Gene Polymorphism Influencing Persistent Bone Resorption in Brown Tumor: A Case Series with Possible Pathogenesis and Potential Therapeutic Approach

Abstract

Brown tumor represents a terminal stage of bone remodeling process due to an imbalance between osteoclastic and osteoblastic activity. It represents a reparative cellular process, rather than a neoplastic process mostly associated with primary or secondary hyperparathyroidism. Although parathyroidectomy is the first treatment of choice for brown tumors, several cases don’t resolve even after normalization of parathyroid hormone levels which leads to surgical intervention. Therefore, to avoid multiple bone surgeries in the same patient, it is crucial to have a conservative approach like targeted therapy which could block certain molecules involved in bone resorption. In this string, we have recognized and quantified three molecules namely sclerostin, MCP-1 and CD73 in brown tumors and correlated their expression with bone resorption pathogenesis and potential therapeutic approach.

Introduction

Brown tumor (BT) also known as osteitis fibrosa cystica and rarely as osteoclastoma, represents terminal stage of bone remodeling process emerging by an imbalance between osteoclastic and osteoblastic activity followed by resorption and fibrous tissue replacement of the bone [1]. It represents a reparative cellular process, rather than a neoplastic process commonly seen in jaw bones and presents as well-demarcated, slowly growing painful osteolytic lesion. The presence of these lesions is mostly associated with primary or secondary hyperparathyroidism (HPT), mostly diagnosed by hypercalcemia and hypophosphatemia on routine serum testing [2].

Long-standing excessive levels of parathyroid hormone (PTH) initiate both fibrous and osteoclastic reactions in the skeleton leading to bone resorption. Parathyroidectomy to control HPT is the first treatment of choice for brown tumors as the normalization of parathyroid function leads to a reduction in size or disappearance of the tumor. However, several cases of brown tumors don’t resolve even after parathyroidectomy and subsequent normalization of PTH level. Moreover, in few cases with multiple bony involvement, it is impractical to do surgeries at various sites in the same patient [3]. So, there is necessity to have a conservative approach like targeted therapy which could block certain molecules involved in bone resorption. Extensive literature search show diverse mechanisms involved in the formation of osteoclasts and excessive PTH production. Among numerous secondary messengers, expression of sclerostin and monocyte chemoattractant protein-1 (MCP-1) have been reported to be more important for the differentiation and function of osteoclasts [4]. So we have quantified the aforementioned markers to elucidate their possible role in the pathogenesis of brown tumor. In addition, cluster of differentiation 73 (CD73) IHC was employed to predict the cell lineage of this entity [5]. In this study, we have compiled our institutional experience of brown tumor cases and discussed panel of genes in addition to immunohistochemical markers that show possible involvement in unique pattern of bone resorption.

Cases Presentation

Case 1

A 30-year-old female reported to outpatient department and complained of swelling and vague bony pain on the right side of the jaw for 4 years. She had gone through multiple fractures in the left distal forearm, right humerus, right clavicle and had left renal pain that was diagnosed to be left ureteric calculi later. Her CT scan revealed large expansile bony overgrowth measuring approx. 45 × 30 mm arising from the right body and ramus of the mandible with diffuse sclerosis and a smaller lesion approx. 30 × 25 mm on the left ramus of the mandible. (Figure 1A and B) Blood findings showed increased serum calcium-13.41 mg/dl, PTH-1062.8 pg/ml, alkaline phosphatase-466.2 IU/L, and urinary creatinine-593.6 mg/dl. USG revealed a well-defined, predominantly hypoechoic lesion showing internal vascularity with abutting common carotid artery and left lobe of thyroid suggestive of parathyroid adenoma. Parathyroid scan also suggested left inferior parathyroid adenoma. Histopathological findings revealed numerous multinucleated giant cells scattered in the background of monomorphic spindle cells with plump nuclei throughout the fibro collagenous connective tissue stroma suggestive of giant cell lesion. Later a final diagnosis of brown tumor was made after clinical correlation. (Fig. 2A)

Fig. 1.

Photomicrograph showing radiographical findings of A, B] case 1; C, D] case 2; E, F] case 3

Fig. 2.

Photomicrograph showing histopathological view under 200X (H&E) A] case 1; B] case 2; C] case 3; IHC expression for D] CD73; E] MCP-1; F] Sclerostin

Case 2

An 8-year-old male patient came to us with chief complaint of mobility and shedding of teeth in the mandibular anterior region since about 4 years. Clinically, a well-defined non-tender bony hard swelling was present in the anterior mandibular region which was radiographically seen as a single well-defined radiolucent lesion and increased follicle space around impacted 33, 43, and 44. CT scan of the mandible showed a well-defined expansile and unilocular lesion involving the right hemimandible causing focal irregularity and erosion of the overlying mandibular cortex. (Figure 1C and D)Serum T3 level was normal whereas T4 (14.33 µg/dl), TSH (12.63µIU/ml) and PTH (72.6pg/ml) levels were found to be increased. Ultrasound examination of the neck showed few enlarged deep upper cervical nodes on either side of the neck measuring 14 × 5 mm. Microscopically, numerous multinucleated giant cells were seen with extensive areas of hemorrhage and hemosiderin pigment confirming the diagnosis of a giant cell lesion. (Fig. 2B)

