DBC2 resistance is achieved by enhancing 26S proteasome-mediated protein degradation

Variants in RHOBTB2 associated with cancer and rare developmental and epileptic encephalopathy

RHOBTB2 is a member of the Rho GTPases subfamily of signaling proteins, known tumor suppressors whose loss of function and decreased expression is associated with cancer onset. Beyond its cancer-related role, RHOBTB2 is implicated in rare neurodevelopmental disorders, specifically RHOBTB2-related disorders, recognized in 2018 as a subtype of developmental and epileptic encephalopathies (DEE). Common symptoms of these disorders include early-onset epilepsy, severe intellectual disability, microcephaly, and movement disorders. Few studies have investigated patient variants associated with RHOBTB2-related disorders, and the impact of these variants on protein function remains unclear. Limited research suggests that the accumulation of RHOBTB2 in neural tissues contributes to the development of DEE. Similarly, preclinical studies indicate that missense variants near or in the BTB domain of RHOBTB2 result in decreased degradation of RHOBTB2 and the onset of DEE, whereas variants in the GTPase domain cause more variable neurodevelopmental symptoms, but do not impair proteasomal degradation of RHOBTB2. However, the exact pathophysiological mechanisms are unclear and may differ across variants. Current treatment approaches for individuals with RHOBTB2-related DEE involve the use of antiseizure medications to decrease seizures; however, no treatments have been identified that address the other symptoms or the underlying pathophysiological mechanisms associated with these disorders. Overall, RHOBTB2 remains an understudied protein with limited information on its function and how it contributes to disease mechanisms. This review provides an overview of the current knowledge of RHOBTB2 function, with an emphasis on its association with neurodevelopmental disorders through an analysis of preclinical studies and case reports that link individual variants with clinical features.

Molecular basis and current insights of atypical Rho small GTPase in cancer

Atypical Rho GTPases are a subtype of the Rho GTPase family that are involved in diverse cellular processes. The typical Rho GTPases, led by RhoA, Rac1 and Cdc42, have been well studied, while relative studies on atypical Rho GTPases are relatively still limited and have great exploration potential. With the increase in studies, current evidence suggests that atypical Rho GTPases regulate multiple biological processes and play important roles in the occurrence and development of human cancers. Therefore, this review mainly discusses the molecular basis of atypical Rho GTPases and their roles in cancer. We summarize the sequence characteristics, subcellular localization and biological functions of each atypical Rho GTPase. Moreover, we review the recent advances and potential mechanisms of atypical Rho GTPases in the development of multiple cancers. A comprehensive understanding and extensive exploration of the biological functions of atypical Rho GTPases and their molecular mechanisms in tumors will provide important insights into the pathophysiology of tumors and the development of cancer therapeutic strategies.

Atypical Rho GTPases of the RhoBTB Subfamily: Roles in Vesicle Trafficking and Tumorigenesis

RhoBTB proteins constitute a subfamily of atypical Rho GTPases represented in mammals by RhoBTB1, RhoBTB2, and RhoBTB3. Their characteristic feature is a carboxyl terminal extension that harbors two BTB domains capable of assembling cullin 3-dependent ubiquitin ligase complexes. The expression of all three RHOBTB genes has been found reduced or abolished in a variety of tumors. They are considered tumor suppressor genes and recent studies have strengthened their implication in tumorigenesis through regulation of the cell cycle and apoptosis. RhoBTB3 is also involved in retrograde transport from endosomes to the Golgi apparatus. One aspect that makes RhoBTB proteins atypical among the Rho GTPases is their proposed mechanism of activation. No specific guanine nucleotide exchange factors or GTPase activating proteins are known. Instead, RhoBTB might be activated through interaction with other proteins that relieve their auto-inhibited conformation and inactivated through auto-ubiquitination and destruction in the proteasome. In this review we discuss our current knowledge on the molecular mechanisms of action of RhoBTB proteins and the implications for tumorigenesis and other pathologic conditions.

