Autor(es):
Martins, Rute 
; Proença, Daniela ; Silva, Bruno ; Barbosa, Cristina ; Silva, Ana Luísa ; Faustino, Paula ; Romão, Luísa
Data: 2012
Identificador Persistente: http://hdl.handle.net/10400.18/887
Origem: Repositório Científico do Instituto Nacional de Saúde
Assunto(s): Doenças Genéticas; Genómica Funcional e Estrutural; HFE; NMD; Alternative Polyadenylation; Gene Expression Regulation
Descrição
Nonsense-mediated decay (NMD) is an mRNA surveillance pathway that selectively recognizes and degrades defective
mRNAs carrying premature translation-termination codons. However, several studies have shown that NMD also targets
physiological transcripts that encode full-length proteins, modulating their expression. Indeed, some features of
physiological mRNAs can render them NMD-sensitive. Human HFE is a MHC class I protein mainly expressed in the liver that,
when mutated, can cause hereditary hemochromatosis, a common genetic disorder of iron metabolism. The HFE gene
structure comprises seven exons; although the sixth exon is 1056 base pairs (bp) long, only the first 41 bp encode for amino
acids. Thus, the remaining downstream 1015 bp sequence corresponds to the HFE 39 untranslated region (UTR), along with
exon seven. Therefore, this 39 UTR encompasses an exon/exon junction, a feature that can make the corresponding
physiological transcript NMD-sensitive. Here, we demonstrate that in UPF1-depleted or in cycloheximide-treated HeLa and
HepG2 cells the HFE transcripts are clearly upregulated, meaning that the physiological HFE mRNA is in fact an NMD-target.
This role of NMD in controlling the HFE expression levels was further confirmed in HeLa cells transiently expressing the HFE
human gene. Besides, we show, by 39-RACE analysis in several human tissues that HFE mRNA expression results from
alternative cleavage and polyadenylation at four different sites – two were previously described and two are novel
polyadenylation sites: one located at exon six, which confers NMD-resistance to the corresponding transcripts, and another
located at exon seven. In addition, we show that the amount of HFE mRNA isoforms resulting from cleavage and
polyadenylation at exon seven, although present in both cell lines, is higher in HepG2 cells. These results reveal that NMD
and alternative polyadenylation may act coordinately to control HFE mRNA levels, possibly varying its protein expression
according to the physiological cellular requirements.
