Because every longevity-control gene eventually is eventually shown to interact with every other longevity-control gene, it is perhaps not surprising that SIRT1 deacetylates WRN, the protein whose gene is mutated in the devastating human progeria Werner’s Syndrome. Both the helicase and exonuclease activities of the WRN protein are more active in the deacetylated state; thus, the longevity-assurance gene (SIRT1) is responsible for boosting the activity of the major player in the cellular response to DNA damage (WRN), which is the way we’d expect it to work.

Two other recent reports describing progress on WRN reveal that the protein plays a significant role in DNA metabolism under normal growth and after DNA damage: WRN is required both for replication fork progression after genotoxic stress as well as suppressing the spontaneous formation of telomeric DNA circles. These latter structures (which remind me of the extrachromosomal ribosomal DNA circles from the early years of the sirtuin field) are associated with both telomere shortening and cellular senescence.

Hutchinson-Gilford progeria syndrome (HGPS) is caused by a mutation in the lamin A/C gene (LMNA); the mutant allele produces a truncated lamin A (“progerin”) that disrupts the nuclear architecture, interferes with cell cycle progression, and may accelerate cellular senescence.

HGPS is dominant, i.e., heterozygotes are fully affected, implying that the ratio of progerin to normal lamin is important. One would predict that more wildtype lamin would titrate out the mutant protein and lessen the severity of the disease. In a counterintuitive finding, Yang et al. demonstrate that the opposite is true: Mice that express only the lamin C isoform, but no lamin A, suffer from much milder progeria than mice expressing both isoforms:

Eliminating the synthesis of mature lamin a reduces disease phenotypes in mice carrying a hutchinson-gilford progeria syndrome allele

Hutchinson-Gilford progeria syndrome is caused by the synthesis of a mutant form of prelamin A, which is generally called progerin. Progerin is targeted to the nuclear rim, where it interferes with the integrity of the nuclear lamina, causes misshapen cell nuclei, and leads to multiple aging-like disease phenotypes. We created a gene-targeted allele yielding exclusively progerin (LmnaHG) and found that heterozygous mice (LmnaHG/+) exhibit many phenotypes of progeria. In this study, we tested the hypothesis that the phenotypes elicited by the LmnaHG allele might be modulated by compositional changes in the nuclear lamina. To explore this hypothesis, we bred mice harboring one LmnaHG allele and one LmnaLCO allele (a mutant allele that produces lamin C but no lamin A). We then compared the phenotypes of LmnaHG/LCO mice (which produce progerin and lamin C) with littermate LmnaHG/+ mice (which produce lamin A, lamin C, and progerin). LmnaHG/LCO mice exhibited improved body weight curves (P < 0.0001), reduced numbers of spontaneous rib fractures (P < 0.0001), and improved survival (P < 0.0001). In addition, LmnaHG/LCO fibroblasts had fewer misshapen nuclei than LmnaHG/+ fibroblasts (P < 0.0001). A likely explanation for these differences was uncovered: the amount of progerin in LmnaHG/LCO fibroblasts and tissues was lower than in LmnaHG/+ fibroblasts and tissues. These studies suggest that compositional changes in the nuclear lamina can influence both the steady-state levels of progerin and the severity of progeria-like disease phenotypes.

Because the animals expressing only lamin C also have lower steady-state levels of progerin, it is possible that the net result is that wildtype lamin is titrating out the mutant protein after all. The mechanism of the progerin downregulation is unclear; the simplest model would seem to be that progerin is stabilized by lamin A but not lamin C.

Regardless, it’s still striking that a trait resulting from dominant mutation in a gene can be ameliorated by lowering levels of the wildtype protein. Does this imply that one could treat HGPS by targeting the wildtype allele of LMNA? Progerin arises at some low level during normal aging, and may contribute to cellular senescence; would lowering levels of lamin A slow the time-dependent accumulation of progerin in otherwise healthy humans?

Earlier this week we learned that mutations in the kinase SCH9, combined with intervention in the RAS and TOR pathways, can extend the chronological lifespan of yeast by as much as tenfold.

