Huntingtin “coats” mitotic chromosomes when PAR break-down is inhibited

Blog Post by Dr. Tamara Maiuri

“What did you discover that you didn’t set out to discover?”

Richard Feynman

Sometimes you find the coolest things when you’re not looking for them. In this case, I wanted to test the effect of PARP or PARG inhibition on the recruitment of huntingtin to sites of micro-irradiation.

Rationale side note: 

-PARP produces poly ADP ribose (PAR) in response to DNA damage; PARG breaks it down

-We know that huntingtin binds PARylated proteins in cells, and we’ve identified a PAR-binding motif within the huntingtin sequence that binds PAR in vitro, but we still don’t know why huntingtin binds PAR.

It’s been awhile since I’ve done a micro-irradiation “stripe” experiment (what with the pandemic and all), so I must not have adjusted the settings properly on the microscope: I was surprised to find no DNA damage stripes after fixing the cells, staining for huntingtin, and returning to the same cells on the scope.

Where are my stripes?

I decided to look for them the old-fashioned way: with my eyes. That is, through the eyepiece instead of using the automated microscope stage and computer screen.

In the cells treated with PARG inhibitor, something caught my eye–not stripes, but unusual staining in mitotic cells. We know that huntingtin localizes to centrosomes and mitotic spindle poles, and mitotic cells are very obvious when looking at huntingtin (phospho-N17)-stained cells. But this was something I’d never seen before: huntingtin was “coating” the mitotic chromosomes. Very cool.

Huntingtin (green) “coating” mitotic chromosomes (blue) in PARG inhibitor-treated cells

I looked for mitotic cells in the PARP inhibitor-treated sample and found that huntingtin staining was excluded from the condensed chromosomes (nothing crazy here–this is what mitotic cells normally look like in the absence of inhibitors). In this case, huntingtin is in red and PARP1 is in green:

What does this mean?

This is strong evidence that huntingtin binds PAR in cells: when the break-down of PAR is inhibited by PARG inhibitor, PAR accumulates on mitotic chromosomes. A quick search on BenchSci returned these images of anti-PAR immunofluorescence (one from 1981!):


Cool to note that the PAR in those old images also lives at nuclear puncta in non-mitotic cells. We’ve seen this too, and identified the puncta as SC35-positive “nuclear speckles”, where huntingtin also lives.

I’ve since repeated the experiment to confirm the effect of PARG inhibition on huntingtin localization to mitotic chromosomes and deposited the results to Zenodo.

Huntingtin (green) and condensed chromosomes (blue) in PARG inhibitor-treated cells

While this data still doesn’t tell us why huntingtin binds PAR, it’s another (rather pretty) piece of the puzzle.

Are you a graduate student of postdoctoral fellow whose research has been disrupted by COVID-19? We are seeking participants for a research study of the impact of research disruption during the COVID-19 pandemic on graduate students and postdoctoral fellows engaging in laboratory based research. The study will involve taking one anonymous online survey. An optional virtual interview is available for those who complete the survey. Principal Investigator: Dr. Ray Truant, Student Co-investigator: Celeste Suart This study has been reviewed by the Hamilton Integrated Research Ethics Board under Project 10832.

Launching a Research Study on the Impact of COVID-19 on Graduate Students & Postdoctoral Fellows

Blog post by Celeste Suart

Like many of you, we here in the Truant lab have had to temporarily pause our research efforts and close the lab in response to COVID-19. This is an important and necessary step to flatten the curve of COVID-19. People are having a variety of reactions to this unexpected situation.

This is in part why we have launched a research study to examine the impact of research disruption during the COVID-19 pandemic on graduate students and postdoctoral fellows. We want to gather this data on the stresses and concerns of students and fellows are experiencing in a systematic way to identify the supports that are needed right now and when the pandemic dissipates.

We also want to identify best practices of how to quickly shut down laboratories, and provide support to students and fellows, that could be used if similar situations arise in the future.

