Abstract PD9-11: Platelet derived growth factor receptor-β signaling: A novel therapeutic target for breast cancer associated brain metastasis

PDGFRβ is a receptor tyrosine kinase found in cells of mesenchymal origin such as fibroblasts and pericytes. Activation of this receptor is dependent on paracrine ligand induction, and its preferred ligand, PDGFB, is released by neighboring epithelial and endothelial cells. While expression of both PDGFRβ and PDGFB has been noted in patient breast tumors for decades, how PDGFB-to-PDGFRβ tumor-stromal signaling mediates breast cancer initiation, progression, and metastasis remains unclear. To test this important research question, we developed a mouse model of mesenchymal-specific PDGFRβ hyper-activation. PDGFRβ mutant mammary glands exhibit increased tertiary side-branching and epithelial proliferation confirming a stromal-specific PDGFRβ effect on neighboring epithelium during normal development. To test the effect of hyper-active mesenchymal PDGFRβ on disease progression, experimental tail vein metastasis assays were performed where we observed prominent brain metastases in 50% of the PDGFRβ mutantmice (n=5/10) with no brain lesions seen in controls (n=0/19). There was no difference in the incidence of lung or liver metastases in the mutant mice suggesting a pro-metastatic function for PDGFRβ in the brain metastatic niche. To rule out dysfunction of the blood brain barrier contributing to the observed metastatic spread, we then intracranially injected mammary tumor cells, and as expected based on our metastasis assay, found that larger tumors formed in the brains of PDGFRβ mutant mice versus controls. To our knowledge, these combined findings are the first example where genetic manipulation of the stroma increases breast cancer associated brain metastases (BCBM). Given that these pre-clinical data suggest that primary breast tumors expressing high PDGFB could preferentially metastasize to the brain, we analyzed PDGFB protein expression in a tissue microarray comprised of HER2-positive and triple negative breast cancer (TNBC) primary tumors (total n=425). While high PDGFB did not correlate with site-independent metastatic recurrence, it was prognostic of brain metastasis, mirroring our mouse data. Evaluation of PDGFB in a small cohort of matched primary breast tumors with associated brain (n=5) and lung metastases (n=2) revealed intense PDGFB staining in 100% of the brain metastases, but only 50% of the lung metastases. These findings further suggest that high primary tumor PDGFBexpression defines a subset of breast cancer patients predisposed to brain metastases and that these patients may benefit from therapeutic inhibition of PDGFRβ signaling. To test this pre-clinically, we treated mice harboring intracranial tumors with the PDGFR specific inhibitor crenolanib. Excitingly, crenolanib treatment significantly inhibited the brain tumor burden in these mice. Combined, our findings to date (1) advocate that primary tumor expression of PDGFB is a novel prognostic biomarker for the development of BCBM and (2) support clinical trial evaluation of PDGFR inhibitors for the prevention and treatment of BCBM. Ongoing studies are evaluating how the PDGFRβ-expressing mesenchymal cells within the brain promote a pro-metastatic niche.

Citation Format: Sizemore GM, Hammer AM, Thies KA, Hildreth BE, Russell LO, Sizemore ST, Trimboli AJ, Kladney RD, Steck SA, Das M, Bolyard CM, Pilarski R, Schoenfield L, Otero J, Chakravarti A, Ringel M, Kaur B, Leone G, Ostrowski MC. Platelet derived growth factor receptor-β signaling: A novel therapeutic target for breast cancer associated brain metastasis [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD9-11.

Abstract P3-03-01: Clinical utility of finding pathogenic mutations beyond BRCA1/2 in breast cancer patients

Background

The clinical utility of germline genetic testing for BRCA1 and BRCA2 has long been established. However, management recommendations for pathogenic variants in other genes, typically included in multigene panels, have only recently been included in consensus guidelines for HBOC. The clinician actions implemented for findings in these genes, and patient follow-up, are not yet well studied. We report interim results from a multi-site study of clinical actions undertaken in patients presenting with HBOC and carrying a pathogenic germline mutation in cancer risk genes other than BRCA1/2.

Methods

We retrospectively examined a cohort of patients with a personal history of HBOC who had been referred for hereditary cancer multigene testing from three major academic medical centers. For patients with pathogenic findings in a non-BRCA1/2 cancer risk gene, ordering clinicians completed a short case report form describing the clinical actions taken in response to the genetic test result, and patient follow-up. Some patients were lost to follow-up and answers of “unknown” were permitted. Genes with positive findings included CHEK2, PALB2, ATM, MUTYH, RAD51C, TP53, MSH6, RAD50, APC, BARD1, BRIP1, MSH2, NF1, NBN, PMS2, and PTEN. Case report forms were available for 77 patients as of our cut off date, and these data were de-identified and summarized for this interim report. Additional cases continue to accrue in this ongoing study.

