Intertumor heterogeneity in vascularity and invasiveness of artificial melanoma brain metastases

The origin of brain malignancies at the blood-brain barrier

Despite improvements in extracranial therapy, survival rate for patients suffering from brain metastases remains very poor. This is coupled with the incidence of brain metastases continuing to rise. In this review, we focus on core contributions of the blood-brain barrier to the origin of brain metastases. We first provide an overview of the structure and function of the blood-brain barrier under physiological conditions. Next, we discuss the emerging idea of a pre-metastatic niche, namely that secreted factors and extracellular vesicles from a primary tumor site are able to travel through the circulation and prime the neurovasculature for metastatic invasion. We then consider the neurotropic mechanisms that circulating tumor cells possess or develop that facilitate disruption of the blood-brain barrier and survival in the brain's parenchyma. Finally, we compare and contrast brain metastases at the blood-brain barrier to the primary brain tumor, glioma, examining the process of vessel co-option that favors the survival and outgrowth of brain malignancies.

Breast Cancer Brain Metastases: Implementation and Characterization of a Mouse Model Relying on Malignant Cells Inoculation in the Carotid Artery

Breast cancer (BC) brain metastases (BCBM) is a severe condition frequently occurring in the triple-negative subtype. The study of BCBM pathogenesis and treatment has been hampered by the difficulty in establishing a reliable animal model that faithfully recapitulates the preferential formation of brain metastases. The injection of BC cells in the carotid artery of mice has been proposed but the procedure is challenging, with the metastatic pattern being scarcely characterized. In this work, we thoroughly describe an improved procedure, highlighting the tricks and challenges of the process, and providing a characterization of the brain and peripheral metastatic pattern at the cellular and molecular level. Triple-negative BC (4T1) cells were inoculated in the common carotid artery of BALB/c mice. Brains and peripheral organs were harvested at 7-14 days for the histological characterization of the metastases' pattern and the immunofluorescence analysis of specific markers. With our surgical procedure, both mouse death and procedure-associated weight loss were negligible. Brain metastases mostly occurred in the hippocampus, while sparse peripheral lesions were only detected in the lungs. Brain-colonizing BC cells presented proliferative (Ki-67) and epithelial (pan-cytokeratin and tomato lectin) features, which account for metastases' establishment. The presented surgical approach constitutes an important and reliable tool for BCBM studies.

Recurrence benefit from supramarginal resection in brain metastases of lung adenocarcinoma

Background

There is growing evidence that brain metastases (BM) have no well-defined boundaries and that conventional microsurgical circumferential dissection of BM is often inadequate to prevent local tumor recurrence. Previous studies have suggested that supramarginal resection can significantly improve local tumor control. We retrospectively analyzed the local tumor control in a series of patients with BM from lung adenocarcinoma.

Methods

We retrospectively analyzed 48 patients with BM for lung adenocarcinoma in Shenzhen Second People’s Hospital from May 2015 to May 2020. 26 resected lesions were located in eloquent areas and underwent standard gross total resection (GTR group); 22 resected lesions were located in ineloquent areas, after standard gross total resection, the periphery was expanded and resected by 5 mm (MTR group). The postoperative tumor recurrence was compared between the two groups.

Results

During the follow-up period, the local recurrence rates in the GTR group and the MTR group were 61.5% and 27.3% (p = 0.022), respectively. Within 6 months after surgery, the local recurrence rates in the GTR group and the MTR group were 42.3% and 13.6% (p = 0.029), respectively. Within 12 months after surgery, the local recurrence rates in the GTR group and the MTR group were 57.7% and 22.7% (p = 0.014), respectively. The median progression-free survival time after surgery was 7.0 months (95% CI 4.0–10.0 months) in the GTR group and 14.0 months (95% CI 11.4–16.6 months) in the MTR group (Log-Rank p = 0.008). Compared with the MTR group, the HR of local recurrence in the GTR group was 3.74 (95% CI 1.38–10.39, p = 0.010). Cox multivariable analysis showed no other factors associated with local recurrence except for the surgical method (p = 0.012).

Conclusions

On the basis of conventional surgical total resection, expanded peripheral resection of 5 mm around the brain metastases of lung adenocarcinoma can significantly reduce the local recurrence rate and prolongs the progression-free survival time.

