The poor proliferation of fibroblasts expressing progerin is the result of both premature senescence and apoptosis [11
], two key features of cells expressing progerin or elevated levels of prelamin A that are likely to contribute to the premature aging phenotype of HGPS. How accumulation of progerin or partially processed prelamin A results in the premature onset of cell dysfunction is unknown but likely linked to alterations in fundamental nuclear processes.
Here, we implemented a stringent screen to identify genes whose altered expression could contribute to the onset of the defective phenotypes induced by dysfunctional prelamin A processing. To this end, we compared the transcriptome of isogenic fibroblasts expressing either the mutant form of lamin A associated with progeria (termed progerin) or elevated levels of wild-type prelamin A, with that of wild-type fibroblasts. We subsequently used the reversion of two phenotypes, cell proliferation and nuclear membrane morphology, towards normal by treatment with farnesyltransferase inhibitors (FTI) and ZMPSTE24 overexpression as a filtering strategy to identify key downstream effectors. This analysis identified FOXQ1, a subtelomeric gene that encodes for a forkhead transcription factor, as the only statistically significant gene whose expression is induced in both cells expressing progerin and cells with elevated levels of wild-type prelamin A, subsequently reduced in both cell types upon treatment with FTI as well as in cells expressing elevated levels of lamin A after ZMPSTE24 overexpression. Strikingly, ectopic expression of FOXQ1 in normal human fibroblasts leads to the development of both features that were used in the filtering strategy (growth defects and alterations in nuclear membrane morphology) in a qualitative and quantitative manner similar to that observed in cells with dysfunctional prelamin A processing. However, we did not observe a significant improvement in growth nor nuclear membrane morphology in progerin-expressing cells after downregulation of FOXQ1 by siRNAs (data not shown), suggesting that altered expression of additional genes contributes to the maintenance of these phenotypes.
Although there is no prior link between FOXQ1 and aging-associated pathologies, there have been studies suggesting that FOXOs, a subfamily of the forkhead transcription factors, play a protective role against age-associated diseases including diabetes, cancer, autoimmune syndromes and neurodegeneration [36
]. FOXOs inhibit mTOR [37
], a potential substrate for targeted therapy [17
]; thus manipulation of FOXOs function could provide clinical benefit for HGPS. Whether FOXQ1 impacts mTOR function remains to be determined.
Lamin A interacts with chromosomes and accumulation of progerin has been shown to cause a decrease in the level of peripheral heterochromatin, loss of heterochromatin associated with the inactive X chromosome [27
], relocalization or decrease levels of markers of heterochromatin including heterochromatin protein 1a (HP1a), histone H3 trimethylated on lysine 9 (H3K9-3me) and histone H3 trimethylated on lysine 27 (H3K27-3me) [20
]. These findings suggest that epigenetic alterations caused by progerin may induce changes in the expression of specific genes, which contribute to the onset of the defective phenotypes. These epigenetic changes could be limited to a gene promoter or, since lamins have been implicated in the positioning of chromosomal domains within the nucleus [41
], comprise large chromosomal regions as a consequence of altered spatial distribution of chromosomes. Indeed, comparative genomic hybridization studies have shown that disruption of lamins function differentially influence the relocalization of chromosomes to nuclear “blebbed” sites [42
]. Interestingly, the gene encoding for FOXQ1 is located on the subtelomeric domain of chromosome arm 6p, a chromosome arm that was consistently overrepresented in nuclear membrane blebs induced by lamin deficiency [42
Changes in gene expression between cells from HGPS patients and normal controls have been reported [30
]. However, very little overlap in the identity of differentially expressed genes have been observed, possibly suggesting that genetic heterogeneity in the cell lines used for these studies influenced the outcome of the analyses. Alternatively, progerin may induce stochastic epigenetic alterations that affect distinct gene sets, each of which contributes in distinct ways to the onset of the progeroid phenotypes.