Progeria


Nuclear DNA repair mutants exhibit progeroid symptoms. There are many types of DNA damage, and, accordingly, we have evolved mechanisms to deal with each type of damage. Nucleotide excision repair removes bulky adducts, and base excision repair removes damaged bases. Mismatch repair fixes nucleotides that aren’t matched in their correct A:T/G:C configuration. Lastly, non-homologous end joining and recombination can fix double stranded breaks. Deficiencies in several of these repair mechanisms have been implicated in aging, and they may play a role in age-related disease.

Repair mechanisms also exist for mitochondrial DNA. Mitochondria have robust base excision repair, and there is new evidence for mismatch repair. But do deficiencies in mtDNA repair play a similar role in aging? We’ve already seen that mitochondrial DNA damage accumulates with age. And calorie restriction, the gold standard of lifespan extension, prevents this increase in damage. A new report in Nucleic Acids Research looks at the controversy over when mitochondria DNA deletions occur: is it during replication or repair? Using a proofreading-deficient mitochondrial DNA polymerase (POLG), which causes premature aging phenotypes and early death, Bailey et al provide supporting evidence that the majority of damage comes from replication pausing and breakage at fragile sites.

This particular mutation in POLG results in high levels of point mutations and linear DNA (mtDNA is normally circular). Most of the linear fragments came from one particular region, and POLG is known to initiate and pause replication at specific mtDNA regions. This indicates a common location of breakage. The authors showed that the POLG mutant mouse had increased replication intermediates compared to wild type. Because of the high number of point mutations, POLG might be stalling at sites it is attempting to repair. In this manner, DNA damage is acting as a checkpoint for replication. POLG mutant mice mtDNA is also more sensitive to the single stranded nuclease S1, indicating chromosomal breakage. These single stranded ends can give rise to deletions through recombination.

The authors argue that the increased level of chromosomal breakage and the replicative pausing in the mutant mouse are responsible for the progeroid symptoms of the POLG mouse. In their view, mitochondrial DNA replication is actually upregulated in order to compensate for the reduction in replication capacity. Because of the high levels in point mutations, ox/phos activity would be decreased, which might lead to an even greater need for mitochondrial DNA replication.

And because DNA processing resources – nucleotide precursors as well as enzymes such as RNase H1, Flap endonuclease, and Brca1 – are shared by mitochondria and the nucleus, it is possible that there is a connection between the POLG mutator mouse and mutations in nuclear DNA repair proteins. The phenotype of DNA repair mutants could be caused not by mutations themselves, but by the effort it takes to prevent DNA mutation from occurring past some threshold which would cause cellular catastrophe. The authors note the similarities between POLG and WRN, a helicase in the nucleus. Like POLG, WRN is involved with both DNA replication and DNA repair. Mutations in WRN cause similar DNA breakage and lead to the human progeroid Werner syndrome.

What do you think? Is it possible that the problem in progeroid models is not due to the DNA damage itself, but to the energy required to prevent a catastrophic collapse of DNA integrity?

ResearchBlogging.orgBailey, L., Cluett, T., Reyes, A., Prolla, T., Poulton, J., Leeuwenburgh, C., & Holt, I. (2009). Mice expressing an error-prone DNA polymerase in mitochondria display elevated replication pausing and chromosomal breakage at fragile sites of mitochondrial DNA Nucleic Acids Research DOI: 10.1093/nar/gkp091

Sirtuins are involved in longevity assurance in organisms as evolutionarily diverse as yeast, worms, and mice. All members of the family have homology to histone deacetylases (HDACs), but each protein has unique characteristics as well. Individual family members have distinct tissue expression profiles, subcellular localization, and substrate specificity. Over the past few years, we’ve begun to learn a great deal about the specific targets and interactions of each sirtuin, and how these interaction contribute to their functions in prolonging lifespan.

The SIRT6 protein, one of seven sirtuins encoded by mammalian genomes, came onto biogerontologists’ radar with a report from Katrin Chua‘s group that its histone H3K9 deacetylase activity is required to maintain telomeric chromatin in a healthy state. Furthermore, SIRT6 is required for the proper localization of the Werner’s syndrome protein, WRN, to telomeres: in the absence of SIRT6, the WRN-telomere association becomes unstable, recapitulating several of the cellular phenotypes of Werner’s progeria. (SIRT6 isn’t the only sirtuin involved in WRN biology: SIRT1, the most well-studied member of the family, appears to directly deacetylate WRN).

