Study of RPC32α, subunit of the RNA polymerase III, in a tumor model

The RNA polymerases are key players of transcription. Eukaryotes have three RNA polymerases (I, II and III). The RNA polymerase III (Pol III) has 17 subunits, one of which exists in two alternative forms: RPC32α and RPC32β. Only one of the two forms can be integrated into the enzymes, thus generating either Pol IIIα or Pol IIIβ. While RPC32β is found in all somatic cells, RPC32α is expressed in stem cells and tumor cells. To date nothing is known of their respective roles. Breast cancer is one of the major public health problems, as it is the most common cancer in women. Several types of breast cancers are distinguished, according to the presence or absence of hormonal receptors. Cancers that test negative for estrogen receptors, progesterone receptors and that do not overexpress the human epidermal growth factor receptor 2, are called triple-negative breast cancers. They tend to have a poor prognosis, due to the aggressive nature of the cancer and the lack of targeted therapies. To study the role of RPC32α, a tumor model needed to be identified. In collaboration with Jean-Paul Feugeas (INSERM UMR 1098) a transcriptomic study was performed on 2627 clinical breast tissue samples. The study showed that RPC32α was overexpressed in triplenegative breast cancer, whereas RPC32β was overexpressed in normal tissue. A study on six breast cancer cell lines and one non-tumorigenic line confirmed the results of the transcriptomic study. The breast cancer model was thus validated. A characterization of different breast cancer cell lines showed that other Pol III subunits were not overexpressed in triple-negative breast cancer. The overexpression of RPC32α was therefore not a mere consequence of a Pol III hyperactivity. An analysis of the transcripts synthesized by Pol III showed that overall the Pol III transcript levels were elevated in triplenegative breast cancer compared to other breast cancer subtypes. In order to study the role of RPC32α in tumorigenesis, several RPC32α knock-out cell lines were created using CRISPR-Cas9. The loss of RPC32α did not induce an increase in transcription of the RNAs of RPC32α or RPC32β. This shows that no feed-back loop exists for RPC32α and that the two homologues are not co-regulated. Various Pol III transcripts showed decreased expression levels in the knock-out cell lines. Yet not all transcripts were reduced in the absence of RPC32α. This indicates that some sort of transcription specificity must exist for Pol IIIα and Pol IIIβ. The knock-out cell lines did not show any alterations in their phenotype or growth rates. However, in soft agar assays the knock-out cell lines produced 85% less colonies than the mother cell line. This proves that RPC32α is necessary for tumorigenic growth in vitro. To find out if RPC32α was also necessary for tumorigenic growth in vivo, knock-out and wild type cells were injected into mice. The mice grafted with knock-out cells showed a slowed onset of tumor growth. After six weeks, the mice injected with knock-out cells had tumors half the size of the mice injected with wild type cells. The primary tumor was ablated and mice were tracked for metastasis. Four weeks later, mice injected with RPC32α knock-out cells had 100 times less metastasis than the control group. These results show that RPC32α is necessary for tumorigenic growth in vitro and in vivo. The protein seems also to be implicated in the formation of metastasis, which are one of the greatest problems in cancer treatment today.