HISTORIA, ETIOLOGÍA, TIPOLOGÍA E INCIDENCIA

FAAP24, in addition to SGK3, initially appeared to be a promising target because it was found to be the most statistically significant target identified and its role in being part of the core complex in the Fanconi Anemia (FA) syndrome made it well placed to potentially play a role in poor prognosis in cancer. The FA pathway is known for being essential in DNA damage repair and consequently important in maintaining genomic stability (Kee and D’Andrea, 2010). It is thought that mutation or silencing of genes core to the FA pathway can lead to the inability for DNA repair to occur efficiently, consequently leading to genomic instability and causing hypersensitivity to DNA damaging agents. To date, the most frequently reported gene studied in the FA pathway is FANCD2, which when monoubiquitinylated causes translocation to nuclear foci for DNA repair to continue (Guo et al, 2014), and this is indicative of drug resistance. FAAP24 is thought to be the binding partner of FANCM, another FA gene forming part of the Fanconi Anemia core complex that is required for normal levels of FANCD2 ubiquitylation and for resistance to DNA crosslinking agents. The FANCM-FAAP24 complex is thought to be directly involved in checkpoint signaling in addition to its repair functions (Ashour et al, 2015).

The studies reported here showed that transiently knocking down FAAP24 by siRNA did not resensitize cells to either cisplatin or doxorubicin. This contrasts with a report suggesting the siRNA knockdown of FAAP24 results in a 2-3-fold sensitivity to cisplatin and mitomycin C compared to the respective control in HeLa cells (Ciccia et al, 2007). It is important to bear in mind that our results are from experiments performed in one isogenic paired cell line and that the effects that other groups have reported may be dependent on the presence of other genomic abnormalities that may not be present the cell lines we used. Interrogation of this gene in other more suitable cell line models would be necessary to make conclusions regarding the role of FAAP24 in contributing to poor prognosis. Given its potential role in checkpoint signaling, future studies could also focus on the mechanism by which FAAP24 induces ATR activation, which results in the phosphorylation of FANCM by ATR on S1045 (Ashour et al, 2015).

3.4.2 MRPS12 validation

MRPS12, a mitochondrial ribosomal protein of the S12 family was also identified as being commonly amplified across the three tumour types studied. Although this finding was not statistically significant using the copy number calls generated using the SNP6 microarray processed by CBS, it was statistically significant when the array comparative genomic hybridization (aCGH) method was employed.

MRPS12 is encoded by nuclear genes and is involved in protein synthesis in the mitochondria. There is functional relevance of ribosomal proteins in tumourigenesis; it is known that both tumour suppressors and oncogenes modulate ribosome protein biosynthesis and initiation of ribosome translation in various models (Ruggero et al, 2003). An example is MYC, a proto-oncogene product, which regulates mature ribosome biogenesis and its overexpression results in the increase of activity and expression of ribosomal components, suggesting that this could be an important mechanism by which MYC regulates growth and thus contributes to tumourigenesis (Boon et al, 2001). PTEN has also been shown to have similar functions in the regulation of ribosome biogenesis (Coelho et al, 2005), suggesting that perturbation in the ribosome biogenesis function may be an important factor, that contributes to carcinogenesis.

The amount of ribosomes produced is a limiting factor for cell growth. It is known the amount of ribosomes produced controls the G1-S transition phase of the cell cycle, which is considered to be the critical restriction point, after which cells are destined to undergo division independent of cell growth. It has been reported that inhibition of ribosomal RNA (rRNA) transcription through the depletion of POLR1A, caused cell cycle arrest (Donati et al, 2011). Therefore, targeting factors of rRNA transcription complex looks like a promising strategy for chemotherapeutic treatment in p53 proficient and deficient tumours. Our results showed that knocking down MRPS12 by siRNA enhanced sensitivity to cisplatin treatment in the PEA1 cells. These studies must be carried out in other suitable cell lines, and future studies that address how modulation of MRPS12 affects the cell cycle would also prove beneficial before conclusions can be drawn regarding its contribution to poor prognosis.

ZBTB32 is a zinc finger and BTB domain containing 32, whose function has not fully been characterized and very little is known about it. It is thought to be a repressor of GATA3 by preventing it from binding to DNA (Yoon et al, 2012). GATA3 has particular significance in breast cancer because it is thought to prevent metastasis by maintaining a luminal phenotype; loss of GATA3 is thought to correlate with poor prognosis in these patients. The preliminary validations carried out as part of this study did not show a significant difference between control and knockdown in the PEA1, however the PEA2 cells showed a significant induction of apoptosis with cisplatin treatment. This was thought to be an interesting observation, as this appeared to be a resistance-specific effect as enhanced sensitivity to cisplatin was not observed in the platinum sensitive PEA1 cells, suggesting that ZBTB32 may have a role in contributing to poor prognosis. Due to time constraints, further characterization of this gene was not possible, however, the data here suggests that further characterization of this gene would prove beneficial in determining its role as a suitable biomarker and therapeutic target. Future studies could focus on further elucidating the mechanism by which ZBTB32 represses GATA3 and to identify further targets that ZBTB32 may affect to get a better understanding of how and why the effects are observed.