Clinical outcomes of multifocal papillary thyroid cancer: A systematic review and meta‐analysis

Objective

Methods

Results

Level of Evidence

Conclusion

Identification of Transcriptional Heterogeneity and Construction of a Prognostic Model for Melanoma Based on Single-Cell and Bulk Transcriptome Analysis

Melanoma is one of the most aggressive and heterogeneous life-threatening cancers. However, the heterogeneity of melanoma and its impact on clinical outcomes are largely unknown. In the present study, intra-tumoral heterogeneity of melanoma cell subpopulations was explored using public single-cell RNA sequencing data. Marker genes, transcription factor regulatory networks, and gene set enrichment analysis were further analyzed. Marker genes of each malignant cluster were screened to create a prognostic risk score, and a nomogram tool was further generated to predict the prognosis of melanoma patients. It was found that malignant cells were divided into six clusters by different marker genes and biological characteristics in which the cell cycling subset was significantly correlated with unfavorable clinical outcomes, and the Wnt signaling pathway-enriched subset may be correlated with the resistance to immunotherapy. Based on the malignant marker genes, melanoma patients in TCGA datasets were divided into three groups which had different survival rates and immune infiltration states. Five malignant cell markers (PSME2, ARID5A, SERPINE2, GPC3, and S100A11) were selected to generate a prognostic risk score. The risk score was associated with overall survival independent of routine clinicopathologic characteristics. The nomogram tool showed good performance with an area under the curve value of 0.802.

Molecular Profiles of Brain Metastases: A Focus on Heterogeneity

Simple Summary

Precision cancer medicine depends on the characterization of tumor samples, usually by a single-tumor biopsy, to administer an optimal therapeutic. However, primary tumors and their metastases are often heterogeneous. A metastatic lesion may harbor a completely different genetic makeup to that of its parent tumor, and a single tumor sampling may be ineffective in selecting the most efficient therapy. Brain metastases, due to their low availability and specific microenvironment, pose a particular challenge for precision medicine. In this review, we highlight the genetic landscape of brain metastases, with a particular focus on their heterogeneity. To illustrate this problem, we present phenotypic alterations in brain metastases originating from lung cancer, breast cancer, and melanoma. This article may help clinicians better understand alterations in brain metastases and the relevance of their heterogeneity.

Abstract

Brain metastasis is a common and devastating clinical entity. Intratumor heterogeneity in brain metastases poses a crucial challenge to precision medicine. However, advances in next-generation sequencing, new insight into the pathophysiology of driver mutations, and the creation of novel tumor models have allowed us to gain better insight into the genetic landscapes of brain metastases, their temporal evolution, and their response to various treatments. A plethora of genomic studies have identified the heterogeneous clonal landscape of tumors and, at the same time, introduced potential targets for precision medicine. As an example, we present phenotypic alterations in brain metastases originating from three malignancies with the highest brain metastasis frequency: lung cancer, breast cancer, and melanoma. We discuss the barriers to precision medicine, tumor heterogeneity, the significance of blood-based biomarkers in tracking clonal evolution, the phylogenetic relationship between primary and metastatic tumors, blood–brain barrier heterogeneity, and limitations to ongoing research.

High-resolution MRI demonstrates that more than 90% of small intracranial melanoma metastases develop in close relationship to the leptomeninges

BACKGROUND

Despite classic teaching that intracranial metastases typically arise at the gray-white matter junction, small intracranial melanoma metastases (IMM) are frequently observed at the interface between the cortex and leptomeninges (ie, "corticomeningeal interface"), suggesting possible leptomeningeal origin.

METHODS

MRI brain examinations of melanoma patients treated at a specialist oncology center from July 2015 to June 2017 were retrospectively reviewed. The MRI examination on which IMM were first visible was identified, utilizing 1 mm volumetric postcontrast imaging prior to local therapy. Individual metastases (up to 10 per patient) were assessed for the presence of leptomeningeal contact, as well as their number, size, and morphology. Lesions ≥10 mm in long axis were excluded, in order to examine early metastatic disease.

RESULTS

Seventy-five patients had evidence of IMM. Fifteen patients had only lesion(s) measuring ≥10 mm at diagnosis, leaving 60 patients. One hundred ninety-two individual metastases were examined (median 2 per patient; interquartile range, 1-4), 174 (91%) demonstrating leptomeningeal contact. A nodular morphology was observed in 154 of 192 (82%), 32 (17%) were ovoid but elongated along the cortex, and 6 (3%) were linear. Only 3 patients (5%) also exhibited a "classic" linear leptomeningeal disease appearance.

