Angela C. Hirbe

Neurofibromatosis type 1: a multidisciplinary approach to care

Neurofibromatosis type 1 is a relatively common inherited disorder. Patients have a high predisposition to develop both benign and malignant tumours. Although many manifestations of neurofibromatosis type 1 affect the nervous system, other organs and tissues can also be affected. Because of the varying features and clinical heterogeneity inherent to this disorder, patients can present to different medical and surgical specialists and, therefore, the association of clinical symptoms with neurofibromatosis type 1 might not be appreciated. Thus, for prompt diagnosis and to provide optimum care for patients with neurofibromatosis type 1, clinicians must be aware of the diverse clinical features of this disorder. We advocate a multidisciplinary approach to care, entailing a dedicated team of specialists throughout the lifetime of the patient. As our understanding of this disorder deepens through basic laboratory and clinical investigations, swift implementation of new effective treatments becomes feasible.

RelB is the NF-κB subunit downstream of NIK responsible for osteoclast differentiation

NF-κB inducing kinase (NIK) is required for osteoclastogenesis in response to pathologic stimuli, and its loss leads to functional blockade of both alternative and classical NF-κB caused by cytoplasmic retention by p100. We now show that deletion of p100 restores the capacity of NIK-deficient osteoclast (OC) precursors to differentiate and normalizes RelB and p65 signaling. Differentiation of NIK−/− precursors is also restored by overexpression of RelB, but not p65. Additionally, RelB−/− precursors fail to form OCs in culture, and this defect is rescued by re-expression of RelB, but not by overexpression of p65. To further support the role of RelB in OCs, we challenged RelB−/− mice with TNF-α in vivo and found a diminished osteoclastogenic response. We then examined tumor-induced osteolysis in both RelB−/− and NIK−/− mice by using the B16 melanoma model. Growth of tumor cells in the bone marrow was similar to WT controls, but the absence of either RelB or NIK completely blocked the tumor-induced loss of trabecular bone. Thus, the alternative NF-κB pathway, culminating in activation of RelB, has a key and specific role in the differentiation of OCs that cannot be compensated for by p65.

Skeletal Complications of Breast Cancer Therapies

Nonsurgical treatment options, such as hormonal therapy, chemotherapy, radiation, and bisphosphonate therapy, are undoubtedly improving outcomes for women with breast cancer; however, these therapies also carry significant skeletal side effects. For example, adjuvant hormonal treatments, such as aromatase inhibitors that disrupt the estrogen-skeleton axis, have the potential to cause decreased bone mineral density. Similarly, chemotherapy often induces primary ovarian failure in premenopausal women, resulting in decreased levels of circulating estrogen and subsequent osteopenia. In both cases, women receiving these therapies are at an increased risk for the development of osteoporosis and skeletal fracture. Furthermore, women undergoing radiation therapy to the upper body may have an increased incidence of rib fracture, and those receiving bisphosphonates may be vulnerable to the development of osteonecrosis of the jaw. Therefore, women with breast cancer who are undergoing any of these therapies should be closely monitored for bone mineral loss and advised of skeletal health maintenance strategies.

Developmental regulation of a neuron-specific neurofibromatosis 1 isoform

We have identified a protein isoform of the neurofibromatosis 1 (NF1) gene (neurofibromin) containing the alternatively spliced exon 9a that is expressed in forebrain neurons. Exon 9a neurofibromin is localized in the cytoplasm, sediments in a P100 fraction, and is expressed throughout the soma and processes in cortical neurons in vitro. Expression of exon 9a neurofibromin is developmentally regulated, with expression first detected after postnatal day 2. The identification of this novel protein isoform with restricted neuronal expression suggests novel functions for neurofibromin in the postmitotic brain that are perhaps relevant to the learning disabilities observed in children with NF1.

