Intracerebral metastases pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Sujit Routray, M.D. [2]

Overview

Intracerebral metastases are different from the cancer that starts in the brain (called primary brain cancer). Primary brain tumors occur much less often than intracerebral metastases. It is estimated that 20–40% of intracerebral tumors are metastatic. Cancers that start in the brain usually remain in one place (solitary mass). If there is more than one tumor in the brain, they are most probably intracerebral metastases. The ability of cancer cells to sever their link to the primary tumor site and commence the metastatic process, once specific functions have been acquired by an appropriate subset of cancer cells. The multistep cascade can be grouped into two stages: migration (intravasation, dissemination, and extravasation) and colonization. Genes involved in the pathogenesis of intracerebral metastases include RHoC, LOX, VEGF, and CSF1. On gross pathology, intracerebral metastases are characterized by single-to-multiple masses typically found in the watershed areas of the brain, that are sharply demarcated from the surrounding parenchyma and usually have a zone of peritumoral edema that is out of proportion with the tumor size. On microscopic histopathological analysis, intracerebral metastases are characterized by tubule formation, well-circumscribed and sharply demarcated from surrounding tissues, with mitoses and nuclear atypia. Intracerebral metastases are demonstrated by positivity to tumor markers such as pankeratin, TTF-1, CK7, and CK20.

Pathophysiology

Pathogenesis

• Intracerebral metastases are different from the cancer that starts in the brain (called primary brain cancer). Primary brain tumors occur much less often than intracerebral metastases. It is estimated that 20–40% of intracerebral tumors are metastatic.
• Cancers that start in the brain usually remain in one place (solitary mass). If there is more than one tumor in the brain, they are most probably intracerebral metastases.
• Parenchymal blood flow is an important determinant of the distribution of metastases to the brain.
• The ability of cancer cells to sever their link to the primary tumor site and commence the metastatic process, once specific functions have been acquired by an appropriate subset of cancer cells. The multistep cascade can be grouped into two stages: migration (intravasation, dissemination, and extravasation) and colonization.

Migration	Colonization
Cellular heterogeneity and proliferation Epithelial-mesenchymal transition (EMT) Interactions with tumor stroma Local Invasion E-cadherin-catenin complex (ECCC), integrins, and other molecules Genetic alterations Dissemination	Organ-specific infiltration The blood brain barrier, function of the brain microenvironment, and brain metastasis Neoangiogenesis and proliferation Cascade-nonspecific contributors to metastasis Overview of microRNAs (miRNAs) and their emerging role in oncogenesis

Migration

1. Cellular heterogeneity and proliferation

• The primary tumor consists of cancer cells which are genetically heterogeneous and have varying potentials to metastasize.
• These include the cell’s ability to invade adjacent tissues, initiate neoangiogenesis, disseminate, and adhere to new tissue substrates while expressing an affinity for the central nervous system.
• Tumor cells have the ability to evade the structural organization present in normal tissues and cells.
• In spite of being exposed to various environmental pressures (hypoxia and nutrient deprivation, low pH, immune and inflammatory mediators, poor blood supply), a subset of tumor cells survive these pressures with the ability to metastasize to distant sites.
• Additionally, the tumor cells are able to evade growth suppressors, which limit cell growth and proliferation, as well as circumvent inhibitors of cell proliferation such as cell cycle checkpoint and DNA damage control systems.
• The tumor cells can also resist apoptosis by the increased expression of antiapoptotic regulators (Bcl-2, Bcl-xL), survival signals (lgf 1/2), and downregulating proapoptotic factors (Bax, Bim, and Puma).
• The primary tumor cells have the ability to acquire genetic and epigenetic mutations, such as DNA methylation and histone modification, allowing the fittest group of cells to survive.
• The microRNA (miRNA) species interactions with pseudogenes may modify gene expression in cancer.
• Various genetic mutations result in the ability of tumor cells to commence the proliferative process. Clonal expansion of these surviving fit cells leads to an acquisition of further changes, making subsequent cell lines progressively more carcinogenic.
• Observations within the primary tumor mass have revealed the presence of heterogeneous cell lines including cancer stem cells (CSCs), partially differentiated progenitor cells, and fully differentiated end-stage cells.
• These appear to recapitulate the same hierarchal patterns in normal tissue types but in an uncontrolled manner.
• These CSCs may be the primary drivers of the enhanced malignant potential of primary tumors, giving origin to their aggressive phenotypes with the ability to degrade the extracellular matrix (ECM), invade blood vessels and lymph nodes, migrate, extravasate, colonize, and renew themselves at their new locations.
• These CSCs can reside in clusters or niches, at two or more locations within the primary tumor cell mass.
• Thus, the key role a cancer stem cell plays in the metastatic cascade cannot be overstated, due to its ability to initiate tumor proliferation and “self-renew” itself at alternative tissue locations. Other observations reveal that, in addition to the abilities discussed, they are also motile, invasive, and are resilient to the apoptotic process.