Hsp90-dependent assembly of the DBC2/RhoBTB2-Cullin3 E3-ligase complex

The expression of the wild-type tumor-suppressor gene DBC2 (Deleted-in-Breast Cancer 2, a.k.a RhoBTB2) is suppressed in many cancers, in addition to breast cancer. In a screen for Cdc37-associated proteins, DBC2 was identified to be a potential client protein of the 90 kDa heat shock protein (Hsp90) chaperone machine. Pull down assays of ectopically expressed DBC2 confirmed that DBC2 associated with Hsp90 and its co-chaperone components in reticulocyte lysate and MCF7 cells. Similar to other atypical Rho GTPases, DBC2 was found to have retained the capacity to bind GTP. The ability of DBC2 to bind GTP was modulated by the Hsp90 ATPase cycle, as demonstrated through the use of the Hsp90 chemical inhibitors, geldanamycin and molybdate. The binding of full length DBC2 to GTP was suppressed in the presence of geldanamycin, while it was enhanced in the presence of molybdate. Furthermore, assembly of DBC2-Cullin3-COP9 E3 ligase complexes was Hsp90-dependent. The data suggest a new paradigm for Hsp90-modulated assembly of a Cul3/DBC2 E3 ubiquitin ligase complex that may extend to other E3 ligase complexes.

Downregulated RhoBTB2 expression contributes to poor outcome in osteosarcoma patients

PURPOSE

Osteosarcoma is a malignant bone tumor. RhoBTB2 protein participated in various cellular activities and influenced pathways responsible for cell cycle and apoptosis. To address its potential as a therapeutic target for osteosarcoma, this study investigated the effect of RhoBTB2 expression on osteosarcoma tissue and cell.

MATERIALS AND METHODS

Real-time PCR and immunohistochemistry analysis were performed to evaluate the level of RhoBTB2 mRNA and protein in 121 osteosarcoma specimens. The relationship of RhoBTB2 expression with clinicopathological parameters of osteosarcoma patients was analyzed using Chi-square test. In addition, a plasmid expressing the RhoBTB2 gene was transfected into human osteosarcoma (HOS) cell using Lipofectamine 2000, and the effects of RhoBTB2 on HOS cell were investigated using 3-(4,5-dimethylthiazolyl)-2,5-diphenyltetrazoliumbromide assay and flow cytometry.

RESULTS

This study reports that RhoBTB2 protein is weakly expressed in osteosarcoma specimens, but highly in normal parts of specimens. RhoBTB2 expression is significantly associated with primary location and local recurrence of osteosarcoma. Overexpression of RhoBTB2 results in significant G1 phase arrest and apoptosis in HOS cell.

CONCLUSION

Taken together, we identified the role RhoBTB2 in osteosarcoma tissue and cell. The results might not only be of relevance for diagnosis and prognosis, but potentially also provide a novel target for osteosarcoma therapies.

Loss of DBC2 expression is an early and progressive event in the development of lung adenocarcinoma

PURPOSE

DBC2 (Deleted in Breast Cancer 2) has been indicated to be a tumor suppressor gene in many cancers including lung adenocarcinoma recently. In this study, we aimed to explore the expression status of DBC2 in different subtypes of lung adenocarcinoma (from pre-invasive to invasive lesions), and to determine if downregulation becomes more marked with pathological progression.

METHODS

We collected 172 tissue samples from different subtypes of lung adenocarcinoma and investigated the frequency of DBC2 loss by immunohistochemistry.

RESULTS

Our results indicated that DBC2 downregulation is a relatively frequent event in lung adenocarcinoma. Moreover, as the adenocarcinoma subtype turns to be more invasive, more downregulation occurred.

CONCLUSION

We conclude that loss of DBC2 expression is an early and progressive event in the pathogenesis of lung adenocarcinoma. Positive DBC2 immunohistochemistry may become an indicator for early stage disease and better prognosis of lung adenocarcinomas.

A novel tumor suppressor gene RhoBTB2 (DBC2): frequent loss of expression in sporadic breast cancer

RhoBTB2 was isolated recently as a tumor suppressor gene from human chromosome 8p21.3. Although RhoBTB2 was found to be frequently lost in breast cancer lines, expression status of RhoBTB2 in sporadic breast cancer tissues and its clinical and prognostic value, however, remain unclear. Tissue samples from breast cancer patients and normal controls and cell samples from cell lines were collected and reverse transcription (RT)-PCR was used to monitor the presence of RhoBTB2 mRNA. The protein expression of RhoBTB2 was detected by immunohistochemical staining. Cumulative survival time was assessed by the Kaplan-Meier method and Cox regression model. We discovered that RhoBTB2 expression was lacking in a breast ductal epithelial carcinoma cell line T-47D but was expressed in other types of tumor cell lines and normal tissues we tested. The results from tissue samples showed that RhoBTB2 was absent in 60% of breast cancers on both the mRNA and protein level. The results from RT-PCR were completely uniform with those from immunohistochemistry. We demonstrated that loss of RhoBTB2 more frequently occurred in postmenopausal patients of age >or=50 yr old and in patients with infiltrating ductal carcinoma of the breast. The prognostic value of RhoBTB2 in breast cancers also be assessed by a long-term follow-up investigation and we found that patients with RhoBTB2-negative breast cancer were linked to poor clinical prognosis. Therefore, the loss of RhoBTB2 expression is a common occurrence in breast cancers and it is an important factor in the development and prognosis of sporadic breast cancer.