To participate in our study, we are asking students and fellows to fill out one anonymous online survey. For those wishing to provide further feedback, we are also conducting optional virtual one-on-one interviews via Zoom. Further details about the procedures involved in both the survey and interview can be found in their associated letters of information. You can find copies of these letters in the hyperlinked text above.

If you are interested in participating in the study, you can access the online survey at the following link:

You can sign-up to receive more information about the interviews following the completion of the online survey.

Thank you in advance for your time and consideration of this study!

This study has been reviewed by the Hamilton Integrated Research Ethics Board (HiREB). If you any have concerns or questions about your rights as a participant or about the way the study is being conducted, call the Office of the Chair, HiREB, at 905.521.2100 x 42013.

Experiments on PAR levels in HD cells and huntingtin PAR binding

Blog post by Dr. Tamara Maiuri

Regulation of PAR levels in HD cells

As reported previously, we found higher levels of poly ADP ribose (PAR) in HD cells compared to controls. This is consistent with higher levels of DNA damage found in HD cells by us and by others

For a little more mechanistic insight, we turned to the major regulators of PAR levels: PARP, which generates PAR, and PARG, which breaks PAR down. To get an idea of how well these two proteins are working in HD cells, I performed dose response curves with inhibitors against each.

Interestingly, despite higher overall PAR levels in HD cells, it takes less PARP inhibitor to reduce the response by half (HD cells have a lower IC50). This suggests that the PARP in HD cells isn’t working as well as it does in normal cells. Seems at odds with the elevated PAR levels, but it could be that there’s so much DNA damage in HD cells, the overall PAR is high even without PARP functioning at its max. These results and the experimental details have been deposited to Zenodo.


IC50 three trials
PARP inhibitor dose response in cells from healthy control (Q21) and HD patients (Q43 and Q50)



As for the PARG inhibition experiments, I didn’t use a large enough range of concentrations to get an accurate reading of the IC50. The results (or lack thereof) were nonetheless deposited to Zenodo. I’m currently repeating the dose-response experiment with a wider range of doses.


More huntingtin PAR-binding experiments

We previously confirmed one region of the huntingtin protein that binds PAR, which we named PBM3. This is a small discovery, but I always find it exciting to uncover something about how nature works. Even more fun is thinking of clever ways to use nature’s tricks to find out more about how nature works! We have some ideas coming down the pipeline for how to use PBM3 as a tool.

The first idea was to use PBM3 to measure the on-off rate of huntingtin PAR binding. Then we could compare normal versus mutant huntingtin to see if mutant huntingtin holds on to PAR more or less tightly than it should. Spoiler alert: this idea didn’t work. 

The first problem was a high degree of variability in PAR binding between technical replicates. The PAR overlay by dot-blotting is good for a “yes or no” answer whether something binds PAR, but it is not quantitative. So I tried improving the reproducibility with slot blotting instead of dot blotting (slot blotting optimization experiments in this Zenodo entry.

The second problem was that the control peptide (a PBM3 mutant that cannot bind PAR), had a non-specific effect in the assay. Not sure how to interpret this, but for now, the only conclusion is that this method is not working (sad face). Suggestions are welcome to improve this protocol (deposited to Zenodo)!

Expts 1 and 2 normalized
PAR overlay assay using PBM3 or mutant peptide to compete with full-length huntingtin PAR binding


Work is also in progress on getting to the bottom of the reproducibility issues between preps of full-length huntingtin:


This work is funded by the HDSA Berman/Topper HD Career Development Fellowship.

Update on the huntingtin PAR-binding story

Blog post by Dr. Tamara Maiuri

One of the major findings of the HDSA-funded project to find huntingtin interactors relevant DNA repair was that many proteins that interact with huntingtin are modified by poly ADP ribose (PAR). Previous posts have described our hypothesis that huntingtin binds poly ADP ribose, our preliminary evidence that it does so, frustrating attempts to understand why it does so, and other confounding results.

I wish this post was about a major breakthrough clarifying everything… but it’s just another normal, incremental piece of the puzzle (aka: science).