2. Epithelial-mesenchymal transition (EMT)

• The epithelial-mesenchymal transition (EMT) describes a temporary, reversible phenomenon wherein cells can dedifferentiate, migrate to a distant focus, and then redifferentiate back to their original cell, forming a new structure.
• Signals activating the EMT can be intrinsic such as gene mutations and extrinsic such as growth factor signaling.
• Transdifferentiation appears to be initiated by release of certain EMT-inducing transcription factors (EMT-TFs) that transform epithelial cells into mesenchymal derivatives, giving these cells the capacity to invade, resist apoptosis, and disseminate.
• Transforming growth factor β (TGFβ), hepatocyte growth factor (HGF), epidermal growth factor (EGF), insulin-like growth factor (IGF), fibroblast growth factor (FGF), and members of the notch signaling family play a role in inducing the EMT pathway.
• The EMT program enables non-CSCs to derive characteristics of the CSC state, which enables them to invade and disseminate from the primary tumor to a distant, metastatic focus.
• Some of these traits include the ability to loosen adherent junctions, express matrix-degrading enzymes, resist apoptosis, and to undergo morphological conversion.
• Using the EMT program, cancer cells can, transiently or for longer time frames, activate themselves and acquire attributes critical to survival and dissemination. To activate the EMT, a certain amount of crosstalk has to exist between the tumor cells and adjacent stromal cells, which are done by various EMT-TFs and signals from within the adjacent tumor stroma.

3. Interactions with tumor stroma

• Progression in cancer also involves activating a number of cells in the adjacent stroma via paracrine signaling.
• These stromal cells including endothelial cells, pericytes, fibroblasts, and leukocytes provide a number of protumorigenic factors which sustain tumor growth.
• The two prominent cell types are the cancer-associated fibroblasts (CAFs) and the pericytes.
• These cells produce growth factors, hormones, and cytokines that promote tumor proliferation.
• CAFs are known to express high amounts of TGFβ, HGF, EGF, FGF, canonical Wnt families, and cytokines like stromal-derived factor-1α (SDF-1α) and interleukin-6 (IL-6).
• Invasion of the cancer cells can be enhanced by stromal macrophages which supply matrix-degrading enzymes such as matrix metalloproteinases and cysteine cathepsin protease.
• The cancer cells release factors such as CSF-1, which stimulates macrophages in the tumor microenvironment with the subsequent release of EGF, which promotes proliferation of the tumor mass.
• In addition, cancer-associated fibroblasts are activated by various inflammatory mediators and induced to produce increased quantities of VEGF, FGF-2, among other cytokines and growth factors, which recruits endothelial progenitor cells thereby promoting angiogenesis. This dynamic stromal environment further stresses the tumor cells, potentially enhancing additional genomic instability, heterogeneity, and epigenetic dysregulation.

4. Local invasion

• Once the phenotypically aggressive clone has developed, spread of the tumor consists of a series of two sequential steps: namely, invasion of the extracellular matrix (ECM) with penetration into the vasculature and hematogenous dissemination to the central nervous system. Tumor expansion causes adjacent ECM compression and modifies lymphatic and blood vessel flow, eventually leading to basement membrane (BM) thinning. Combined with the various molecular and cellular events, this leads to eventual tumor metastasis.
• To reach the circulation, tumor cells must penetrate the basement membrane, traverse the extracellular connective tissue matrix (ECM) tissue, and then breach the vascular basement membrane (VBM) to enter the circulation.
• The process is dependent on a number of protein complexes that regulate cellular interactions and proteolytic enzymes, with degradation of the ECM, which permits extravasation.