Another paper from the same lab has also shown that mutation in SCH9 can suppress the genomic instability that accompanies loss of function in SGS1. Recall that SGS1 is the yeast homolog of the RecQ-like helicase WRN, which is mutated in the human progeria Werner’s syndrome. From Madia et al.:

Longevity mutation in SCH9 prevents recombination errors and premature genomic instability in a Werner/Bloom model system

Werner and Bloom syndromes are human diseases characterized by premature age-related defects including elevated cancer incidence. Using a novel Saccharomyces cerevisiae model system for aging and cancer, we show that cells lacking the RecQ helicase SGS1 (WRN and BLM homologue) undergo premature age-related changes, including reduced life span under stress and calorie restriction (CR), G1 arrest defects, dedifferentiation, elevated recombination errors, and age-dependent increase in DNA mutations. Lack of SGS1 results in a 110-fold increase in gross chromosomal rearrangement frequency during aging of nondividing cells compared with that generated during the initial population expansion. This underscores the central role of aging in genomic instability. The deletion of SCH9 (homologous to AKT and S6K), but not CR, protects against the age-dependent defects in sgs1-delta by inhibiting error-prone recombination and preventing DNA damage and dedifferentiation. The conserved function of Akt/S6k homologues in lifespan regulation raises the possibility that modulation of the IGF-I–Akt–56K pathway can protect against premature aging syndromes in mammals.

Mutation of SCH9, which extends lifespan on its own, suppresses the longevity-shortening phenotype of SGS1 deletion. Since calorie restriction (CR) has no effect on chromosomal rearrangements in the SGS1 mutants, this study has parsed the contributions of SCH9 and CR to at least one molecular correlate of aging (specifically, genome stability).

The authors argue that their results further demonstrate the importance of genome stability to the aging process, a point on which certain researchers focusing on mammalian aging would agree. The question remains, however: Does genomic instability shorten lifespan by increasing transcriptional noise, as suggested by the Vijg lab’s stochasticity experiments, or via another mechanism?

Late last year we learned that the p53 tumor suppressor response to DNA damage dwindles with age, possibly providing an explanation for the exponentially increasing risk of cancer in old mammals. Another recent paper visits the other side of the coin: Moore et al. (from Larry Donehower‘s lab) describe how hyperactive p53 could contribute to premature aging — thereby shortening lifespan even as it prevents tumors:

Aging-associated truncated form of p53 interacts with wild-type p53 and alters p53 stability, localization, and activity

Evidence has accumulated that p53, a prototypical tumor suppressor, may also influence aspects of organismal aging. We have previously described a p53 mutant mouse model, the p53+/m mouse, which is cancer resistant yet exhibits reduced longevity and premature aging phenotypes. p53+/m mice express one full length p53 allele and one truncated p53 allele that is translated into a C-terminal fragment of p53 termed the M protein. The augmented cancer resistance and premature aging phenotypes in the p53+/m mice are consistent with a hyperactive p53 state. To determine how the M protein could increase p53 activity, we examined the M protein in various cellular contexts. Here, we show that embryo fibroblasts from p53+/m mice exhibit reduced proliferation and cell cycle progression compared to embryo fibroblasts from p53+/− mice (with equivalent wild-type p53 dosage). The M protein interacts with wild-type p53, increases its stability, and facilitates its nuclear localization in the absence of stress. Despite increasing p53 stability, the M protein does not disrupt p53–Mdm2 interactions and does not prevent p53 ubiquitination. These results suggest molecular mechanisms by which the M protein could influence the aging and cancer resistance phenotypes in the p53+/m mouse.

The mouse mutation used in this study resembles the p44 allele characterized by Scrable and co-workers in the sense that it is constitutively active. It is therefore likely to stimulate apoptosis and diminish proliferative potential, thereby decreasing the regenerative capacity of a tissue, even in the absence of genotoxic damage. Consistent with this, the p44 mouse also shows signs of segmental progeria. In contrast, when p53 is present in an extra copy but regulated normally (i.e., only activated when the cell detects damage), both tumorigenesis and aging are slowed.

Taken together, results from these two types of mutants suggest that p53 is only a gerontogene when it is activated all the time (similar, perhaps, to the case of cells suffering from chronic DNA damage, as in DNA repair mutants).

Note the subtle clash with the idea, discussed here previously, that tumor suppression inevitably trades off with regenerative capacity: Although inappropriately active p53 does accelerate aging by prematurely culling undamaged cells, properly regulated (but high-copy) p53 appears to be able to block cancer without crippling cellular replenishment pathways.

The relationship between progerias (syndromes that mimic multiple aspects of aging) and aging per se remains controversial: some conditions are best thought of “segmental” progerias (in that they model aging only in specific organs or cell types), whereas others model the natural aging process very closely in the majority of tissues. Chief among the latter are Werner’s Syndrome (WS) and Hutchinson-Gilford Progeria Syndrome (HGPS).