The good news is that one of the potential PAR-binding motifs we identified by sequence analysis does in fact bind PAR as a peptide in an in vitro PAR overlay assay. Furthermore, if the critical arginine residues are mutated to alanines, then the peptide no longer binds. This data has been deposited to Zenodo.

Blog post 18 image

The bad news is that initial, reproducible PAR binding results with full length huntingtin and an N-terminal fragment have become less reproducible with different preparations of huntingtin protein. A full analysis of which preps bind PAR and which do not, along with some preliminary results suggesting it could be due to a heparin purification step, can be found here. I’m continuing to work with Dr. Rachel Harding (huntingtin protein super-producer) to get to the bottom of it.

The jury is still out on the physiological function of huntingtin PAR binding. Some of our results suggest a role for PAR in huntingtin chromatin binding, but this is confounded by the apparent trapping effect of PARP inhibitors on huntingtin. Given the identification of a potential PAR binding motif within huntingtin (PBM3), I decided to try a chromatin retention assay with fragments of huntingtin containing this sequence.

Preliminary results are… you guessed it: confusing. Two fragments of huntingtin bearing the PBM3 sequence actually had reduced chromatin binding in response to oxidative stress. Experimental details and results have been deposited to Zenodo.

It’s too early to make hard conclusions, but the huntingtin 1208-1810 and 1775-2413 fragments may actually bind chromatin under untreated conditions and release upon KBrO3 treatment. This is difficult to interpret since endogenous full length huntingtin tends toward chromatin retention upon oxidative stress. If the potential PAR binding motif PBM3 plays any role in this behaviour, mutating it within these fragments will tell.

So that’s the plan.


Huntingtin chromatin retention in PARP knockout cells

Blog post by Dr. Tamara Maiuri

Another quick report on some year-old data that missed the boat on Zenodo deposits.

How to measure huntingtin chromatin retention

The method for measuring huntingtin chromatin retention upon oxidative stress makes use of a fluorescent huntingtin-specific intrabody, so we can follow the movement of endogenous huntingtin.

The method is described in this Zenodo post, including all the necessary controls to make sure the huntingtin chromatin retention is specific to huntingtin, and specific to oxidative stress conditions. Briefly, the YFP-tagged huntingtin-specific intrabody is co-transfected with H2B-mCherry (transfection control and marker of chromatin). After treatment, soluble proteins are extracted and cells are fixed and imaged. CellProfiler [1] is used to identify nuclei by the H2B-mCherry signal, and the nuclear YFP signal remaining bound to chromatin is quantified for several hundred cells.

Is PARP activity required for huntingtin chromatin retention?

Once these conditions were established, I was able to apply them to ask a biological question: is PARP activity required for huntingtin chromatin retention?

The Keith Caldecott lab at University of Sussex kindly provided us with a panel of PARP knock out cell lines. These are RPE1 cells with either PARP1 or PARP2 or both PARP1/PARP2 deleted by CRISPR/Cas9 [2].

When I first did this experiment, I was somewhat confused by the results. Huntingtin chromatin retention was diminished in PARP1 knockout cells and in PARP2 knockout cells, suggesting a role for PAR in the recruitment of huntingtin to chromatin. This would be expected based on our previous results that huntingtin binds PAR and acts as a scaffold for DNA repair proteins [3].

Blog post 15 fig1 copy


However, the double knockout cells seemed to retain huntingtin at chromatin more efficiently than either single knockout cell line. This didn’t make sense until I noticed that the huntingtin intrabody expression pattern in the double knockout cells had very bright nuclear and cytoplasmic staining (almost like inclusions) compared to the diffuse nuclear staining seen in wild type cells. To take a closer look, I compared the expression patterns by high magnification microscopy.

Blog post 15 fig2

In wild type RPE1 cells, the intrabody is recruited to the chromatin in response to KBrO3 and displays diffuse nuclear localization. A small proportion of cells also have bright punctate staining in the nucleus and cytoplasm. In PARP1/2 double knockout cells, on the other hand, the intrabody forms large inclusions, some of which may be positioned over the nucleus. If so, in the quantification process, these inclusions would be scored as nuclear nucHCB2 intensity within the confines of the H2B-mCherry-delineated nucleus, skewing the results.