5. E-cadherin-catenin complex (ECCC), integrins, and other molecules

• The E-cadherin-catenin molecular complex is essential to maintain a normal and tumoral cytoarchitecture.
• It is a necessary mediator of cell-cell adhesion that, among other functions, determines the polarity of normal (and tumor) cells and their organization into tissues.
• Cadherin molecules are integral cell membrane glycoproteins that interact in a homophilic manner with one another.
• They have a stable extracellular fragment and possess a cytoplasmic undercoat protein of one or more proteins called catenins.
• In the process of tumor metastasis, tumor clones become discohesive, fail to adhere to one another, and develop a more disordered cytoarchitecture which allows these cells to separate from the tumor mass.
• E-cadherin maintains cell adhesion by anchoring its cytoplasmic domain to actin cytoskeleton via α-catenin and β-catenin.
• Infiltrating malignancies have mutations in the genes for α-catenin, β-catenins, and E-cadherin, thus decreasing the expression of this complex. This has been correlated with invasion, metastasis, and an unfavorable prognosis.
• Furthermore, DNA hypermethylation of the promoter region of E-cadherin can diminish or silence its expression, thereby disturbing ECCC function, and is a common event in intracerebral metastases.
• N-cadherin is another molecule connected to the cellular cytoskeleton via α-catenin and β-catenins in a manner similar to E-cadherin.
• One of the hallmarks of the EMT described above is a cadherin switch, with loss of epithelial E-cadherin and gain of mesenchymal N-cadherin functions. This induces loss of epithelial cellular affinity, while at the same time increasing the affinity of cells for the mesenchymal cells like fibroblasts. Gain-of-function mutations in N-cadherin also trigger increased migration and invasion in tumors.
• Integrins are another family of major adhesion and signaling receptor proteins linking the ECM to the cellular actin cytoskeletal structure called focal adhesions and play an important role in mediating cell migration and invasion. They trigger a variety of signal transduction pathways and regulate cytoskeletal organization, specific gene expression, control of growth, and apoptosis.
• Integrins induce the release of a key mediator in signaling known as focal adhesion kinase (FAK). FAK is a ubiquitously expressed nonreceptor cytoplasmic tyrosine kinase, thought to play a key role in migration and proliferation, by providing abnormal signals for survival, EMT, invasion, and angiogenesis. FAK may also play an important role in the regulation of CSCs. Dephosphorylation and inhibition of FAK at the Y397 locus via the activated Ras oncogene promotes tumor migration by facilitating focal adhesion at the leading edge of tumor cells.
• The ability of tumor cells to escape the primary site is dependent on their ability to remodel the ECM. This remodeling occurs by breaking down or degrading the ECM via proteolytic enzymes, thus creating a pathway for invasion. The advancing edge of tumor cells posses the ability to carry out this proteolytic activity by releasing signals that promote cell proliferation and angiogenesis in the metastatic cascade. Neurotrophins (NTs) promote brain invasion by enhancing the production of heparinase, which is an ECM proteolytic enzyme. Heparinase is a β-d-glucuronidase that cleaves the heparin sulfate chain of the ECM. It is the prominent heparin sulfate degradative enzyme and is known to destroy both the ECM and the blood-brain barrier. Evidence suggests the presence of NTs at the tumor-brain interface in melanomas, and reports have suggested a role for the p75 NT receptor in brain metastasis.
• Matrix metalloproteinases (MMPs) are members of a family of zinc-dependent endopeptidases that function at physiological pH and help remodeling human connective tissue at low levels. About 25 human family members have been identified, and they have been grouped according to their substrate on which they act, namely collagenase, stromelysin, matrilysin, and gelatinase.
• They also play a critical role in the EMT and tumor microenvironment. Cytokines and inducers present on the surface of tumor cells in the ECM regulate their expression.
• Once these MMPs are induced and stimulated, they aid in breakdown of type I collagen, fibronectin, and laminin in the ECM and enhance tumor cell migration. MMP activity correlates with invasiveness, metastasis, and poor prognosis.
• MMP-2 may be identified in all intracerebral metastases regardless of site of origin. Moreover, MMP-2 activity correlated inversely with survival.
• The urokinase-type plasminogen activator (uPA) system consists of uPA, its receptor (uPAR), and plasminogen. The uPA binds to the receptor uPA-R (CD87), the activity of which is regulated by the action of plasminogen activator inhibitor type 1 and 2 (PAI-1/2) on the cell membrane and causes urokinase to convert plasminogen to plasmin.
• The proteolytic activity of plasmin then degrades components of the ECM including fibrin, fibronectin, proteoglycans, and laminin. Further, plasmin activates other proteolytic enzymes with resultant local invasion and migration. There is a high level of uPA in metastatic tumors, correlates with necrosis and edema, and there is an inverse correlation with a tumor’s levels of uPA and survival. Additionally, high levels of uPA and absent tissue plasminogen activator (tPA) correlate with aggressiveness and decreased survival.
• More recent evidence describes the role of “invadopodia”, which are three-dimensional protrusive processes, compared to the two-dimensional lamellipodia and filopodia, in metastatic invasion. Invadopodia appear to share a number of structural and functional features with filopodia, but spatially focus on proteolytic secretion, remodeling the ECM matrix, and establishing tracts supporting subsequent invasion.
• Integrins play a major role in organizing the components, triggering the formation of invadopodia. α3β1 activation promotes Src-dependent tyrosine phosphorylation of p190RhoGAP, via RhoGTPases family, which activates invadopodia and invasion.
• Integrins also appear to focus proteolytic activity to the region of these processes, as in melanoma cells, where collagen-induced α3β1 association with the serine protease "seprase" (surface-expressed protease) enhances the activity of matrix-degrading enzymes focally at the invadopodia.
• Numerous cancer cell lines such as melanoma, breast cancer, glioma, and head and neck cancer have shown the presence of invadopodia. A number of other molecules, such as EGF, HGF, or TGF-β can induce their formation as well.
• The release of tumor-released chemokines such as CSF-1 and PIGF attract tumor-associated macrophages (TAM) to the microenvironment, which in turn release multiple factors stimulating invadopodia. In addition, a family of proteins called aquaporins may also facilitate migration.
• Aquaporin-dependent tumor angiogenesis and metastases enhance water transport in the lamellipodia of migrating cells.
• Studies on brain-specific breast metastasis reveal that increased expression of KCNMA1, a gene encoding for a big conductance type potassium channel (BKCa) that is upregulated in breast cancer, leads to greater invasiveness and transendothelial migration.