Metastatic susceptibility locus, an 8p hot-spot for tumour progression disrupted in colorectal liver metastases: 13 candidate genes examined at the DNA, mRNA and protein level

Background

Mortality from colorectal cancer is mainly due to metastatic liver disease. Improved understanding of the molecular events underlying metastasis is crucial for the development of new methods for early detection and treatment of colorectal cancer. Loss of chromosome 8p is frequently seen in colorectal cancer and implicated in later stage disease and metastasis, although a single metastasis suppressor gene has yet to be identified. We therefore examined 8p for genes involved in colorectal cancer progression.

Methods

Loss of heterozygosity analyses were used to map genetic loss in colorectal liver metastases. Candidate genes in the region of loss were investigated in clinical samples from 44 patients, including 6 with matched colon normal, colon tumour and liver metastasis. We investigated gene disruption at the level of DNA, mRNA and protein using a combination of mutation, semi-quantitative real-time PCR, western blotting and immunohistochemical analyses.

Results

We mapped a 2 Mb region of 8p21-22 with loss of heterozygosity in 73% of samples; 8/11 liver metastasis samples had loss which was not present in the corresponding matched primary colon tumour. 13 candidate genes were identified for further analysis. Both up and down-regulation of 8p21-22 gene expression was associated with metastasis. ADAMDEC1 mRNA and protein expression decreased during both tumourigenesis and tumour progression. Increased STC1 and LOXL2 mRNA expression occurred during tumourigenesis. Liver metastases with low DcR1/TNFRSF10C mRNA expression were more likely to present with extrahepatic metastases (p = 0.005). A novel germline truncating mutation of DR5/TNFRSF10B was identified, and DR4/TNFRSF10A SNP rs4872077 was associated with the development of liver metastases (p = 0.02).

Conclusion

Our data confirm that genes on 8p21-22 are dysregulated during colorectal cancer progression. Interestingly, however, instead of harbouring a single candidate colorectal metastasis suppressor 8p21-22 appears to be a hot-spot for tumour progression, encoding at least 13 genes with a putative role in carcinoma development. Thus, we propose that this region of 8p comprises a metastatic susceptibility locus involved in tumour progression whose disruption increases metastatic potential.

Rho GTPases of the RhoBTB subfamily and tumorigenesis

RhoBTB proteins constitute a subfamily of atypical members within the Rho family of small guanosine triphosphatases (GTPases). Their most salient feature is their domain architecture: a GTPase domain (in most cases, non-functional) is followed by a prolinerich region, a tandem of 2 broadcomplex, tramtrack, bric a brac (BTB) domains, and a conserved Cterminal region. In humans, the RhoBTB subfamily consists of 3 isoforms: RhoBTB1, RhoBTB2, and RhoBTB3. Orthologs are present in several other eukaryotes, such as Drosophila and Dictyostelium, but have been lost in plants and fungi. Interest in RhoBTB arose when RHOBTB2 was identified as the gene homozygously deleted in breast cancer samples and was proposed as a candidate tumor suppressor gene, a property that has been extended to RHOBTB1. The functions of RhoBTB proteins have not been defined yet, but may be related to the roles of BTB domains in the recruitment of cullin3, a component of a family of ubiquitin ligases. A model emerges in which RhoBTB proteins are required to maintain constant levels of putative substrates involved in cell cycle regulation or vesicle transport through targeting for degradation in the 26S proteasome. RhoBTB proteins are engrossing the list of Rho GTPases involved in tumorigenesis. Unlike typical Rho GTPases (usually overexpressed or hyperactive), RhoBTB proteins appear to play a part in the carcinogenic process through a mechanism that involves the decreased or abolished expression of the corresponding genes, or more rarely, mutations that result in impaired functioning of the protein, presumably leading to the accumulation of RhoBTB substrates and alterations of the cellular homeostasis.