Shortening of the 3′ untranslated region: an important mechanism leading to overexpression of HMGA2 in serous ovarian cancer

Abstract

Background Oncofetal protein high-mobility-group AT-hook protein 2 (HMGA2) is reactivated in serous ovarian cancer (SOC) and its overexpression correlates with poor prognosis. To explore the mechanism, we investigated whether HMGA2 could avoid microRNA regulation due to gene truncation or 3′ UTR shortening by alternative polyadenylation. Methods Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to evaluate the abundance of different regions of HMGA2 mRNA in 46 SOC samples. Rapid amplification of cDNA 3′ ends (3′ RACE) and Southern blotting were used to confirm the shortening of 3′ untranslated region (UTR). 5′ RACE and Southern blotting were used to prove the mRNA decay. Results No significant difference in the ratio of the stable coding region to the fragile region was observed between SOC and control normal fallopian tubes, indicating that the HMGA2 gene is not truncated in SOC. Varying degrees of 3′ UTR shortening in SOC samples were observed by comparing the abundance of the proximal region and distal region of the HMGA2 3′ UTR. The ratio of the proximal to the distal region of the 3′ UTR correlated significantly with expression of the HMGA2 coding region in SOC (r=0.579, P <0.01). Moreover, although the abundance of the HMGA2 coding region varied, all samples, including the very low expressed samples, exhibit relatively high levels of the proximal 3′ UTR region, suggesting a dynamic decay of HMGA2 mRNA from the 5′ end. The shortening of 3′ UTR and the decay from the 5′ end were confirmed by 3′ RACE, 5′ RACE and subsequent Southern blotting. Conclusion Heterogeneous 3′ UTR lengths render HMGA2 susceptible to different levels of negative regulation by microRNAs, which represents an important mechanism of HMGA2 reactivation in SOC.