6. Genetic Alterations

• Several known tumor suppressor genes (TSGs) function at the level of escape and migration/intravasation. The best known of these is the KiSS1 gene on chromosome 1.
• KiSS1 encodes metastin, which is a ligand of the orphan G protein couples receptor hOT7T175. Lee et al. have found that the forced expression of KiSS1 suppressed both melanoma and breast metastasis.
• KAI1 (CD82), a TSG on chromosome 11p11.2, regulates adhesion, migration, growth, and differentiation of tumor cell lines.
• KAI1 expression is inversely correlated with prostate cancer progression as well as breast and melanoma metastasis. Additionally, KAI1 is known to be associated with the epidermal growth factor receptor (EGFR) and is thought to affect the Rho GTPase pathway resulting in suppression of lamellipodia formation and migration.
• Hypermethylation of the TSG DRG1 inhibits both liver metastasis and colorectal carcinoma invasion. Conversely, overexpression of DRG1 has been linked to resistance to irinotecan chemotherapy.
• In addition to the suppressor genes responsible for invasion and metastasis, there are a number of promoter genes responsible for invasion and metastasis as well.
• Genetic activation or inactivation of promoter/suppressor genes in human cancer can be the result of mutations, deletions, loss of heterozygosity, multiplication, and translocation.
• The same genes that are responsible for normal cellular functioning, signaling, signal transduction, modulating, and mediating cellular response are frequently the genes that enhance invasion and metastasis when altered by genetic or epigenetic dysfunction.
• These changes within the primary tumor microenvironment give rise to an “active seed” ready to implant itself in a fertile environmental “soil”. These cellular modifications enable the next steps of migration (dissemination and extravasation).

7. Dissemination

• Once a cancer cell has breached its microenvironment and arrived at the vasculature (intracerebral metastasis) or lymphatic system (other sites), the tumor cell must survive its exposure to high shear forces and varied stress patterns.
• Tumor cells respond by reenforcing their cytoskeleton and increasing the ability to adhere to the vascular wall.
• On adhering to endothelium of target tissue, the tumor cells behave like macrophages, creating pseudopodia and penetrating the cell-cell junctions, driven by dynamic remodeling of the cellular cytoskeleton.
• There are a subset of circulating tumor cells which maintain their physical plasticity and although much larger in diameter (20–30 μ) than lung capillaries (~8 μ), can survive the sieving action of lung capillaries. These cells can be found either growing as clumps in the lung or colonizing other organ sites. Cancer cells in circulation appear to attract platelets because of their expressed surface tissue proteins and these protect the cells from the immune system.
• Once these mobile cancer cells get lodged in a secondary organ tissue site, there are two pathways for colonization. One is mediated by cellular diapedesis, extravasation, and proliferation of the tumor cell mass, whereas the other consists of accumulation of tumor cells within the site of obstruction in the foreign tissue vascular bed, wherein they proliferate prior to their rupture into the adjacent stroma where they begin to grow.