The underlying mutation in the two diseases are quite different: WS is due to a mutation in a DNA helicase involved in repair, whereas HGPS is caused by a dominant mutation in lamin A/C, a which is critical to nuclear structure (and consequently in gene regulation). While the diseases have distinct phenotypes and ages of onset, they are both widely considered good models of accelerated aging. What, if anything, is the common currency beteween the two?

In a recent review article, Cox and Faragher argue that premature cellular senescence is likely to be important in both WS and HGPS:

From old organisms to new molecules: integrative biology and therapeutic targets in accelerated human ageing

Although some molecular pathways that have been proposed to contribute to ageing have been discovered using classical biochemistry and genetics, the complex, polygenic and stochastic nature of ageing is such that the process as a whole is not immediately amenable to biochemical analysis. Thus, attempts have been made to elucidate the causes of monogenic progeroid disorders that recapitulate some, if not all, features of normal ageing in the hope that this may contribute to our understanding of normal human ageing. Two canonical progeroid disorders are Werner’s syndrome and Hutchinson-Gilford progeroid syndrome (also known as progeria). Because such disorders are essentially phenocopies of ageing, rather than ageing itself, advances made in understanding their pathogenesis must always be contextualised within theories proposed to help explain how the normal process operates. One such possible ageing mechanism is described by the cell senescence hypothesis of ageing. Here, we discuss this hypothesis and demonstrate that it provides a plausible explanation for many of the ageing phenotypes seen in Werner’s syndrome and Hutchinson-Gilford progeriod syndrome.

The idea that cellular senescence is important in normal aging happens to be favored in my current lab, and increasingly seems to be the mainstream position (see especially Devil’s bargain: Tradeoffs between stem cell maintenance and tumor suppression, and also here and here). According to this model, senescence (which permanently growth-arrests old and damaged cells) prevents individual cells from forming tumors, but persistent senescent cells embark on a highly anti-social program of gene expression that can surrounding tissues and may contribute to age-related decline in tissue function. Many experiments remain to be done before we can claim confidently that senescence plays a causative role in aging; for now, the most I can say is that the existing data are consistent with the idea.

Lecture-hall cartoon depictions aside, the nucleus is not a balloon. Rather, its architecture is defined by a specialized cytoskeleton (the nuclear lamina) consisting of proteins known as lamins. These proteins give structure to the nuclear envelope during interphase; during cell division, a complex dance of phosphorylation and dephosphorylation causes them to disassemble (allowing for chromosome segregation into the daughter nuclei) and then reassemble following cytokinesis.

Derangement of the lamina might be expected to cause serious trouble for the cell. Indeed, an extensive body of work over the past 5 years has clearly demonstrated that a particular mutation in the lamin A gene (LMNA) results in severe nuclear malformation. In humans, this mutation gives rise to a devastating disease, Hutchinson-Gilford progeria syndrome (HGPS). (For earlier articles on HGPS, see here and here).

Two new papers, published back-to-back in the most recent issue of PNAS, describe the impact of the HGPS lamin A mutant protein (also called “progerin”) on progression of the cell cycle and the results of mitosis.

Dechat et al. find that progerin perturbs the cell cycle at multiple stages, both at entry into S phase and progression into M. The mechanisms are related: An uncleaved farnesylated moiety causes progerin to tightly associate with membranes, drastically changing its solubility profile and distribution; the mutant therefore interferes both with nuclear disassembly (during M, prior to chromosome segregation) and reassembly (after G1, before DNA synthesis begins).

Alterations in mitosis and cell cycle progression caused by a mutant lamin A known to accelerate human aging

Mutations in the gene encoding nuclear lamin A (LA) cause the premature aging disease Hutchinson–Gilford Progeria Syndrome. The most common of these mutations results in the expression of a mutant LA, with a 50-aa deletion within its C terminus. In this study, we demonstrate that this deletion leads to a stable farnesylation and carboxymethylation of the mutant LA (LAΔ50/progerin). These modifications cause an abnormal association of LAΔ50/progerin with membranes during mitosis, which delays the onset and progression of cytokinesis. Furthermore, we demonstrate that the targeting of nuclear envelope/lamina components into daughter cell nuclei in early G1 is impaired in cells expressing LAΔ50/progerin. The mutant LA also appears to be responsible for defects in the retinoblastoma protein-mediated transition into S-phase, most likely by inhibiting the hyperphosphorylation of retinoblastoma protein by cyclin D1/cdk4. These results provide insights into the mechanisms responsible for premature aging and also shed light on the role of lamins in the normal process of human aging.

