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.)

DNA repair mutants often show a dramatic accelerated aging phenotype, with affected animals developing signs of frailty (wrinkling, graying, osteoporosis, heart disease, senility, you name it) much more rapidly than their wildtype cohorts. In some cases, these mutants age (“segmentally,” which is to say, not in every part of the body equally) at a rate ten or twenty times faster than normal.

In contrast, small body size due to genetic dwarfism and calorie restriction (CR) are both known to extend lifespan in many organisms (at least when they’re safe and warm in the lab, and in some species possibly only when they’re inbred).

One would therefore expect that animals with DNA repair-related progeroid syndromes would be dissimilar in every regard from dwarf and CR animals — but I wouldn’t have set it up this way if that expectation weren’t wrong. Recent results from a multi-lab collaboration suggest that both certain kinds of (life-shortening) DNA repair deficiencies trigger the same alterations in metabolism that are activated during (life-prolonging) CR. From van de Ven et al.:

… Contrary to expectations, neither accelerated senescence nor acute oxidative stress hypersensitivity was detected in primary fibroblast or erythroblast cultures from multiple progeroid mouse models for defects in the nucleotide excision DNA repair pathway, which share premature aging features including postnatal growth retardation, cerebellar ataxia, and death before weaning. Instead, we report a prominent phenotypic overlap with long-lived dwarfism and calorie restriction during postnatal development (2 wk of age), including reduced size, reduced body temperature, hypoglycemia, and perturbation of the growth hormone/insulin-like growth factor 1 neuroendocrine axis. These symptoms were also present at 2 wk of age in a novel progeroid nucleotide excision repair-deficient mouse model (XPDG602D/R722W/XPA−/−) that survived weaning with high penetrance. However, despite persistent cachectic dwarfism, blood glucose and serum insulin-like growth factor 1 levels returned to normal by 10 wk, with hypoglycemia reappearing near premature death at 5 mo of age. These data strongly suggest changes in energy metabolism as part of an adaptive response during the stressful period of postnatal growth. … Specific (but not all) types of genome instability may thus engage a conserved response to stress that evolved to cope with environmental pressures such as food shortage.

So it would seem that both calorie-restricted and DNA-repair-deficient animals converge on some of the same physiological strategies, albeit with drastically different eventual results. How best to rationalize this?

Short-lived repair mutants seem to divert resources away from growth and into maintenance and repair — because they need more repair. Long-lived dwarf mutants and CR animals divert resources away from growth and into maintenance and repair — because they don’t necessarily have (or don’t believe they have) enough food to rapidly mature to adulthood. In both cases, we can understand the change in priorities as an attempt to wait out temporarily adverse conditions, waiting for things to change for the better so that the process of maturing to adulthood and sexual maturity can begin again. The DNA repair mutants, sadly, don’t know that there’s nothing temporary about their adversity: because the repair machinery itself has been compromised, no improvement will result no matter how many resources they throw at the problem — so even though they might be on the same track as the CR animals, they’ll never make it to the desired destination.

As is often the case with new data, mysteries abound: The DNA repair mutants that seemed to turn on the CR-related metabolic changes were all deficient in nucleotide excision repair (NER). A mutant in a different pathway, double-strand break repair (DSBR) showed signs of premature aging but no indication of the alterations in growth hormone release that was seen in the NER mutants. The rationalization is therefore not simply that the animals need more repair in general. Wild speculation: perhaps NER mutant cells are more prone to apoptosis, requiring active regeneration of lost cells, whereas DSBR mutations allow cells to persist, albeit in a senescent form?

It slays me when there are more reviews than primary papers coming out on a subject — one would think that the ratio of reviews to primary articles should be far less than 1, and that journal editors might reasonably be expected to (say) search PubMed, in order to determine whether the field needs yet another review, before soliciting yet another one.

But on the other hand, there are times when something generally new is happening, and everyone needs to get their heads around it. When that happens, a lot of different perspectives on the same data can be helpful.

Lately I’ve noticed that Hutchinson-Gilford progeria is getting a healthy share of reviews. The lamin A/C mutation story is a genuinely new mechanism for progeria, so I appreciate why there is so much interest. Hence I’ll provide some links to three recent review articles, and leave the determination of overkill as an exercise for the reader.

Altered splicing in prelamin A-associated premature aging phenotypes, Sandre-Giovannoli & Lévy:

… The major pathophysiological mechanism involved in progeria is an aberrant splicing of pre-mRNAs issued from the LMNA gene, due to a de novo heterozygous point mutation, leading to the production and accumulation of truncated lamin A precursors. Aberrant splicing of prelamin A pre-mRNAs causing the production of more extensively truncated precursors is involved in the allelic disease restrictive dermopathy. … In functional terms, all these conditions share the same pathophysiological basis: intranuclear accumulation of lamin A precursors, which cannot be fully processed (due to primary or secondary events) and exert toxic, dominant negative effects on nuclear homeostasis. … In any case, the impact of these mutations on pre-mRNA splicing has rarely been assessed. These disorders affect different tissues and organs, mainly including bone, skin, striated muscles, adipose tissue, vessels, and peripheral nerves in isolated or combined fashions, giving rise to syndromes whose severity ranges from mild to perinatally lethal. In this chapter we … envisage possible targeted therapeutic strategies on the basis of recent research results.