Colonization

1. Organ-specific infiltration

• Subsequent to intravasation and dissemination, special mechanisms are necessary to extravasate and colonize secondary sites.
• The metastatic deposits occur in certain organ tissues because of the influence of hematogenous dynamics, for example, colon cancer metastasis preferentially metastasizing to the liver because of mesenteric circulation and large vascular sinusoids.
• The overexpression of the cell adhesion molecule, metadherin, in breast cancer makes it easier for tumor cells to target and adhere to endothelial lining in the lung parenchyma, making it possible for these endothelial-adhesive interactions to enhance the possibility of brain metastasis.
• Although the exact causes of preferential metastatic sites have not been clearly elucidated, one theory states that direct neurotropic interactions with yet undiscovered brain homing mechanisms result in intracerebral metastases. “Vascular co-option”, a term put forward by Carbonell et al., describes the ability of metastatic cells to grow along the preexisting vessels much before overt secondaries are detected. Once adherent to the vascular basement membrane, the tumor cells can extravasate into the parenchyma, the vascular basement membrane thus being the “soil” for intracerebral metastases. Saito et al. demonstrated that the pia-glial membrane along the external surface of blood vessels serves as a scaffold for the angiocentric spread of metastatic cells.
• The tumor cells function like macrophages within the vasculature and during extravasation, express CD11b, Iba1, F4/80, CD68, CD45, and CXCR which are proteins normally expressed specifically by the macrophages. The ability of the tumor cells to mimic macrophages may enable them to evade the immune system while in the vasculature.

2. The blood brain barrier, function of the brain microenvironment, and brain metastasis

• Passage of the tumor cells across the blood-brain barrier occurs via mechanisms which have not yet been delineated fully.
• Recently, three proteins that mediate breast metastasis to the brain and lungs have been described, cyclooxygenase 2 or COX2 (also known as PTGS2), EGFR, ligand and heparin-binding EGF-like growth factor (HBEGF). These proteins facilitate extravasation through nonfenestrated blood vessels and enhance colonization.
• Other molecules targeting organ specific colonization may also be expressed by the cancer cells. These molecules include ezrin (an intracellular protein necessary for the survival of osteosarcoma cells in the lung) and serine-threonine kinase 11 (STK11 or LKB1, a metastasis suppressor gene which regulates NEDD9 in lung cancer).

3. Neoangiogenesis and proliferation

• A key component of both primary and secondary (metastatic) tumor growth at any site is angiogenesis. The growth may occur by utilizing preexisting vasculature or co-opting these vessels rather than inducing new vessel formation (neoangiogenesis).
• Kusters et al. observed that growth of the melanoma metastatic tumor up to 3 mm could occur without inducing the angiogenic switch.
• Carbonell et al. have also observed that beta-1 integrin, expressed by the metastatic tumor cell line, is the key molecule to co-opt adjacent blood vessels to the growing tumor.
• Various angiogenic factors have been scrutinized as viable targets for treatment. Vascular endothelial growth factor (VEGF) is the most commonly recognized angiogenic factor. VEGF expression in breast cancer plays a role in metastasis and inhibition with a tyrosine kinase receptor inhibitor-reduced growth and angiogenesis.
• MMP-9/gelatinase B complex, a member of the MMP family and PAI-1, a uPA cell surface receptor may play roles in angiogenesis. The role in angiogenesis and uniqueness of plexin D1 (PLXND1) expression was explored in the tumor cells and vasculogenesis. Neoplastic cells expressed plexin D1 as well as tumor vasculature, while its expression in nonneoplastic tissue was restricted to a small subset of activated macrophages, which suggests that plexin D1 may play a significant role in tumor angiogenesis.
• Overexpression of hexokinase 2 (HK2), which plays a key role in glucose metabolism and apoptosis, may also influence intracerebral metastases in breast and other cancers. It has been observed that both mRNA and protein levels of HK2 are elevated in intracerebral metastatic derivative cell lines compared to the parental cell line in vitro. Knockdown of expression reduced cell proliferation, which implies that HK2 contributes to the proliferation and growth of breast cancer metastasis. Finally, increased expression of HK2 is associated with poor survival after craniotomy.
• At least two tumor suppressor genes that function at the proliferation level of the metastatic cascade have been described. The first, NM23 regulates cell growth by encoding for a nucleotide diphosphate protein kinase that interacts with menin, a protein encoded by MEN1. NM23 is thought to reduce signal transduction and thereby decrease anchorage independent colonization, invasion, and motility. In melanoma, decreased expression is correlated with increased brain metastasis.
• Another tumor suppressor gene, BRMS1, located at 11q13, is altered in many melanomas and breast cancers. BRMS1 prevents disseminated tumor cell growth by restoring the normal gap junction phenotype and maintaining cell-to-cell communication in the primary tumor. Seraj et al. found an inverse correlation between the expression of BRMS1 and the metastatic potential in melanoma.