So I acquired a Z-stack to see whether the inclusions are within or above the nucleus:

Blog post 15 fig3

Sure enough, the bright inclusions are positioned above the nucleus. This means that huntingtin localization is somehow dysregulated in PARP1/2 double knock out cells. Cytoplasmic inclusion formation skews CellProfiler results when inclusions are positioned over the nucleus. Transient transfection of the huntingtin-specific intrabody is therefore not the method of choice for measuring huntingtin chromatin retention in these cells.

The experimental details and results of huntingtin chromatin retention for all three cell lines have now been deposited to Zenodo.

In the end, what we have are a few more clues. The fact that huntingtin chromatin retention is diminished in PARP1 KO cells and PARP2 KO cells is worth investigating further. The fact that huntingtin localization is dysregulated in the double KO cells is also worth pursuing, but it makes these cells unusable for further experiments of this type.




  1. Carpenter AE, Jones TR, Lamprecht MR, Clarke C, Kang IH, Friman O, et al. CellProfiler: image analysis software for identifying and quantifying cell phenotypes. Genome Biol. 2006;7: R100.
  2. Hanzlikova H, Gittens W, Krejcikova K, Zeng Z, Caldecott KW. Overlapping roles for PARP1 and PARP2 in the recruitment of endogenous XRCC1 and PNKP into oxidized chromatin. Nucleic Acids Res. 2017;45: 2546–2557.
  3. Maiuri T, Mocle AJ, Hung CL, Xia J, van Roon-Mom WMC, Truant R. Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex. Hum Mol Genet. 2017;26: 395–406.

Detecting poly ADP ribose

Blog post by Dr. Tamara Maiuri

As mentioned in the year-end review post, there were a few experiments I forgot to deposit to Zenodo.

Here is one: optimizing the method of detecting poly-ADP ribose (PAR) by immunofluorescence. Nothing ground-breaking in this report, but it was the key to detecting the increased PAR levels in HD patient fibroblasts.

Trying to detect PAR by immunofluorescence was very frustrating until I went to the CSHL meeting on The PARP Family and ADP-ribosylation last year. There I met Dr. Hana Hanzlikova, previously from Prof. Keith Caldecott’s lab at University of Sussex and now at the Institute of Molecular Genetics of the ASCR in Prague.

Dr. Hanzlikova gave me two important tips: you need to block the degradation of PAR using PARG inhibitors (because PAR turnover is too fast to catch it in action), and the best detection reagent is MABE1016, a macro domain fused to rabbit Fc tag.  Sure enough, these conditions allowed us to visualize PAR formation in response to oxidative stress. The conditions and results have now been deposited to Zenodo.

The Year in Review

Blog post by Dr. Tamara Maiuri

Most researchers would agree, it feels like we just can’t get enough experiments done, and those, not fast enough. But it’s useful to pause the benchwork from time to time and review a project–what worked, what didn’t, and what we may have missed in our rush to move on to the next thing.

That’s what I’ve done at the end of Year 2 of the HDSA Berman-Topper Fellowship. One of the requirements of the fellowship is that you summarize your work in a report at the end of each year–a daunting task, but well worth the time. It certainly put things in perspective. In a nutshell, what the report says is:

The major finding of the first year of the Berman-Topper fellowship was that many proteins that interact with the huntingtin protein bear a molecule involved in the DNA repair process: poly ADP ribose (PAR). This led us to investigate a connection between huntingtin and PAR signaling in the second year of the fellowship. Methods for detecting PAR were optimized, and a hyper-PAR phenotype was identified in HD patient fibroblasts and in HD mouse striatal precursor cells. The huntingtin-PAR interaction was confirmed in cells and in vitro.

Elevated PAR levels may arise from unrepaired DNA damage and lead to cellular energy deficits, both of which have been observed in many HD models and tissues. Our results introduce the exciting possibility that inhibitors of poly ADP ribose polymerase (PARP), drugs that have already been through clinical trials for a number of cancers, can be explored as a therapeutic option for HD. This is an avenue that has not been considered in the past, and shows promise for other neurodegenerative diseases including Parkinson’s, Alzheimer’s, and ALS.