Case 3

A 38-year-old female patient complained of pain and bony hard swelling in the left maxillary posterior region for 3 months. CT scan revealed a large expansile bony overgrowth measuring approx. 20 × 15 mm with diffuse sclerosis in the left posterior maxilla. (Figure 1E F) Blood investigation showed elevated serum PTH level of 550.8 pg/ml and calcium to be 14.12 mg/dl. Microscopically, numerous multinucleated giant cells along with multiple foci of irregular bony trabeculae suspended in hypercellular connective tissue stroma along with extravasated blood were seen as suggestive of giant cell lesion. Therefore, the final diagnosis of brown tumor was made after association with high PTH levels. (Fig. 2C)

Discussion

In this era of modern medicine, understanding the molecular background of any disease has become possible due to the availability of sophisticated diagnostic modalities. The deeper knowledge about a pathology at its root level would aid in devising a specific treatment protocol with better outcomes, especially in lesions that appear as a secondary manifestation of systemic abnormalities. Brown tumor is one such a disease, which is characterised by osteolytic bone lesions due to excessive secretion of PTH with underlying parathyroid neoplasm [6]. These lesions usually exhibit multifocal involvement and mostly seen in ribs, clavicle, pelvis, long bones and mandible causing pain [7]. Due to its uncommon occurrence, clinical and radiological findings often lead to misdiagnoses as malignant bone tumors resulting in inaccurate diagnosis and improper management. Even, if the definitive diagnosis is achieved, the routine surgical treatment would lead to disfigurement and increased financial burden. It becomes more complicated while treating lesions involving multiple bones where surgery is not feasible. These circumstances necessitate an exploration for newer alternative treatment options producing least damage and having preferable prognosis.

PTH-mediated effect on bone is governed by multiple molecules including chemokines, cytokines, and several growth factors for the regulation of bone homeostasis. Recent studies suggest that some effects of PTH on bone are exhibited by its binding to osteocytic receptors resulting in inhibition of expression of SOST gene. This gene encodes sclerostin, a protein that is exclusively expressed in osteocytes, which decrease bone formation by inhibition of Wnt signaling pathway in osteoblasts [8]. The literature search reveals that in humans, PTH causes a significant decrease in circulating sclerostin, but in the present case series, strong immunopositivity for sclerostin was observed in the pathological tissue. It strongly demonstrate that chronic hypersecretion of PTH could end up in the mutation of SOST gene leading to overactivation of sclerostin and subsequent bone resorption.

Another unavoidable and least explored protein in the osteolytic pathway is MCP-1, which is a chemokine acting in an autocrine-paracrine manner to regulate osteoclast formation. It is also seen to be highly expressed at sites of osteoporotic resorption and plays a crucial role in macrophage recruitment, osteoclast differentiation and bone resorption [9]. Siddiqui et al. [10] in their study revealed that in hyperparathyroidism, continuous upregulation of MCP-1 expression is required for PTH-mediated catabolic effects on bone. Our cases also showed an increased immunopositivity for MCP-1, supporting its pivotal role of it in the pathogenesis of PTH mediated bone resorption.

CD73 is a glycophosphatidylinositol anchored enzyme that generates extracellular adenosine and shows its origin from periodontal ligament cells. It is characterized by the properties of self-renewal, multilineage differentiation and is found to be exhibited by reactive giant cell lesions of jaws [11]. The immunonegativity of CD73 in all the included cases represent the fact that the origin of giant cells in brown tumor does not show periodontal stem cell lineage. Instead, they originate from mischievous tumor-like cells which are responsible for their aggressiveness and unpredictable behavior. A thorough research in this string could help to establish the true origin of this condition.

Although brown tumors are rare, it should be considered as a differential diagnosis while reporting osteolytic lesions of jaws to avoid overtreatment. It usually exhibits multifocal bone involvement and resolves after parathyroidectomy, but few exceptional cases do not respond to routine procedures. The gene modulation caused by PTH on secondary messengers like MCP1 and sclerostin leads to continued bone resorption even after parathyroidectomy, which needs to be treated in a different way. Targeting the aforementioned biomarkers in a specific way could efficiently hamper the osteoclastic activity and do wonders in managing those cases with desirable outcomes.

Conclusion

In conclusion, we suggest that sclerostin and MCP1 might be the perfect candidate to delineate brown tumors and to detect their aggressiveness that might help in choosing alternate treatment options, especially in PTH resistant bone lesions. More multicentric experimental studies are needed to validate our results and to consider using it as a biomarker for targeted therapy. Further studies are warranted in this streamline to establish a specific molecular blocker regime to manage these kinds of lesions with minimal burden to the affected individuals.

Authors’ Contributions

All the authors contributed significantly to this manuscript. All authors read and approved the final manuscript.

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Declarations

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The authors declare that they have no known conflict of interest.

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