CONCLUSIONS

Most IMM measuring between 2 and 9 mm in diameter are corticomeningeal nodules. These data raise the hypothesis that deeper parenchymal extension of IMM occurs secondarily. If the leptomeninges provide a preferential site for establishment of IMM, further investigation of the underlying biology of this phenomenon may provide opportunities for novel therapeutic strategies for patients with IMM.

The Network of Cytokines in Brain Metastases

Brain metastases are the most common of all intracranial tumors and a major cause of death in patients with cancer. Cytokines, including chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors are key regulators in the formation of brain metastases. They regulate the infiltration of different cellular subsets into the tumor microenvironment and affect the therapeutic outcomes in patients. Elucidating the cancer cell-cytokine interactions in the setting of brain metastases is crucial for the development of more accurate diagnostics and efficacious therapies. In this review, we focus on cytokines that are found in the tumor microenvironment of brain metastases and elaborate on their trends of expression, regulation, and roles in cellular recruitment and tumorigenesis. We also explore how cytokines can alter the anti-tumor response in the context of brain metastases and discuss ways through which cytokine networks can be manipulated for diagnosis and treatment.

Bevacizumab treatment of meningeal melanoma metastases

Background

Melanoma patients with metastatic growth in the meninges have poor prognosis and few treatment options. Although treatment with BRAF inhibitors or immune checkpoint inhibitors has provided promising results, most patients with advanced melanoma are resistant to these treatments and develop severe side effects. Novel treatment strategies are needed for patients with meningeal melanoma metastases, and the potential of antiangiogenic therapy was investigated in this preclinical study.

Methods

Two GFP-transfected melanoma models (A-07 and D-12) differing substantially in VEGF-A expression were included in the study, and the anti-VEGF-A antibody bevacizumab was used as therapeutic agent. Meningeal metastases were initiated in BALB/c nu/nu mice by intracranial inoculation of melanoma cells, and bevacizumab treatment was given twice a week in i.p. doses of 10 mg/kg until the mice became moribund. Therapeutic effects were evaluated by determining tumor host survival time, assessing tumor growth and angiogenic activity by quantitative analyses of histological preparations, and measuring the expression of angiogenesis-related genes by quantitative PCR.

Results

Meningeal A-07 melanomas showed higher expression of VEGF-A than meningeal D-12 melanomas, whereas the expression of ANGPT2 and IL8, two important angiogenesis drivers in melanoma, was much higher in D-12 than in A-07 tumors. Bevacizumab treatment inhibited tumor angiogenesis and prolonged host survival in mice with A-07 tumors but not in mice with D-12 tumors. Meningeal A-07 tumors in bevacizumab-treated mice compensated for the reduced VEGF-A activity by up-regulating a large number of angiogenesis-related genes, including ANGPT2 and its receptors TIE1 and TIE2. Melanoma cells migrated from meningeal tumors into the cerebrum, where they initiated metastatic growth by vessel co-option. In the A-07 model, the density of cerebral micrometastases was higher in bevacizumab-treated than in untreated mice, either because bevacizumab treatment increased mouse survival or induced increased tumor gene expression.

Conclusions

The development of antiangiogenic strategies for the treatment of meningeal melanoma metastases is a challenging task because the outcome of treatment will depend on the angiogenic signature of the tumor tissue, treatment-induced alterations of the angiogenic signature, and the treatment sensitivity of metastatic lesions in other intracranial sites.

Vessel co-option and resistance to anti-angiogenic therapy

Vessel co-option is a non-angiogenic mechanism of tumour vascularisation in which cancer cells utilise pre-existing blood vessels instead of inducing new blood vessel formation. Vessel co-option has been observed across a range of different tumour types, in both primary cancers and metastatic disease. Importantly, vessel co-option is now implicated as a major mechanism that mediates resistance to conventional anti-angiogenic drugs and this may help to explain the limited efficacy of this therapeutic approach in certain clinical settings. This includes the use of anti-angiogenic drugs to treat advanced-stage/metastatic disease, treatment in the adjuvant setting and the treatment of primary disease. In this article, we review the available evidence linking vessel co-option with resistance to anti-angiogenic therapy in numerous tumour types, including breast, colorectal, lung and pancreatic cancer, glioblastoma, melanoma, hepatocellular carcinoma, and renal cell carcinoma. The finding that vessel co-option is a significant mechanism of resistance to anti-angiogenic therapy may have important implications for the future of anti-cancer therapy, including (a) predicting response to anti-angiogenic drugs, (b) the need to develop therapies that target both angiogenesis and vessel co-option in tumours, and (c) predicting the response to other therapeutic modalities, including immunotherapy.