4. Cascade-nonspecific contributors to metastasis

• There are certain molecular contributions that cannot be attributed to a specific step in the cascade, either because they are active at every level. Zeb-1, the zinc finger E-box homeobox transcription factor, is overexpressed in metastatic cancers.
• This overexpression leads to epithelial-mesenchymal transition and increased metastasis.
• Mutation of Zeb-1 leads to decrease in the proliferation of progenitor cells.

5. Overview of microRNAs (miRNAs) and their emerging role in oncogenesis

• There is an important role of microRNAs in cell and tissue development, proliferation, and motility via their ability to repress mRNA translation or induce mRNA degradation.
• The dysregulated expression of a single miRNA can cause a cascade of silencing events capable of eliciting disease development in humans, which includes cancer.

Gallery

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  _{(a) Formation of metastatic tumor cell lines at primary sites like breast, lung, and skin (melanoma) seen as the red nodes. Metastasis from these primary sites then spreads to the brain via the circulatory system (red arrows) and also to adjacent sites like the liver, bone, lung, and lymph nodes (black arrows). The inset shows the primary site of melanoma cells proliferating and migrating towards the vasculature, subsequently disseminating to secondary organ sites.}

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  _{(b) The metastatic tumor cells detach from the primary site and penetrate the adjacent parenchyma to reach the blood vessels. On reaching the vessels, the cells invade and enter the circulation (intravasation) and then disseminate within the vascular system (left half of figure). These cells eventually adhere to secondary sites “soil” to then extravasate out of the blood vessels and for colonies of metastatic cells (right half of figure).}

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  _{Schematic of the metastatic cascade. Cascade of events taking place at the primary site during oncogenesis, illustrating the steps creating the neoplastic cell line followed by clonal expansion and survival of the fittest cells, becoming the invasive and metastatic phenotype.}

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  _{Subsets of cancer cells at the primary site develop an invasive phenotype; survive environmental pressures (hypoxia and nutrient deprivation, low pH, immune and inflammatory mediators, and poor blood supply) gaining the ability to metastasize to distant sites. These cancer cells can evade growth suppressors and circumvent inhibitors of cell proliferation to intravasate and disseminate to various other sites.}

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  _{Overexpression of the adhesion molecules makes it easy for tumor cells to target and adhere to endothelial lining in the parenchyma, making it possible for these endothelial-adhesive interactions to enhance the possibility of brain metastasis. Direct neurotropic interactions with brain homing mechanisms result in brain metastases. “Vascular co-option” is the ability of metastatic cells to grow along the preexisting vessels, and once adherent to the vascular basement membrane, tumor cells can extravasate into the parenchyma, the vascular basement membrane thus being the “soil” for brain metastases.}

Genetics

Genes involved in the pathogenesis of intracerebral metastases are tabulated below:

Gene	Cancer site (primary)	Role and implications	Chromosome location
RHoC	Melanoma	Regulates remodeling of actin cytoskeleton during morphogenesis and motility Important in tumor cell invasion	1p21-p13
LOX	Breast Head and neck cancer	Increases invasiveness of hypoxic human cancer cells through cell matrix adhesion and focal adhesion kinase activity	5q23.1-q23.2
VEGF	Lung Breast Melanoma Colon	Angiogenic growth factor Inhibition decreases brain metastasis formation; reduces blood vessel formation and cell proliferation; increases apoptosis	6p21.1
CSF1	Breast Lung	Stimulate macrophage proliferation and subsequent release of growth factors	1p13.3
ID1	Breast Lung	Involved in matrix remodeling, intracellular signaling, and angiogenesis	20q11.21
TWIST1	Breast Gastric Rhabdomyosarcoma Melanoma Hepatocellular	Causes loss of E-cadherin mediated cell-cell adhesion, activates mesenchymal markers, and induces cell motility by promoting epithelial-mesenchymal transition	7p21.1
MET	Renal cell cancer	Affects a wide range of biological activity depending on the cell target, varying from mitogenesis, morphogenesis, and motogenesis	7q31.2
MMP-9	Colorectal Breast Melanoma Chondrosarcoma	Extracellular matrix degradation, tissue remodeling	20q13.12
NEDD9	Melanoma	Acquisition of a metastatic potential	6p24.2
LEF1	Lung	Transcriptional effecter—WNT pathway; predilection for brain metastasis Knockdown inhibits brain metastasis, decreases colony formation; in vitro decreases invasion	4q25
HOXB9	Lung Breast	Homeobox gene family; critical for embryonic segmentation and patterning. Also a TCF4 target Knockdown in vitro decreased invasion and colony formation; in vivo appears to inhibit brain metastasis	17q21.32
BMP4	Lung Colorectal	Plays an essential role in embryonic development and may be an essential component of the epithelial-mesenchymal transition	14q22.2
STAT3	Melanoma	Cell signaling transcription factor Reduction suppresses brain metastasis; decreases angiogenesis in vivo and cellular invasion in vitro	17q21.2

Gross Pathology

• On gross pathology, intracerebral metastases are characterized by single-to-multiple masses typically found in the watershed areas of the brain, that are sharply demarcated from the surrounding parenchyma and usually have a zone of peritumoral edema that is out of proportion with the tumor size.
• Common intracranial sites associated with intracerebral metastases include:

• Cerebrum (80%)
• Cerebellum (15%)
• Brain stem (5% )

Gallery

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  _{This solitary brain metastasis from thyroid papillary carcinoma resulted in neurological symptoms. The thyroid primary was clinically occult. (Courtesy of Dr. Nikola Kostich, Minneapolis, MN.).}

Microscopic Pathology

The histopathological appearance of intracerebral metastases may vary with the type of primary tumor. Common findings are listed below:

• Tubule formation/glands
• Well-circumscribed and sharply demarcated from surrounding tissue (with the exception of melanoma metastasis)
• Mitoses
• Nuclear atypia

• Nuclear hyperchromasia
• Variation of nuclear size
• Variation of nuclear shape

Gallery

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  _{Very low magnification micrograph demonstrating metastatic adenocarcinoma that from a colorectal primary, i.e. colorectal carcinoma, by immunostains on HPS stain. The cerebellum seen on the image has Bergmann gliosis and Purkinje cell loss.}

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  _{High magnification micrograph demonstrating metastatic adenocarcinoma that is from a colorectal primary, i.e. colorectal carcinoma, by immunostains on HPS stain. The cerebellum has Bergmann gliosis and Purkinje cell loss.}

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  _{High magnification micrograph of a brain metastasis on HPS stain demonstrating normal brain tissue on the left and tumor cells on the right. The sharp demarcation between tumor and normal is typical of brain metastases.}

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  _{Adenocarcinoma infiltrating the brain in a case of lung cancer on H&E stain.}

Immunohistochemistry

• The immunohistochemistry profile of intracerebral metastases may vary with the type of the primary tumor.
• Intracerebral metastases are demonstrated by positivity to tumor markers such as:

• General brain metastases: Pankeratin +ve, GFAP -ve
• Lung adenocarcinoma and small cell lung carcinoma: TTF-1 +ve, CK7 +ve, CK20 -ve
• Breast carcinoma: CK7 +ve, ER +ve, PR +ve, BRST2 +ve/-ve
• Colorectal carcinoma: CK20 +ve, CDX2 +ve, TTF-1 -ve, CK7 -ve
• Clear cell renal cell carcinoma: PAX8 +ve, vimentin +ve, CD10 +ve, CK7 -ve, CK20 -ve
• Melanoma: S-100 +ve, HMB-45 +ve, melan-A +ve.

Gallery

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  _{Immunohistochemistry profile of intracerebral metastases from an adenocarcinoma of lung (primary) demonstrating positivity to CK7, CK20, and TTF1.}

References

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