CD8+ T cells Regulate Bone Tumor Burden Independent of Osteoclast Resorption

Blockade of osteoclast (OC) activity efficiently decreases tumor burden as well as associated bone erosion in immune-compromised animals bearing human osteolytic cancers. In this study, we showed that modulation of antitumor T-cell responses alters tumor growth in bone, regardless of OC status, by using genetic and pharmacologic models. PLCγ2(-/-) mice, with dysfunctional OCs and impaired dendritic cell (DC)-mediated T-cell activation, had increased bone tumor burden despite protection from bone loss. In contrast, Lyn(-/-) mice, with more numerous OCs and a hyperactive myeloid population leading to increased T-cell responses, had reduced tumor growth in bone despite enhanced osteolysis. The unexpected tumor/bone phenotype observed in PLCγ2(-/-) and Lyn(-/-) mice was transplantable, suggesting the involvement of an immune component. Consistent with this hypothesis, T-cell activation diminished skeletal metastasis whereas T-cell depletion enhanced it, even in the presence of zoledronic acid, a potent antiresorptive agent. Importantly, injection of antigen-specific wild-type cytotoxic CD8(+) T cells in PLCγ2(-/-) mice or CD8(+) T-cell depletion in Lyn(-/-) mice normalized tumor growth in bone. Our findings show the important contribution of CD8(+) T cells in the regulation of bone metastases regardless of OC status, thus including T cells as critical regulators of tumor growth in bone.

The CXCR4/SDF-1 Chemokine Axis: A Potential Therapeutic Target for Bone Metastases?

Chemokines and chemokine receptors play diverse roles in homeostasis. The chemokine stromal cell-derived factor 1 (SDF-1) and its receptor CXCR4 have critical functions in the immune, circulatory, and central nervous systems and have also been implicated in tumor biology and metastasis. Here we review the current data regarding the role of the CXCR4/SDF-1 chemokine axis in the development of bone metastases, derived from tumor models of breast or prostate cancers. There is substantial evidence that CXCR4 and SDF-1 directly influence the survival and proliferation of tumor cells. In regards to bone metastases, the CXCR4/SDF-1 axis also appears to facilitate tumor cell recruitment to the bone marrow microenvironment via a homing mechanism. This makes disruption of the chemokine axis an attractive therapeutic target for the prevention of tumor cell spread to bone. However, within the bone microenvironment, SDF-1 and CXCR4 appear to have conflicting roles. While genetic disruption of CXCR4 enhances osteoclast activity and therefore stimulates tumor cell growth in the bone - likely via release of bone-derived growth factors - SDF-1 has been shown to have either a stimulatory effect or no effect on osteoclasts. In short, the effects of the CXCR4/SDF-1 axis on tumor cell growth within the bone are not yet fully defined. Further, there are theoretical risks that blockade of this chemokine axis could impair immune function or mobilize tumor cells leading to other sites of metastasis. As such, caution should be taken when designing therapeutic strategies targeting this chemokine axis.

The bisphosphonate zoledronic acid decreases tumor growth in bone in mice with defective osteoclasts

Bisphosphonates (BPs), bone targeted drugs that disrupt osteoclast function, are routinely used to treat complications of bone metastasis. Studies in preclinical models of cancer have shown that BPs reduce skeletal tumor burden and increase survival. Similarly, we observed in the present study that administration of the Nitrogen-containing BP (N-BP), zoledronic acid (ZA) to osteolytic tumor-bearing Tax+ mice beginning at 6 months of age led to resolution of radiographic skeletal lesions. N-BPs inhibit farnesyl diphosphate (FPP) synthase, thereby inhibiting protein prenylation and causing cellular toxicity. We found that ZA decreased Tax+ tumor and B16 melanoma viability and caused the accumulation of unprenylated Rap1a proteins in vitro. However, it is presently unclear whether N-BPs exert anti-tumor effects in bone independent of inhibition of osteoclast (OC) function in vivo. Therefore, we evaluated the impact of treatment with ZA on B16 melanoma bone tumor burden in irradiated mice transplanted with splenic cells from src(-/-) mice, which have non-functioning OCs. OC-defective mice treated with ZA demonstrated a significant 88% decrease in tumor growth in bone compared to vehicle-treated OC-defective mice. These data support an osteoclast-independent role for N-BP therapy in bone metastasis.

Disruption of CXCR4 enhances osteoclastogenesis and tumor growth in bone

CXCR4 regulates hematopoietic and tumor cell homing to bone, but its role during osteoclast (OC) development is unknown. We investigated the role of CXCR4 in osteoclastogenesis and in a model of bone metastasis. Compared with controls, mice reconstituted with CXCR4 null hematopoietic cells exhibited elevated markers of bone resorption, increased OC perimeter along bone, and increased bone loss. CXCR4−/− OCs demonstrated accelerated differentiation and enhanced bone resorption in vitro. Furthermore, tumor growth specifically in bone was significantly increased in mice reconstituted with CXCR4−/− hematopoietic cells. Finally, enhancement of bone tumor growth in the absence of CXCR4 was abrogated with the OC inhibitor, zoledronic acid. These data demonstrate that disruption of CXCR4 enhances osteoclastogenesis and suggest that inhibition of CXCR4 may enhance established skeletal tumor burden by increasing OC activity.

