate controls. Ki67 mmunohistochemistry was also performed on these specimens.

Results:The rs5820483 allele was heterozygous in 10 specimens, homozygous for the major allele in 11 and homozygous for the minor allele in 7. The intensity of p-IGF-1R was higher in major homozygous cases (140±54 SD) than in either the minor homozygous (99±24) or heterozygous cases (95±44). The frequency of Ki-67+ cells was higher in the major homozygous case (3.1±2.9 SD) than in either the minor homozygous (1.5±1.9 SD) or heterozygous cases (2.2±2.5 SD). However, neither p-IGF-IR immunoreactivity nor frequency of Ki-67+ cells was statistically different between groups.

Conclusion: Breast cancer risk in BRCA1 mutation carriers is modified by common genetic variation at the corresponding locus. We have identified a variation at rs5820483 that affects the isoform ratio. Our preliminary analysis suggests that IGF-1 activity increases in those women homozygous for the major allele, concomitant with increased Ki-67. Corroboration of this analysis in a larger series is ongoing. A statistically significant difference might have fundamental implications for cancer prevention in those carriers.

Citation Format: Singh B, Bochaca II, Ruiz de Garibay G, Damiola F, Mazoyer S, Antoniou A, Chenevix-Trench G, Pujana MA, Barcellos-Hoff MH, Kleinberg D. The BRCA-1 polymorphism (major homozygous rs5820483) is associated with higher expression of phosphorylated -IGF-1 receptor. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P2-10-01.