What is needed at this point is rigorous pre-clinical testing of

  • Whether PARP inhibitors affect HD model phenotypes
  • Safety profiles of PARP inhibitor drugs that would make promising therapeutic leads

Preliminary experiments suggest that PARP inhibitors, which trap PARP at DNA, are co-trapping huntingtin at DNA. This is not likely to be beneficial as it may interfere with huntingtin function. As such, PARP lowering strategies will also be considered in the final year of the fellowship. Complete characterization of the mechanism and function of huntingtin PAR binding is also underway.


The full report, with links to all the raw data, is available on Zenodo. I have also deposited last year’s year-end report and the shorter quarterly reports for 2017-2018.

In reviewing the year’s data, I realized that I failed to deposit a few experiments to Zenodo. I’m doing so now, and I’ll post about them in the coming weeks.

Why the heck does huntingtin bind PAR?

Blog post by Dr. Tamara Maiuri

I’ve previously described our hypothesis that huntingtin binds poly ADP ribose and our preliminary evidence that it does so. I now have a few more pieces of data to add to that story.

How does huntingtin bind PAR?

Full length huntingtin, as well as a fragment spanning amino acids 78-426, can bind PAR in vitro in a PAR overlay assay. In an attempt to narrow down which part of the huntingtin protein might bind PAR, I looked at the huntingtin sequence to identify potential PAR-binding motifs (PBMs). Peptides representing four of the most promising PAR-binding motifs (PBMs) were tested for their ability to bind PAR in vitro. PBM3 peptide, which corresponds to amino acids 1782-1804 of the huntingtin sequence (with the PBM at 1790-1798), was the only peptide to bind PAR. The experiment required optimization to maintain peptides on the nitrocellulose membrane. These optimization steps and the results have been deposited to Zenodo.

I was somewhat surprised to find that PBM1 did not bind PAR. That was the only PBM that I could find within the 78-426 fragment, which can bind PAR. It could be that the 78-426 fragment is binding PAR non-specifically, or it could bind PAR through a different motif.

What’s the point of huntingtin PAR-binding?

Current efforts are focused on figuring out the physiological relevance of huntingtin PAR binding. So far, no dice. We asked the following questions:

Is it to stick to chromatin in response to oxidative stress?


Truant lab student Carlos Barba tested endogenous huntingtin chromatin recruitment upon enzymatic inhibition of PARP by veliparib. The amount of endogenous huntingtin bound to chromatin after a Triton X-100 wash, which increases upon KBrO3 treatment, was not reduced by inhibition of PARP with veliparib:


2019-04-01 Endogenous huntingtin CRA veliparib copy


Similarly, the amount of transfected huntingtin 1-586 fragment bound to chromatin after a Triton X-100 wash, which increases upon KBrO3 treatment, was not reduced by inhibition of PARP with veliparib (data deposited to Zenodo).

We also found that endogenous huntingtin was still recruited to chromatin in PARP1/PARP2 knockout cells [1] upon oxidative stress. The knockout cells were highly susceptible to stress, so it’s not clear whether huntingtin was bound to chromatin as part of the DNA repair process (which would lead to recovery) or as part of a cell death process. Enzymatic inhibition of PARP3 in the PARP1/PARP2 knockout cells was toxic.

An unexpected result

When I reported to Ray my lack of success in determining why the heck huntingtin binds PAR, he suggested I try looking at the dynamics of huntingtin chromatin retention using Fluorescence Recovery After Photobleaching (FRAP) of the YFP-tagged intrabody that recognizes endogenous huntingtin [2]. I found that KBrO3 slows huntingtin mobility, as would be expected upon huntingtin chromatin binding. If huntingtin uses PAR binding as a mechanism of chromatin retention, then inhibition of PAR formation should restore huntingtin mobility. In contrast, veliparib significantly reduced huntingtin mobility beyond the degree caused by KBrO3 alone. PARP inhibitors are known to “trap” PARP at chromatin, so it’s possible that huntingtin is also being trapped. I will repeat these experiments using PARP knock down strategies to try and clarify what’s happening, and deposit everything to Zenodo.