Human laminopathies: nuclei gone genetically awry, Capell & Collins:

Few genes have generated as much recent interest as LMNA, LMNB1 and LMNB2, which encode the components of the nuclear lamina. Over 180 mutations in these genes are associated with at least 13 known diseases – the laminopathies. In particular, the study of LMNA, its products and the phenotypes that result from its mutation have provided important insights into subjects ranging from transcriptional regulation, the cell biology of the nuclear lamina and mechanisms of ageing. Recent studies have begun the difficult task of correlating the genotypes of laminopathies with their phenotypes, and potential therapeutic strategies using existing drugs, modified oligonucleotides and RNAi are showing real promise for the treatment of these diseases.

Human progeroid syndromes, aging and cancer: new genetic and epigenetic insights into old questions, Ramírez et al.:

… With the progress that has been made in understanding the etiologies of these conditions in the past decade, potential therapeutic options have begun to move from the realm of improbability to initial stages of testing. Among these syndromes, relevant advances have recently been made in Werner syndrome, one of several progeroid syndromes characterized by defective DNA helicases, and Hutchinson-Gilford progeria syndrome, which is characterized by aberrant processing of the nuclear envelope protein lamin A. Although best known for their causative roles in these illnesses, Werner protein and lamin A have also recently emerged as key players vulnerable to epigenetic changes that contribute to tumorigenesis and aging. These advances further demonstrate that understanding progeroid syndromes and introducing adequate treatments will not only prove beneficial to patients suffering from these dramatic diseases, but will also provide new mechanistic insights into cancer and normal aging processes.

That last line is the money shot, and an important reason why the study of these rare diseases is important (beyond the value to patients and their families, which we also must not forget): To the extent that progerias represent accelerations of bona fide aging, treatments for these diseases might help us design therapeutics that slow or arrest aspects of normal aging.

Premature aging syndromes in model organisms and humans provide insight into the mechanisms of natural aging. To the extent that progerias represent true acceleration of wildtype aging processes, they also draw our attention to genes that might someday be targeted by lifespan-enhancement strategies.

An excellent recent review from Navarro et al. covers a lot of ground on this subject, discussing many different human progeroid syndromes and focusing especially on the laminopathies, exemplified by Hutchinson-Gilford progeria:

Progeroid syndromes (PSs) constitute a group of disorders characterized by clinical features mimicking physiological aging at an early age. In some of these syndromes, biological hallmarks of aging are also present, whereas in others, a link with physiological aging, if any, remains to be elucidated. … [A]ll the characterized PSs enter in the field of rare monogenic disorders and several causative genes have been identified. These can be separated in subcategories corresponding to (i) genes encoding DNA repair factors, in particular, DNA helicases, and (ii) genes affecting the structure or post-translational maturation of lamin A, a major nuclear component. In addition, several animal models featuring premature aging have abnormal mitochondrial function or signal transduction between membrane receptors, nuclear regulatory proteins and mitochondria: no human pathological counterpart of these alterations has been found to date. … Recently, several studies allowed to establish a functional link between DNA repair and A-type lamins-associated syndromes, evidencing a relation between these syndromes, physiological aging and cancer. …

While I’m on the subject: There’s been some very good recent news on the Hutchinson-Gilford front. Farnesyltransferase inhibitors, a class of drugs originally developed to attack the Ras pathway in cancer, are showing great promise in reversing the nuclear abnormalities resulting from the underlying lamin A/C mutation. Often rare diseases don’t get very much attention from pharmaceutical developers, so it’s nice to know that HGPS patients and their families have some hope for a meaningful treatment.

Here are three of the noteworthy aging-related reviews from the past week.

For the remainder of this coming week, I’ll be devoting this space to a bumper crop of recent papers about Sir2/SIRT1/sirtuin, so be sure to check back.

Hutchison-Gilford progeria: A-type lamin networks in light of laminopathic diseases, Vlcek and Foisner:

… Mutations in A-type lamins cause a variety of diseases from muscular dystrophy and lipodystrophy to systemic diseases such as premature ageing syndromes. The molecular basis of these diseases is still unknown. Here we summarize known interactions of A-type lamins with components of the nuclear envelope and the nucleoplasm and discuss their potential involvement in the etiology and molecular mechanisms of the diseases.

Mitochondrial mutation in worm and human: Long-lived C. elegans Mitochondrial mutants as a model for human mitochondrial-associated diseases, Ventura et al.:

… Over 400 mutations in mitochondrial DNA result directly in pathology and many more disorders associated with mitochondrial dysfunction arise from mutations in nuclear DNA. It is counter-intuitive then, that a class of mitochondrially defective mutants in the nematode Caenorhabditis elegans, the so called Mit (Mitochondrial) mutants, in fact live longer than wild-type animals. In this review, we will reconcile this paradox and provide support for the idea that the Mit mutants are in fact an excellent model for studying human mitochondrial associated diseases (HMADs). …

Animal models and theories of aging: How genetic analysis tests theories of animal aging, Siegfried Hekimi:

Each animal species displays a specific life span, rate of aging and pattern of development of age-dependent diseases. The genetic bases of these related features are being studied experimentally in invertebrate and vertebrate model systems as well as in humans through medical records. Three types of mutants are being analyzed: … Here, I analyze some of what we know today and discuss what we should try to find out in the future to understand the aging phenomenon.

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