84525

HOPX

CAMEO|HOD|HOP|LAGY|NECC1|OB1|SMAP31|TOTO;HOP homeobox;HOPX;HOP homeobox

homeodomain-only protein|lung cancer-associated Y protein|not expressed in choriocarcinoma clone 1|not expressed in choriocarcinoma protein 1|odd homeobox 1 protein|odd homeobox protein 1

4q12

protein-coding

Promoter DNA hypermethylation with gene silencing is a common feature of human cancer, and cancer-prone methylation is believed to be a landmark of tumor suppressor genes (TSG). Identification of novel methylated genes would not only aid in the development of tumor markers but also elucidate the biological behavior of human cancers. We identified several epigenetically silenced candidate TSGs by pharmacologic unmasking of esophageal squamous cell carcinoma (ESCC) cell lines by demethylating agents (5-aza-2-deoxycitidine and trichostatin A) combined with ESCC expression profiles using expression microarray. HOP/OB1/NECC1 was identified as an epigenetically silenced candidate TSG and further examined for (a) expression status, (b) methylation status, and (c) functional involvement in cancer cell lines. (a) The HOP gene encodes two putative promoters (promoters A and B) associated with two open reading frames (HOPalpha and HOPbeta, respectively), and HOPalpha and HOPbeta were both down-regulated in ESCC independently. (b) Promoter B harbors dense CpG islands, in which we found dense methylation in a cancer-prone manner (55% in tumor tissues by TaqMan methylation-specific PCR), whereas promoter A does not harbor CpG islands. HOPbeta silencing was associated with DNA methylation of promoter B in nine ESCC cell lines tested, and reactivated by optimal conditions of demethylating agents, whereas HOPalpha silencing was not reactivated by such treatments. Forced expression of HOP suppressed tumorigenesis in soft agar in four different squamous cell carcinoma cell lines. More convincingly, RNA interference knockdown of HOP in TE2 cells showed drastic restoration of the oncogenic phenotype. In conclusion, HOP is a putative TSG that harbors tumor inhibitory activity, and we for the first time showed that the final shutdown process of HOP expression is linked to promoter DNA hypermethylation under the double control of the discrete promoter regions in cancer.

BACKGROUND: Homeodomain proteins play critical roles in shaping the development of the embryonic central nervous system in mammals. After birth, neurogenic activities are relegated to stem cell niches, which include the subgranular layer of the dentate gyrus of the hippocampus. Here, we have analyzed the function of HOP (Homeodomain only protein) in this stem cell niche and in human glioblastomas. RESULTS: We find that HOP is strongly expressed by radial astrocytes of the dentate gyrus in mice, which are stem cells that give rise to hippocampal granular neurons throughout adulthood. Deletion or down-regulation of HOP results in a decrease of apoptosis of these stem cells without changes in proliferation, and in an increase in the number of newly formed granule neurons. We also find that human glioblastomas largely lack HOP expression and that reintroduction of HOP function in glioma cells cultured as gliomaspheres leads to enhanced apoptosis in a subset of cases. In these cells, HOP function decreases clonogenicity. CONCLUSION: These data suggest that HOP participates in the regulation of the adult mouse hippocampal stem cell niche by negatively affecting cell survival. In addition, HOP may work as a tumor suppressor in a subset of glioblastomas. HOP function thus appears to be critical in the adult brain in a region of continued plasticity, and its deregulation may contribute to disease.