By: Lise Munsie, PhD
In stem cell research…
Generating GABAergic medium-sized spiny neurons (MSNs) from human pluripotent stem cell (hPSC) sources is a rapidly evolving technique that holds much promise for HD modeling and treatment. Arber and colleagues describe a new method of differentiating hPSCs to MSNs using Activin A (activin).1 Activin is part of the transforming growth factor beta signaling pathway, involved in neurogenesis and neuronal cell type fate. Arber’s paper describes the ability of activin to produce a much higher proportion of MSNs than current techniques, and the ability of these neurons to survive in grafts in rodent models of HD.
Dr. Ihn Sik Seong’s group makes an elegant cell model of HD using embryonic stem cells, producing six isogenic lines from one lineage. One line expresses two copies of wild-type mouse huntingtin (Htt) (Q7), an Htt null line, while four heterozygous lines express one Q7 allele and one CAG expanded allele of 20, 50, 91, or 111 CAG repeats.2 This paper explores histone modifications in the presence of mHtt and Htt, based on research indicating that Htt acts as a polycomb recessive complex-2 facilitator. The authors show subtle alterations to the chromatin landscape and histone methylation states depending on whether Htt or mHtt is present, and whether cells are in a pluripotent or differentiated state.
Mattis and his group used HD-patient-derived induced pluripotent stem cells produced from non-integrating technology to assess how HD and juvenile-onset HD CAG repeat lengths affect neuronal differentiation.3 The group found a significantly higher proportion of nestin-positive cells in cultures derived from JHD patients after 42 days of differentiation. Cells with juvenile-onset HD CAG repeat length suffer an increase in cell death with brain-derived neurotrophic factor withdrawal, due to glutamate toxicity. The authors posit that this increase may be mediated through the TrkB receptor.
3Mattis VB, Tom C, Akimov S, et al. HD iPSC-derived neural progenitors accumulate in culture and are susceptible to BDNF withdrawal due to glutamate toxicity. Hum Mol Genet. 2015 Mar 3. pii: ddv080. [Epub ahead of print]
In the clinic…
A recent study assesses the clinical manifestation of HD in patients who express intermediate CAG repeat lengths.1 A group led by Panegyres examined subjects who had repeats in the 27–35 range (at risk for prodromal HD), 36–39 range (mixed penetrance), or 40+ range (HD). No patients with CAG repeat length below 35 developed manifest HD. Smoking and being older than 65 correlated with manifest HD among those with intermediate repeat lengths. Of note, a high level of education was associated with lower odds of manifest HD, and the authors propose several hypotheses about environmental, cognitive or epigenetic factors.
A paper by Hobbs and colleagues in a group led by Prof. Sarah Tabrizi assessed candidate outcomes in HD patients in the imaging, clinical and cognitive streams, at both short (six-month) and longer (15-month) time periods, in order to further the guidelines for assessing outcomes in HD clinical trials.2 This is the first study to report significant effect size in short time periods. The strongest longitudinal changes were present in caudate atrophy and ventricular expansion. Small effect sizes were noted in other imaging and clinical measures that will be useful over longer time periods.
To date, there have been few studies assessing changes in the hypothalamus, a brain structure that may be responsible for the sleep, emotional and metabolic changes associated with HD. Hypothalamic changes are noted up to 15 years before predicted onset of manifest HD. In a recent PLoS One article, 3 Gabery and colleagues explored whether alterations to hypothalamic volume may be linked to these symptoms. Using 3T-MRI image data from IMAGE-HD, the group compared hypothalamic volume between patients before and after onset of manifest HD. The group did not find any statistically significant alterations in hypothalamic volume, indicating that the hypothalamic changes noted in previous studies are not likely due to atrophy.
1Panegyres PK, Shu CC, Chen HY, Paulsen JS. Factors influencing the clinical expression of intermediate CAG repeat length mutations of the Huntington’s disease gene. J Neurol, 2015. 262(2): p. 277-84.
2Hobbs NZ, Farmer RE, Rees EM, et al. Short-interval observational data to inform clinical trial design in Huntington’s disease. J Neurol Neurosurg Psychiatry. 2015 Feb 10. pii: jnnp-2014-309768. doi: 10.1136/jnnp-2014-309768. [Epub ahead of print]
3Gabery S, Georgiou-Karistianis N, Lundh SH, et al. Volumetric analysis of the hypothalamus in Huntington Disease using 3T MRI: the IMAGE-HD Study. PLoS One, 2015. 10(2): p. e0117593.
In the pipeline…
Wild and colleagues report in the Journal of Clinical Investigation on an ultra-sensitive method of quantifying mHTT levels in the cerebrospinal fluid (CSF) of HD patients.1 Using a single-molecule counting immunoassay based on the MW1 antibody specific for the expanded CAG tract, the group demonstrated the ability to detect mHTT in a disease-and onset-dependent manner in the CSF of HD patients in two cohorts. This assay will be useful as a pharmacodynamic biomarker in clinical trials, specifically in trials assessing HTT-lowering strategies.
It has been shown that allele-specific silencing of mHTT can be successful using small interfering RNA (siRNA) targeting the expanded CAG tract, or single nucleotide polymorphisms associated with the mHTT allele. The Davidson group aimed to explore whether specificity for the mHTT allele was maintained when previously tested siRNA sequences were moved into the group’s artificial mRNA expression system.2 This mRNA expression system may be more amenable to sustained delivery of knockdown than previously tested methods of in vivo knockdown. The group tested their system both in vitro and in vivo, and found that only some of the previously tested sequences were efficacious long term.
A report published in Nature Chemical Biology by Jimenez-Sanchez and colleagues describes a new therapeutic HD target extracted from an siRNA screen focusing on genes that are amenable to small molecule modulation.3 The most significant suppressor of mHTT toxicity was glutaminyl-peptide cyclotransferase (QPCT), an enzyme with glutaminyl cyclase activity. The knockdown of this protein inhibits mHTT aggregation, while overexpression exacerbates aggregation. Using pharmacophore models and other known structures, the group designed QPCT inhibitors. These inhibitors have positive effects in several models of HD, and with further development may become clinically relevant.
1Wild EJ, Boggio R, Langbehn D, et al. Quantification of mutant huntingtin protein in cerebrospinal fluid from Huntington’s disease patients. J Clin Invest. 2015 Apr 6. pii: 80743. doi: 10.1172/JCI80743. [Epub ahead of print]
2Monteys AM, Wilson MJ, Boudreau RL, et al. Artificial miRNAs targeting mutant huntingtin show preferential silencing in vitro and in vivo. Mol Ther Nucleic Acids. 2015 Apr 7;4:e234. doi: 10.1038/ mtna.2015.7.
3Jimenez-Sanchez M, Lam W, Hannus M, et al. siRNA screen identifies QPCT as a druggable target for Huntington’s disease. Nat Chem Biol. 2015 May;11(5):347-54.