A second association between SIRT6 and aging has been revealed by a new study from the Chua lab: SIRT6 associates with the transcription factor NF-κB and deacetylates histones at NF-κB-bound promoters, causing them to become less active. Genetic suppression studies suggest that SIRT6’s influence on lifespan might be primarily mediated by NF-κB. Kawahara et al.:

SIRT6 Links Histone H3 Lysine 9 Deacetylation to NF-κB-Dependent Gene Expression and Organismal Life Span

Members of the sirtuin (SIRT) family of NAD-dependent deacetylases promote longevity in multiple organisms. Deficiency of mammalian SIRT6 leads to shortened life span and an aging-like phenotype in mice, but the underlying molecular mechanisms are unclear. Here we show that SIRT6 functions at chromatin to attenuate NF-κB signaling. SIRT6 interacts with the NF-κB RELA subunit and deacetylates histone H3 lysine 9 (H3K9) at NF-κB target gene promoters. In SIRT6-deficient cells, hyperacetylation of H3K9 at these target promoters is associated with increased RELA promoter occupancy and enhanced NF-κB-dependent modulation of gene expression, apoptosis, and cellular senescence. Computational genomics analyses revealed increased activity of NF-κB-driven gene expression programs in multiple Sirt6-deficient tissues in vivo. Moreover, haploinsufficiency of RelA rescues the early lethality and degenerative syndrome of Sirt6-deficient mice. We propose that SIRT6 attenuates NF-κB signaling via H3K9 deacetylation at chromatin, and hyperactive NF-κB signaling may contribute to premature and normal aging.

NF-κB has been widely implicated in the aging process, especially in the context of inflammatory transcription resulting in “inflammaging.” Indeed, a very recent study has suggested that knocking down NF-κB activity is sufficient to reverse the effects of chronological aging in the skin, at least at the level of gene expression, possibly by blocking inflammatory transcription and allowing the tissue’s natural regenerative capacity to proceed without obstacle.

As with the WRN story, this isn’t the first time a sirtuin has been implicated in regulating the activity of NF-κB — but also as with WRN, the mechanisms of sirtuin action are distinct. Studies of chronic obstructive pulmonary disease have revealed that SIRT1 directly deacetylates NF-κB, reducing its activity. In contrast, SIRT6 appears to associated with NF-κB but then exploit this interaction to “follow” the transcription factor to promoters, where it deacetylates histone H3K9 and facilitates formation of a closed or inactive chromatin state. Kind of a neat team: SIRT1 directly deacetylates proteins of interest, while SIRT6 acts in the same location but operates on chromatin. Working together, the proteins may well have greater than additive impact.

Thus, there is partial redundancy of ultimate function, even though the proteins operate via different mechanisms. This might actually make it easier to intervene favorably in the affected processes, if separate agonists of SIRT1 and SIRT6 end up having a synergistic effect at target promoters (and telomeres).

(There’s also a nice preview/summary piece in the same issue of Cell, by Gioacchino Natoli.)

Here is the next in what will likely be a long series of semi-regular review roundups — links, without extensive further comment, to the reviews I found most intriguing over the past few weeks months (I went on hiatus during the winter holidays). For the previous foray into the secondary literature, see here.

Alzheimer’s:

Apoptosis & cancer:

Calorie restriction:

Diabetes:

Klotho:

Sirtuins:

Stem cells:

Telomeres:

Sometimes I feel like our field produces review articles faster than it produces good ideas. Certainly, biogerontology generates more reviews in a given week than truly significant papers, but the same might be said of any discipline.

I’ve been ambivalent about how to deal with reviews — I’ve considered ignoring them altogether, only covering the “important ones,” link-dumping a bunch of them whenever I was too lazy to write a real post, and various other hybrid strategies. Ignoring them seemed most attractive, since our main mission at Ouroboros is to review the primary literature, so reviewing reviews seemed pointless and derivative.

But a recent reader inquiry (from one of our junior colleagues who basically wanted me to do some of their homework for them; my response was basically “read a review and make up your own mind”) reminded me of the importance of review articles: They’re a great way for scientists who aren’t already expert in a field to figure out where the important questions are. The best ones also juxtapose the most current efforts in creative and interesting ways, adding value by pointing out non-obvious connections between subfields. If read closely and attentively, reviews can be the source of great inspiration.

So rather than treating the elements of the secondary literature like second-class citizens, I’m going to start a quasi-regular feature wherein I (or one of the other writers) compile a list of the most important and interesting reviews of the last couple of weeks, and link to them without much further comment (thereby avoiding the vaguely ridiculous feeling of reviewing reviews, which would make one — what? — the “tertiary literature”?). You, the reader, can do what you wish with them. This new feature of Ouroboros begins…NOW!

Autophagy:

CR & IGF-I:

DNA damage & gene expression:

Immunology:

Insulin:

Progeria:

Stochasticity:

TOR signaling:

Yeast:

Like I said, I’ll do something like this every couple of weeks, or whenever the review folder gets full. That way we’ll never fall too far behind.