Cao et al. demonstrate that the mutant protein forms aggregates during mitosis (at a time when the wildtype lamin is happily soluble, waiting for the nuclear envelope to coalesce after cytokineses). As a result, both chromosome segregation and separation of the daughter nuclei is aberrant, and binucleated cells frequently form.

A lamin A protein isoform overexpressed in Hutchinson–Gilford progeria syndrome interferes with mitosis in progeria and normal cells

Hutchinson–Gilford progeria syndrome (HGPS) is a rare genetic disorder characterized by dramatic premature aging. Classic HGPS is caused by a de novo point mutation in exon 11 (residue 1824, C -> T) of the LMNA gene, activating a cryptic splice donor and resulting in a mutant lamin A (LA) protein termed “progerin/LAΔ50″ that lacks the normal cleavage site to remove a C-terminal farnesyl group. During interphase, irreversibly farnesylated progerin/LAΔ50 anchors to the nuclear membrane and causes characteristic nuclear blebbing. Progerin/LAΔ50’s localization and behavior during mitosis, however, are completely unknown. Here, we report that progerin/LAΔ50 mislocalizes into insoluble cytoplasmic aggregates and membranes during mitosis and causes abnormal chromosome segregation and binucleation. These phenotypes are largely rescued with either farnesyltransferase inhibitors or a farnesylation-incompetent mutant progerin/LAΔ50. Furthermore, we demonstrate that small amounts of progerin/LAΔ50 exist in normal fibroblasts, and a significant percentage of these progerin/LAΔ50-expressing normal cells are binucleated, implicating progerin/LAΔ50 as causing similar mitotic defects in the normal aging process. Our findings present evidence of mitotic abnormality in HGPS and may shed light on the general phenomenon of aging.

In the penultimate sentence of that abstract lies our connection to normal aging. Progerin is generated at some rate in normal cells (probably because the splice site ablated in the mutant is not 100% efficient), and when it accumulates to a detectable level, the same cell cycle interference and mitotic failures seen in HGPS can occur in an otherwise normal cell.

If the binucleated progeny of these cell divisions escape apoptosis, they will presumably undergo some type of permanent cell cycle arrest (e.g. senescence), and persist in the tissue — unable to divide further, but certainly available to cause damage to their microenvironment and contribute to age-related decline in tissue function.

The distribution of progerin expression isn’t smooth: There’s a small population of highly progerin-positive cells, and almost no detectable expression in the vast majority of cells (see their Figure 5). If progerin expression is a truly sporadic event, one would expect to see more intermediate cases. The presence of a few very bright cells against a dark background suggests that there might be some sort of positive feedback at play: Could expression of progerin result in expression of even more expression, perhaps by interference with mRNA splicing?

Yesterday we discussed a paper that described short-lived DNA repair-deficient mutants undergoing physiological changes strongly reminiscent of long-lived genetic dwarf or calorie restricted (CR) animals.

Here is another paper that addresses the issue, Niedernhofer et al.:

XPF-ERCC1 endonuclease is required for repair of helix-distorting DNA lesions and cytotoxic DNA interstrand crosslinks. Mild mutations in XPF cause the cancer-prone syndrome xeroderma pigmentosum. A patient presented with a severe XPF mutation leading to profound crosslink sensitivity and dramatic progeroid symptoms. It is not known how unrepaired DNA damage accelerates ageing or its relevance to natural ageing. Here we show a highly significant correlation between the liver transcriptome of old mice and a mouse model of this progeroid syndrome. Expression data from XPF-ERCC1-deficient mice indicate increased cell death and anti-oxidant defences, a shift towards anabolism and reduced growth hormone/insulin-like growth factor 1 (IGF1) signalling, a known regulator of lifespan. Similar changes are seen in wild-type mice in response to chronic genotoxic stress, caloric restriction, or with ageing. We conclude that unrepaired cytotoxic DNA damage induces a highly conserved metabolic response mediated by the IGF1/insulin pathway, which re-allocates resources from growth to somatic preservation and life extension. This highlights a causal contribution of DNA damage to ageing and demonstrates that ageing and end-of-life fitness are determined both by stochastic damage, which is the cause of functional decline, and genetics, which determines the rates of damage accumulation and decline.

The authors of this paper add to the evidence that chronic DNA damage can induce the same metabolic changes as life-extending mutations or treatments. Furthermore, they expand the story by demonstrating a strong correlation between the transcriptome of progeroid DNA repair mutants and aged wildtype mice — arguing, in effect, that the syndrome exhibited by the mutant is a good model of aging. (This last bit is an important piece of the puzzle, sincemany types of mutations can shorten lifespan without actually accelerating the aging process per se.)

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