Results

In 57% (44/77) of cases, clinicians reported that counseling and/or clinical management recommendations were changed in response to the genetic test findings. Management changes included modification of imaging surveillance (38%), considered or recommended surgical prophylaxis (12%), modified surgical plan for an existing malignancy (5%), and for one patient each: inclusion in a research trial for PARP inhibitors, modification of colonoscopy schedule, and screening for cancers other than existing malignancy. Clinicians indicated that genetic test results changed management in 48% of patients, did not change management in 29%, and had unknown impact for 23%.

Clinicians also reported that counseling and/or management for the patients' family members was changed in 67% (52/77) of cases, including family variant testing. 27% (21/77) of the patient families had cascade genetic testing, and one or more new carriers were identified in 47% (10) of the tested families. In 58% of cases, the impact of management recommendations on family members was unknown as of the case report date.

Conclusions

Pathogenic variants in non-BRCA genes are present in about 3-11% of patients with a history of HBOC. This study suggests that genetic test results in cancer genes beyond BRCA1/2 changed clinical management for a majority of patients and their family members, led to identification of new carriers, and directly impacted treatment decisions. In almost half of these patients, genetic test results impacted their health outcome, including those reported to be disease free after undergoing interventional or prophylactic surgery informed by their genetic variant. More research is needed to improve the implementation of genetic testing based management recommendations for patients and their family members.

Citation Format: Esplin ED, Michalski S, Yang S, Hampel H, Jeter J, Sweet K, Pilarski R, Pearlman R, Shane K, Brock P, Westman J, Chittenden A, Stopfer J, Schneider K, Sacca R, Stickevers S, Kipnis L, Koeller D, Gaonkar S, Sotelo J, Vaccari E, Cochrane S, Champine M, Espinel W, Lincoln SE, Nussbaum RL. Clinical utility of finding pathogenic mutations beyond BRCA1/2 in breast cancer patients [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-03-01.

Comprehensive review of BAP1 tumor predisposition syndrome with report of two new cases

The BRCA1-associated protein-1 (BAP1) tumor predisposition syndrome (BAP1-TPDS) is a recently identified hereditary cancer syndrome. Germline mutations in this tumor suppressor gene predispose families to the development of various malignancies. The molecular functions of the gene as well as the clinical phenotype of the syndrome are still being clarified. We sought to conduct a comprehensive review of published research into BAP1-TPDS to more thoroughly delineate the clinical implications of germline BAP1 mutations. We also report two additional families with germline BAP1 mutations. Current evidence demonstrates that germline BAP1 mutations predispose families to uveal melanoma, renal cell carcinoma, malignant mesothelioma, cutaneous melanoma, and possibly to a range of other cancers as well. Some of these cancers tend to be more aggressive, have a propensity to metastasize, and onset earlier in life in patients with BAP1 mutations as compared to non-predisposed patients with equivalent cancers. Although further research is necessary, this information can aid in the management, diagnosis, and therapy of these patients and their families, and highlights the importance of genetic counseling.

Subset of individuals with autism spectrum disorders and extreme macrocephaly associated with germline PTEN tumour suppressor gene mutations

The genetic aetiology of autism remains elusive. Occasionally, individuals with Cowden syndrome (a cancer syndrome) and other related hamartoma disorders such as Bannayan-Riley-Ruvalcaba syndrome, Proteus syndrome, and Proteus-like conditions, are characterised by germline PTEN mutations, and may have neurobehavioural features resembling autism as well as overgrowth and macrocephaly. Therefore, we undertook PTEN gene mutation analysis in 18 subjects mainly prospectively ascertained with autism spectrum disorder and macrocephaly. Of these 18 autistic subjects (13 males and five females; ages 3.1-18.4 years) with a head circumference range from 2.5 to 8.0 standard deviations above the mean, three males (17%) carried germline PTEN mutations. These three probands had previously undescribed PTEN mutations: H93R (exon 4), D252G (exon 7), and F241S (exon 7). They had the larger head circumference measurements amongst all our study subjects. The three residues altered in our patients were highly evolutionarily conserved. We suggest that PTEN gene testing be considered for patients with autistic behaviour and extreme macrocephaly. The gene findings may impact on recurrence risks as well as medical management for the patient.