Due to its pathological variation and rare occurrence, the development of end-stage liver disease is difficult to predict in ZZ individuals. Although SNPs were recently implicated in the etiology of the PI Z-associated lung and liver diseases (Chappell et al. 2008
; Demeo et al. 2008
), findings from the present study represent a functionally-defined genetic modifier of the end-stage pathology. Based on the results of this study, we predict that lower levels of ERManI in individuals homozygous for the A allele at rs4567 generate a conditional hypomorphic allele for ERManI that impairs the liver's capacity to deal with the accumulation of misfolded alpha1-antitrypsin, likely accelerating the rate at which a tolerable threshold is surpassed, resulting in the liver failure. However, it is worth noting that the rs4567(A)-mediated translational suppression of ERManI itself could not be the only modifier, because several older ZZ individuals homozygous for rs4567(A) did not exhibit an accelerated onset of end-stage liver disease. Moreover, not all the affected ZZ infants exhibited this genotype. Importantly, our conclusions need confirmation with carefully collected liver tissue from the analysis of a larger group of affected ZZ patients before the role of ERManI can be substantiated.
The suppression of ERManI occurs when Pi Z or AAT are over-expressed, suggesting that cellular stress is likely required. Previous studies have shown that overexpression of Pi Z does not activate the unfolded response (UPR), but rather augments the NFκB and autophagy pathways (Teckman and Perlmutter 2000
; Teckman et al. 2002
; Hidvegi et al. 2005
). It is possible that certain components of either pathway are actually responsible for the suppression of ERManI translation. The exact mechanism for the above suppression is unknown. However, the in silico
analysis (MicroInspector web server (http://www.imbb.forth.gr/microinspector
)) of sequences flanking rs4567, has identified alternative microRNA binding sites with constitution of the A or G allele (). The possibility that certain microRNAs are responsible for the rs4567(A)-mediated translational suppression of ERManI is currently under investigation.
Figure 6 Predicted binding of miR-205 to rs4567 in ERManI. The predicted binding pattern between mRNA sequences of ERManI is shown. The nucleotide at rs4567 (red) and flanking sequences (black) are shown, as is the sequence for mature human miR-205 (blue). The (more ...)
The frequency of homozygous A allele at rs4567 is ~ 28% in the general Caucasian population. Therefore, under stress conditions similar to Pi Z accumulation, these individuals may not be able to cope with ER stress as efficiently as those homozygous for the G allele, due to the suppression of ERManI translation. This may eventually lead to other pathological conditions, and it will therefore be interesting to determine whether the genotype is associated with other conformational diseases.
Although present within the 3'UTR of ERManI, we cannot dismiss the possibility that rs4567 might also reside in a regulatory region of another gene involved in glycoprotein quality control. It will be interesting to eventually determine whether the genes upstream or downstream of ERManI are also affected by the nucleotide at rs4567.
The present study demonstrates the utility of functional studies to validate the contribution of a SNP in disease pathogenesis, and introduces a novel paradigm in which a subtle defect in the multilevel regulation of gene expression can modify a classical gain-of-toxic-function disorder. Whether a similar modality might undermine pathologies associated with other conformational disorders is still unknown. However, the present findings provide an exciting prospect for early prognosis and hold considerable promise for tapping a new avenue for therapeutic intervention.