Implications of differential transcription start site selection on chronic myeloid leukemia and prostate cancer cell protein expression.

The relevance of minor transcription start sites in broad promoters is not well understood. We have studied AGAP2 expression in prostate cancer and chronic myeloid leukemia (CML), showing transcription is initiated from alternative transcription start sites (TSSs) within a single TSS cluster, producing cancer-type-specific AGAP2 mRNAs with small differences in their 5' UTR length. Interestingly, in the CML cell lines where the 5' UTR is longer, AGAP2 protein levels are lower. We demonstrate that the selection of an upstream TSS involved the formation of a G quadruplex in the 5' UTR, decreasing polysome formation. After developing a bioinformatics pipeline to query data from the FANTOM project and the NCl-60 human tumor cell lines screen, we found HK1 expression can also be regulated by the same mechanism. Overall, we present compelling data supporting TSS selection within a TSS cluster play a role in protein expression and should not be ignored.