The soluble protein Klotho appears to be an anti-aging factor, since mice deficient in the Klotho gene show signs of premature aging. However, the validity of Klotho-/- mutants as a model of progeria is controversial: many of the pathological features of the mutant phenotype can be attributed to hypervitaminosis D, and can be reversed by eliminating vitamin D from the diet. (Biogerontologists are generally more skeptical of progeria than increased longevity, since there are lots of ways to shorten lifespan that don’t involve bona fide accelerating of the aging process, whereas there are far fewer ways to lengthen lifespan without slowing that process down.)

Another blow against the idea of Klotho as a regulator of lifespan comes from Brownstein et al., who show that increased circulating levels of Klotho protein (pursuant to a chromosomal translocation that activates the gene) are associated with hyperparathyroidism and rickets (n.b. that the latter is a classic symptom of, wait for it, vitamin D deficiency).

This is interesting from an endocrinological standpoint because rickets is often the result of hypophosphatemia (low phosphate –> weak bones), whereas hyperparathyroidism is usually associated with hyperphosphatemia.

But from a biogerontological perspective, it’s both interesting and sad: Specifically, it’s a strike against the idea that we might be able to supplement aging mammals (like ourselves) with increased levels of Klotho in order to forestall aging. Of course, it’s possible that the very elevated levels of the protein seen in this study are totally off the charts, and that more modest doses might have a salubrious effect — but more and more, the most reasonable interpretation of the data is that Klotho is involved in mineral and vitamin metabolism in a way that doesn’t have much to do with lifespan; that the levels of Klotho are already more or less optimized (since a syndrome results from either deficiency or elevation relative to wildtype); and that the “progeria” of Klotho-/- is simply a compelling mimic of accelerated aging rather than a legitimate model of the process.

Unsurprisingly, but consistent with the idea that Werner’s syndrome is a good model for accelerated aging, WRN mutant mice are more prone than wildtype mice to develop obesity, insulin resistance, and diabetes when they are fed a diabetogenic diet. Moore et al. argue that the increased risk of these age-related diseases is a result of impaired glucose homeostasis and fat metabolism in the mutants.

Sometimes, biogerontologists get so enraptured by the molecular and cell-biological details of our studies that we forget what might be called the “human element” — the experience of an individual who is going through the process of aging. This process can be associated with significant discomfort and indignity, so it is all the more tragic when it occurs too early, as in the case of progeroid syndromes such as Hutchinson-Gilford progeria syndrome (HGPS).

I’ve generally focused on the molecular underpinnings of the disease, especially on recent findings that mutations in the lamin A gene are responsible for HGPS, possibly by causing accelerated cellular senescence or interfering with the cell cycle. What I haven’t done is talk much about what it’s like to have the condition.

Therefore, I read with interest this article by Merideth et a great many al., in which the authors report a detailed study of the clinical progression of this rare and devastating genetic disease:

Phenotype and course of Hutchinson-Gilford progeria syndrome

BACKGROUND: Hutchinson-Gilford progeria syndrome is a rare, sporadic, autosomal dominant syndrome that involves premature aging, generally leading to death at approximately 13 years of age due to myocardial infarction or stroke. The genetic basis of most cases of this syndrome is a change from glycine GGC to glycine GGT in codon 608 of the lamin A (LMNA) gene, which activates a cryptic splice donor site to produce abnormal lamin A; this disrupts the nuclear membrane and alters transcription. METHODS: We enrolled 15 children between 1 and 17 years of age, representing nearly half of the world’s known patients with Hutchinson-Gilford progeria syndrome, in a comprehensive clinical protocol between February 2005 and May 2006. RESULTS: Clinical investigations confirmed sclerotic skin, joint contractures, bone abnormalities, alopecia, and growth impairment in all 15 patients; cardiovascular and central nervous system sequelae were also documented. Previously unrecognized findings included prolonged prothrombin times, elevated platelet counts and serum phosphorus levels, measured reductions in joint range of motion, low-frequency conductive hearing loss, and functional oral deficits. Growth impairment was not related to inadequate nutrition, insulin unresponsiveness, or growth hormone deficiency. Growth hormone treatment in a few patients increased height growth by 10% and weight growth by 50%. Cardiovascular studies revealed diminishing vascular function with age, including elevated blood pressure, reduced vascular compliance, decreased ankle-brachial indexes, and adventitial thickening. CONCLUSIONS: Establishing the detailed phenotype of Hutchinson-Gilford progeria syndrome is important because advances in understanding this syndrome may offer insight into normal aging. Abnormal lamin A (progerin) appears to accumulate with aging in normal cells.

The situation sounds grim, and it is. Happily, the news is not all bad: as a result of the identification of the disease gene and a brilliant insight into the biochemistry of the underlying pathology, HGPS patients are currently showing promising responses to treatment with farnesyltransferase inhibitors. These compounds, originally designed as antitumor drugs, have demonstrated beneficial effects on some (but not all, as pointed out in an article under discussion at Longevity Meme) of the symptoms of HGPS.

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