Enhanced inhibition of clonogenic survival of human medulloblastoma cells by multimodal treatment with ionizing irradiation, epigenetic modifiers, and differentiation-inducing drugs
Our group has previously demonstrated a high frequency of allelic loss on the short arm of chromosome 8 and identified a region of homozygous deletion of 1.41 Mb, flanked by D8S520 and D8S1130, on 8p23.1 in medulloblastomas, suggesting the presence of a tumor suppressor gene in this critical deletion region. The aim of our study was to investigate whether PinX1, a newly identified gene whose product is a potent inhibitor of telomerase, is the target gene in the homozygous deletion region identified in medulloblastomas. We assessed for alterations in gene sequence and transcript expression of PinX1, as well as the correlation between PinX1 expression and telomerase activity in a series of 52 medulloblastomas, 3 medulloblastoma cell lines (D283, D341 and Daoy) and 4 primitive neuroectodermal tumors (PNETs). Direct sequence analysis of all 7 exons and splice junctions of the PinX1 gene revealed no somatic mutations but 11 genetic polymorphisms. Transcript expression of PinX1, as evaluated by reverse transcription-polymerase chain reaction, in microdissected tumors and normal cerebellum showed 2 transcript variants, corresponding to the full-length form and an alternative spliced variant lacking exon 6, in all samples. This result indicated that PinX1 expression was not suppressed in medulloblastomas. Using the telomeric repeat amplification protocol (TRAP) assay, 13 of 19 (68%) medulloblastomas, 1 of 2 PNETs and all 3 cell lines showed telomerase activity. There is no significant correlation between PinX1 transcript expression and telomerase activity, but our results showed that telomerase activation is involved in medulloblastomas. Taken together, our results suggest that PinX1 does not play a major role in the oncogenesis of medulloblastomas.