2019-04-01 FRAP copy


Is it to get to huntingtin stress bodies (HSBs)?


HSBs are spots in the cytoplasm where huntingtin accumulates upon cell stress [3]. There are many types of stress bodies in cells, and some of them use PAR in their formation [4]. I tested whether HSBs could still form in the presence of a PARP inhibitor and found it had no effect on HSB formation.


Is it to interact with PARylated proteins?

The plot thickens…

We set out on this PAR journey because we found that a large proportion of the huntingtin interacting proteins we identified are also part of PARylated protein databases. To make sure that huntingtin does indeed physically interact with PARylated proteins, I performed co-immunoprecipitation experiments. Not surprisingly, huntingtin pulls down many PARylated proteins (experiments deposited to Zenodo). One of those proteins is the PARP enzyme itself, although the conditions promoting huntingtin-PARP interaction were variable (see Zenodo entry). What came as a surprise was that inhibition of PARP with veliparib actually increased the interaction between huntingtin and PARylated proteins, including PARP. It’s possible that this is related to the slowed huntingtin mobility observed in FRAP experiments upon veliparib treatment.

Is it to get into or out of nuclear speckles?

Not sure yet.

Nuclear speckles, or SC35 domains, are sub-nuclear regions of low DNA and high protein where huntingtin lives [5]. We have found PAR at these sites [2], where it is thought to nucleate liquid-liquid phase transition of proteins into this “membraneless organelle” [6]. I am currently conducting experiments to find out whether huntingtin binds PAR to localize to these sites.

Any other ideas?

If anyone has other suggestions for figuring out why the heck huntingtin binds PAR, I am all ears!


  1. Hanzlikova H, Gittens W, Krejcikova K, Zeng Z, Caldecott KW. Overlapping roles for PARP1 and PARP2 in the recruitment of endogenous XRCC1 and PNKP into oxidized chromatin. Nucleic Acids Res. 2017;45: 2546–2557.
  2. Maiuri T, Mocle AJ, Hung CL, Xia J, van Roon-Mom WMC, Truant R. Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex. Hum Mol Genet. 2017;26: 395–406.
  3. Nath S, Munsie LN, Truant R. A huntingtin-mediated fast stress response halting endosomal trafficking is defective in Huntington’s disease. Hum Mol Genet. 2015;24: 450–462.
  4. Leung AKL, Vyas S, Rood JE, Bhutkar A, Sharp PA, Chang P. Poly(ADP-ribose) regulates stress responses and microRNA activity in the cytoplasm. Mol Cell. 2011;42: 489–499.
  5. Hung CL-K, Maiuri T, Bowie LE, Gotesman R, Son S, Falcone M, et al. A Patient-Derived Cellular Model for Huntington’s Disease Reveals Phenotypes at Clinically Relevant CAG Lengths. Mol Biol Cell. 2018; mbcE18090590.
  6. Leung AKL. Poly(ADP-ribose): an organizer of cellular architecture. J Cell Biol. 2014;205: 613–619.

Could cancer drugs help with HD?

Blog post by Dr. Tamara Maiuri

I would like to start this post by saying that the ideas below are PURELY SPECULATIVE at this point. But I’m excited about them and feel they definitely warrant further investigation.

So here’s the deal: In a study funded by the HDSA Berman/Topper fellowship, we have found that the huntingtin protein interacts with proteins modified by poly ADP ribose (PAR), and that PAR levels are elevated in cells from HD patients.


What is PAR?

PAR is a branched chain-like molecule that acts as a recruitment scaffold for DNA repair proteins. It’s made by an enzyme called poly ADP ribose polymerase (PARP) upon DNA damage (we and others also see elevated levels of DNA damage in HD patient cells [1–3]).


What is PARP?

There are several PARP enzymes, but let’s concentrate on PARP1 for now. PARP1 is a major player in the DNA damage response.

