Background
According to the World Health Organization, it is proposed that benign fibro-osseous lesions be divided into 3 categories, including fibrous dysplasia, ossifying fibroma, and osseous dysplasia [
1,
2]. The most common forms of BFOL are ossifying fibroma (OF) and fibrous dysplasia (FD) and they manifest different clinical courses and should be treated differently [
3].
Ossifying fibroma is a benign tumor thought to arise from the periodontal ligament, which potential of continuous growth if not treated [
4,
5]. It is a slow-growing, generally lack of symptoms, but can cause serious cosmetic and functional problems. [
6] OF can occur in almost any bone in the craniofacial region, predominantly in the premolar-molar region of the mandible, with adult women more frequently affected [
7]. Radio-graphically, OF usually presents a well-circumscribed radiolucency with varying degrees of calcification that rarely erodes or displaces teeth [
4]. Ossifying fibromas are typically encountered as solitary lesions, multiple and/or familial lesions rarely present in clinic [
8]. In some instances, OF can be destructive and risk for recurrence, as a result of which completely surgical enucleating is needed [
9].
FDs are genetic and non-inheritable, caused by missense mutations that occur post-zygotically in the gene,
GNAS, which is located on chromosome 20q13 and codes for the a-subunit of the stimulatory G-protein, Gs [
10]. The mutations generate a somatic mosaic and the resulting proteins display reduced GTPase activity and, consequently, increased activation of adenylyl cyclase. Therefore, mutated cells constitutively generate high levels of cAMP and have a high rate of proliferation [
11]. The differential diagnostic value of
GNAS in FD is well recognized. In previous studies, our research group demonstrated that, in a total of 307 cases of FD and 23 cases of OF, the mutation rate of
GNAS in patients with FD was approximately 86 % (264/307), while
GNAS mutations were not found in patients with OF. [
12] Little is known about the etiology of OF.
Hyperparathyroidism-jaw tumor (HPT-JT) syndrome is an autosomal dominant, multiple neoplasia syndrome which is characterized by parathyroid adenoma or carcinoma, ossifying fibroma of the jaws, renal and uterine lesions [
13,
14]. Between 30 and 40 % of individuals with HPT-JT also develop OFs, which are distinct from the “brown” tumors associated with severe hyperparathyroidism.. The HPT-JT locus was mapped to chromosome 1q24–q32; the putative gene, designated first as hyperparathyroidism type 2 (
HRPT2) and then as
CDC73, encodes a 531 amino acid protein called parafibromin. [
15] Parafibromin is ubiquitously expressed and is evolutionarily conserved. It is the human homologue of yeast Cdc73, which is a component of the yeast RNA polymerase II/Paf1 complex that is important for histone modification and post-transcriptional events [
16,
17]. Mutations in
CDC73 can be detected in approximately 58 % of probands with clinical features of HPT-JT syndrome [
15]. In one series, which included four cases with OFs, mutations in
CDC73 were found in two of the cases [
18].
Collectively, these data suggested that CDC73 may be a key factor in the etiology of HPT–JT syndrome and its related tumors, including OFs. The prevalence and range of CDC73 mutations in sporadic OFs remain to be determined, in view of the limited number of cases examined to date. The goal of the present study was to identify CDC73 mutations in a group of Chinese patients presenting with sporadic OFs and to examine the possible role of CDC73 in the pathogenesis and diagnosis of OFs.
Discussion
Parafibromin, the product of
CDC73, is believed to be important for embryonic development and tumorigenesis [
15]. Homozygous
CDC73 knockout mutations in mice led to in utero death by stage E6.5. Conditional knockout of both
CDC73 alleles in E8.5 or older mice resulted in the retardation of embryonic growth and increased apoptosis, whereas in adult mice it resulted in cachexia and death within 20 days [
22]. Parafibromin is a ubiquitously expressed nuclear protein [
21]. In humans, parafibromin interacts with RNA polymerase II via the human PAF1 complex, which also includes human Paf1, CTR9, and Leo1. This complex regulates a number of key transcriptional events, including transcription initiation, transcript elongation, and post-transcriptional events, including mRNA maturation and maintenance of poly (A) tail length [
16,
17].
Mutations in
CDC73 have been frequently detected in patients with HPT-JT and also occur in 20–29 % of individuals with apparently sporadic parathyroid carcinoma [
23]. However, the frequency of
CDC73 mutations in sporadic parathyroid adenomas is low, at 0–4 %, indicating that
CDC73 mutations likely confer an aggressive growth potential and may result in malignant transformation of parathyroid cells [
24].
CDC73 mutations have been detected in between 0 and 33 % of individuals with familial isolated primary hyperparathyroidism [
23]. Somatic
CDC73 mutations have also been found in sporadic renal tumors [
25].
The frequency of
CDC73 mutations in individuals with sporadic OF of the jaws has not been extensively studied. In the present study, two somatic mutations were identified in 40 cases of OF (5 %). This result suggested a possible pathogenetic role for
CDC73 in some cases of sporadic OFs. Previously, a few cases of sporadic OF was found to harbor a mutation in
CDC73 [
18,
26]. Further studies are needed to clarified the exact frequency of CDC73 mutation in sporadic OF due to limitation of sample size.
The CDC73 mutations identified in this study included two novel somatic mutations (c.13_16delCTTA and c.8_10delACGinsCT), which caused frameshifts and premature truncations of the protein. This finding is consistent with previous studies showing that over 75 % of the reported CDC73 mutations are frameshift or nonsense mutations that are predicted to result in either the truncation of the parafibromin protein or in loss of the translated protein through nonsense-mediated mRNA decay.
Somatic mutations of
CDC73 were predominantly located in exons 1 and 2 (55 and 21 %, respectively), in contrast to germline mutations (22 and 16 %, respectively) mainly detected in HPT-JT, which were predominantly located in exon 7 (30 %). Only one somatic mutation has been detected in exon 7 to date [
24]. The two mutations that we identified in OF both occurred in exon 1, and a previous study on OF [
18] also identified a somatic mutation in exon 1. The basis of the differences in the distribution of the germline and somatic
CDC73 mutations remains to be elucidated.
In vitro studies have shown that parafibromin acts as a tumor suppressor. Overexpression of parafibromin inhibits the proliferation of NIH3T3 and HEK293 cells, increases G1 arrest and apoptosis in HeLa cells, and downregulates expression of the cell cycle regulator, cyclin D1 [
27‐
29]. Likewise, RNAi-mediated inhibition of parafibromin expression in HeLa cells resulted in increased S-phase entry with reduction in basal apoptosis and increase in expression of the proto-oncogene, c-myc [
30,
31].
According to Knudson’s two-hit model for tumor suppressor genes [
32] two mutations, one occurring in each of the two alleles of a gene, or one mutation in one allele of a tumor suppressor gene accompanied by the allelic loss of the remaining wild-type allele, are required to trigger neoplasm formation. Biallelic inactivation in
CDC73 has been detected in tumors in HPT-JT kindred and in sporadic parathyroid carcinomas and renal carcinomas [
15]. Since parafibromin is ubiquitously expressed, biallelic inactivation of
CDC73 can be detected immunohistochemically by the diffuse loss of nuclear expression. This is a common feature in parathyroid carcinoma [
33,
34].
However, the two-hit model may not apply to sporadic OF. First, only one mutation was detected in each case, even in the case with two somatic mutations reported by Pimenta [
18]. Second, normal parafibromin expression was detected by immunohistochemistry. The effect of mutated
CDC73 on sporadic OFs needs to be studied further.
Acknowledgements
The authors gratefully acknowledge the patients for their cooperation.