Vessel co-option in cancer

All solid tumours require a vascular supply in order to progress. Although the ability to induce angiogenesis (new blood vessel growth) has long been regarded as essential to this purpose, thus far, anti-angiogenic therapies have shown only modest efficacy in patients. Importantly, overshadowed by the literature on tumour angiogenesis is a long-standing, but continually emerging, body of research indicating that tumours can grow instead by hijacking pre-existing blood vessels of the surrounding nonmalignant tissue. This process, termed vessel co-option, is a frequently overlooked mechanism of tumour vascularization that can influence disease progression, metastasis and response to treatment. In this Review, we describe the evidence that tumours located at numerous anatomical sites can exploit vessel co-option. We also discuss the proposed molecular mechanisms involved and the multifaceted implications of vessel co-option for patient outcomes.

Deciphering mechanisms of brain metastasis in melanoma - the gist of the matter

Metastasis to distant organs and particularly the brain still represents the most serious obstacle in melanoma therapies. Melanoma cells acquire a phenotype to metastasize to the brain and successfully grow there through complex mechanisms determined by microenvironmental than rather genetic cues. There do appear to be some prerequisites, including the presence of oncogenic BRAF or NRAS mutations and a loss of PTEN. Further mediators of the brain metastatic phenotype appear to be the high activation of the PI3K/AKT or STAT3 pathway or high levels of PLEKHA5 and MMP2 in metastatic cells. A yet undefined subset of brain metastases exhibit a high level of expression of CD271 that is associated with stemness, migration and survival. Hence, CD271 expression may determine specific properties of brain metastatic melanoma cells. Environmental cues - in particular those provided by brain parenchymal cells such as astrocytes - seem to help specifically guide melanoma cells that express CCR4 or CD271, potential "homing receptors". Upon entering the brain, these cells interact with brain parenchyma cells and are thereby reprogrammed to adopt a neurological phenotype. Several lines of evidence suggest that current therapies may have a negative effect by activating a program that drives tumor cells toward stemness and metastasis. Yet significant improvements have expanded the therapeutic options for treating brain metastases from melanoma, by combining potent BRAF inhibitors such as dabrafenib with checkpoint inhibitors or stereotactic surgery. Further progress toward developing new therapeutic strategies will require a more profound understanding of the mechanisms that underlie brain metastasis in melanoma.

Foe or friend? Janus-faces of the neurovascular unit in the formation of brain metastases

Despite the potential obstacle represented by the blood-brain barrier for extravasating malignant cells, metastases are more frequent than primary tumors in the central nervous system. Not only tightly interconnected endothelial cells can hinder metastasis formation, other cells of the brain microenvironment (like astrocytes and microglia) can also be very hostile, destroying the large majority of metastatic cells. However, malignant cells that are able to overcome these harmful mechanisms may benefit from the shielding and even support provided by cerebral endothelial cells, astrocytes and microglia, rendering the brain a sanctuary site against anti-tumor strategies. Thus, cells of the neurovascular unit have a Janus-faced attitude towards brain metastatic cells, being both destructive and protective. In this review, we present the main mechanisms of brain metastasis formation, including those involved in extravasation through the brain vasculature and survival in the cerebral environment.

Molecular insights into melanoma brain metastases

Substantial proportions of patients with metastatic melanoma develop brain metastases during the course of their disease, often resulting in significant morbidity and death. Despite recent advances with BRAF/MEK and immune-checkpoint inhibitors in the treatment of patients who have melanoma with extracerebral metastases, patients who have melanoma brain metastases still have poor overall survival, highlighting the need for further therapy options. A deeper understanding of the molecular pathways involved in the development of melanoma brain metastases is required to develop more brain-specific therapies. Here, the authors summarize the currently known preclinical data and describe steps involved in the development of melanoma brain metastases. Only by knowing the molecular background is it possible to design new therapeutic agents that can be used to improve the outcome of patients with melanoma brain metastases. Cancer 2017;123:2163-75. © 2017 American Cancer Society.