PARP1 uses a building material called NAD+ to generate the branched PAR chains that recruit DNA repair factors. The problem is, NAD+ is needed for other things in the cell, especially for converting food into energy. So PARP1 activity is great for a quick response to clear up some DNA damage, but if the damage is too great (or if it’s prolonged), then energy levels in the cell get depleted.


PARP hyper-activation

There are several negative consequences if PARP activity goes on too long. Let’s compare a list of these negative consequences to what is seen in HD models and patients (NB: these phenotypes are very much interrelated):

PARP hyper-activation phenotypes

HD phenotypes

ATP depletion [4–7] ATP depletion [8–13]
Mitochondrial dysfunction [14–17] Mitochondrial dysfunction (reviewed in [18])
Energy crisis [19,20] Energy crisis (reviewed in [21])
Cell death (through parthanatos) [22,23] Cell death through apoptosis, necrosis (parthanatos not yet tested) (reviewed in [24])
Neuroinflammation [25–29] Neuroinflammation [30–32]
Transcriptional and chromatin changes (reviewed in [33]) Transcriptional and chromatin changes (reviewed in [34,35])
Autophagy and protein clearance (reviewed in [36,37]) Autophagy and protein clearance (reviewed in [38,39])

Just looking at this list, even if we had no indication that PAR is dysregulated in HD, I would say it’s worth looking into! Turns out, PARP hyper-activation has been linked to other neurodegenerative diseases (reviewed in [40]), most recently Parkinson’s [41].


The good news

The cancer field has done decades of work to understand how to inhibit PARPs. I didn’t think this would be much use to us in the HD field, since the point of cancer drugs is to KILL TUMOUR CELLS, while we are looking for drugs to KEEP NEURONS ALIVE.

I was disabused of this notion when I read a very nice review article called Opportunities for the repurposing of PARP inhibitors for the therapy of non-oncological diseases [42]. The article recommends that for chronic, non-oncological indications where there is a significant unmet need, we should consider taking PARP inhibitors to trial.


Criteria for deciding whether to try PARP inhibitors for a non-oncological indication

Q: Is there preclinical data demonstrating the efficacy of PARP inhibition in clinically relevant preclinical models?
A: Not yet, but:
  • PARP inhibition is beneficial in R6/2 mice [43,44]
  • Truant lab student Carlos Barba is currently testing PARP inhibition in TruHD cells (and STHdh cells, although not as clinically relevant)
  • We have also asked Chris Ross’s lab to test PARP inhibition in neurons expressing the huntingtin 1-586 fragment (this is an overexpression system, but at least it is in neurons)


Q: Is there human data confirming activation of PARP in the target organ?
A: Not yet, but:


Q: Would the duration of treatment be short, to limit potential side effects?
A: Unfortunately, no. But intermittent administration (“drug holidays”) could be an option.


Q: Are existing therapeutic alternatives insufficient?
A: Yes.


Q: Is HD severe enough to justify an attempt for novel therapies, especially in light of the potential “genotoxic baggage” that comes with PARP inhibition?
A: Hell yes.


Q: Would a trial be logistically feasible?
A: The HD community has proven that they are very capable of designing, recruiting, and running clinical trials.

Preclinical studies should:

  • Use the drug that will eventually be trialed
  • Use the most clinically relevant models
  • Document the drug effects on DNA and chromosomal integrity (for ideas about safety)

The Truant lab works with preclinical systems. We are doing our best to make sure these preclinical studies are done properly so they can be most informative going forward. All of my results will be shared openly through this blog and our Zenodo Community. I welcome any comments or suggestions about how we might test PARP and PAR in HD systems. And now, back to the bench!