Morphologic and immunohistochemical features of malignant peripheral nerve sheath tumors and cellular schwannomas.

Cellular schwannoma is an uncommon, but well-recognized, benign peripheral nerve sheath tumor, which can be misdiagnosed as malignant peripheral nerve sheath tumor. To develop consensus diagnostic criteria for cellular schwannoma, we reviewed 115 malignant peripheral nerve sheath tumor and 26 cellular schwannoma cases from two institutions. Clinical data were retrieved from the electronic medical records, and morphologic features, maximal mitotic counts, Ki67 labeling indices, and immunohistochemical profiles (SOX10, SOX2, p75NTR, p16, p53, EGFR, and neurofibromin) were assessed. Several features distinguish cellular schwannoma from malignant peripheral nerve sheath tumor. First, in contrast to patients with malignant peripheral nerve sheath tumor, no metastases or disease-specific deaths were found in patients with cellular schwannoma. More specifically, 5-year progression-free survival rates were 100 and 18%, and 5-year disease-specific survival rates were 100 and 32% for cellular schwannoma and malignant peripheral nerve sheath tumor, respectively. Second, the presence of Schwannian whorls, a peritumoral capsule, subcapsular lymphocytes, macrophage-rich infiltrates, and the absence of fascicles favored the diagnosis of cellular schwannoma, while the presence of perivascular hypercellularity, tumor herniation into vascular lumens, and necrosis favor malignant peripheral nerve sheath tumor. Third, complete loss of SOX10, neurofibromin or p16 expression, or the presence of EGFR immunoreactivity was specific for malignant peripheral nerve sheath tumor (P<0.001 for each). Expression of p75NTR was observed in 80% of malignant peripheral nerve sheath tumors compared with 31% of cellular schwannomas (P<0.001). Fourth, Ki-67 labeling indices ≥20% were highly predictive of malignant peripheral nerve sheath tumor (87% sensitivity and 96% specificity). Taken together, the combinations of these histopathological and immunohistochemical features provide useful criteria to distinguish between malignant peripheral nerve sheath tumor and cellular schwannoma with high sensitivity and specificity. Additional retrospective and prospective multicenter studies with larger data sets will be required to validate these findings.

The ADP receptor P2RY12 regulates osteoclast function and pathologic bone remodeling

The adenosine diphosphate (ADP) receptor P2RY12 (purinergic receptor P2Y, G protein coupled, 12) plays a critical role in platelet aggregation, and P2RY12 inhibitors are used clinically to prevent cardiac and cerebral thrombotic events. Extracellular ADP has also been shown to increase osteoclast (OC) activity, but the role of P2RY12 in OC biology is unknown. Here, we examined the role of mouse P2RY12 in OC function. Mice lacking P2ry12 had decreased OC activity and were partially protected from age-associated bone loss. P2ry12-/- OCs exhibited intact differentiation markers, but diminished resorptive function. Extracellular ADP enhanced OC adhesion and resorptive activity of WT, but not P2ry12-/-, OCs. In platelets, ADP stimulation of P2RY12 resulted in GTPase Ras-related protein (RAP1) activation and subsequent αIIbβ3 integrin activation. Likewise, we found that ADP stimulation induced RAP1 activation in WT and integrin β3 gene knockout (Itgb3-/-) OCs, but its effects were substantially blunted in P2ry12-/- OCs. In vivo, P2ry12-/- mice were partially protected from pathologic bone loss associated with serum transfer arthritis, tumor growth in bone, and ovariectomy-induced osteoporosis: all conditions associated with increased extracellular ADP. Finally, mice treated with the clinical inhibitor of P2RY12, clopidogrel, were protected from pathologic osteolysis. These results demonstrate that P2RY12 is the primary ADP receptor in OCs and suggest that P2RY12 inhibition is a potential therapeutic target for pathologic bone loss.