  1. Maiuri T, Mocle AJ, Hung CL, Xia J, van Roon-Mom WMC, Truant R. Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex. Hum Mol Genet. 2017;26: 395–406.
  2. Askeland G, Dosoudilova Z, Rodinova M, Klempir J, Liskova I, Kuśnierczyk A, et al. Increased nuclear DNA damage precedes mitochondrial dysfunction in peripheral blood mononuclear cells from Huntington’s disease patients. Sci Rep. 2018;8: 9817.
  3. Castaldo I, De Rosa M, Romano A, Zuchegna C, Squitieri F, Mechelli R, et al. DNA damage signatures in peripheral blood cells as biomarkers in prodromal huntington disease. Ann Neurol. 2019;85: 296–301.
  4. Sims JL, Berger SJ, Berger NA. Poly(ADP-ribose) Polymerase inhibitors preserve nicotinamide adenine dinucleotide and adenosine 5’-triphosphate pools in DNA-damaged cells: mechanism of stimulation of unscheduled DNA synthesis. Biochemistry. 1983;22: 5188–5194.
  5. Berger NA, Sims JL, Catino DM, Berger SJ. Poly (ADP-ribose) polymerase mediates the suicide response to massive DNA damage: studies in normal and DNA-repair defective cells. Princess Takamatsu Symp. 1983. pp. 219–226.
  6. Alano CC, Garnier P, Ying W, Higashi Y, Kauppinen TM, Swanson RA. NAD+ depletion is necessary and sufficient for poly(ADP-ribose) polymerase-1-mediated neuronal death. J Neurosci. 2010;30: 2967–2978.
  7. Ha HC, Snyder SH. Poly(ADP-ribose) polymerase is a mediator of necrotic cell death by ATP depletion. Proc Natl Acad Sci U S A. 1999;96: 13978–13982.
  8. Mochel F, Durant B, Meng X, O’Callaghan J, Yu H, Brouillet E, et al. Early alterations of brain cellular energy homeostasis in Huntington disease models. J Biol Chem. 2012;287: 1361–1370.
  9. Hung CL-K, Maiuri T, Bowie LE, Gotesman R, Son S, Falcone M, et al. A Patient-Derived Cellular Model for Huntington’s Disease Reveals Phenotypes at Clinically Relevant CAG Lengths. Mol Biol Cell. 2018; mbcE18090590.
  10. Milakovic T, Johnson GVW. Mitochondrial respiration and ATP production are significantly impaired in striatal cells expressing mutant huntingtin. J Biol Chem. ASBMB; 2005; Available:
  11. Trettel F, Rigamonti D, Hilditch-Maguire P, Wheeler VC, Sharp AH, Persichetti F, et al. Dominant phenotypes produced by the HD mutation in STHdh(Q111) striatal cells. Hum Mol Genet. 2000;9: 2799–2809.
  12. Seong IS, Ivanova E, Lee J-M, Choo YS, Fossale E, Anderson M, et al. HD CAG repeat implicates a dominant property of huntingtin in mitochondrial energy metabolism. Hum Mol Genet. 2005;14: 2871–2880.
  13. Jędrak P, Mozolewski P, Węgrzyn G, Więckowski MR. Mitochondrial alterations accompanied by oxidative stress conditions in skin fibroblasts of Huntington’s disease patients. Metab Brain Dis. 2018; doi:10.1007/s11011-018-0308-1
  14. Lai Y-C, Baker JS, Donti T, Graham BH, Craigen WJ, Anderson AE. Mitochondrial Dysfunction Mediated by Poly(ADP-Ribose) Polymerase-1 Activation Contributes to Hippocampal Neuronal Damage Following Status Epilepticus. Int J Mol Sci. 2017;18. doi:10.3390/ijms18071502
  15. Lehmann S, Costa AC, Celardo I, Loh SHY, Martins LM. Parp mutations protect against mitochondrial dysfunction and neurodegeneration in a PARKIN model of Parkinson’s disease. Cell Death Dis. 2016;7: e2166.
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PNAS Erratum

In July 2018 we published a manuscript in PNAS,

N6-Furfuryladenine is protective in Huntington’s disease models by signaling huntingtin phosphorylation

After publication, we were made aware of an error in a structure cartoon in Figure 3.

The compound was correct, the representation was wrong. The control compound is 9-deaza-kinetin. This compound is similar to N6FFA, but cannot be salvaged. The correct structure is below, within the corrected figure.


The Editors at PNAS were informed, but they decided a published erratum was not necessary.