The HD Induced Pluripotent Stem Cell (iPSC) consortium differentiated HD patient iPSCs into neuronal cultures.1 RNA-seq and ChIPseq analysis from these cultures indicated subtle alterations in expression and epigenetic signature of genes involved in neural development, function, and striatal maturation in the presence of mHTT. The small molecule isoxazole-9 is known to target some of the disrupted gene networks, and when tested in this system, was able to normalize some of the genetic phenotypes. This compound also showed beneficial effects in HD-related pathologies in the R6/2 mouse model, indicating that some of these pathways are linked to pathology. Yu and Tanese used mouse embryonic stem cells provided by the Zeitland group to investigate HTT involvement in differentiation.2 The lines they used came from the same background, but were either huntingtin-null (Htt-null) or had tagged wildtype Htt (Q7) or mHtt (Q140).
The group found that the presence of HTT, either wildtype or mutant, allowed all three germ layers to form, but Htt knockout blocked the ability of the cells to differentiate down the ectoderm pathway to form neural stem cells (NSC). Although mHTT still allowed NSC formation, it inhibited further differentiation into neurons. RNA-seq data from these lines revealed that MaoA and Oligo1/2 downregulation and upregulation of the PRC2 repressor complex may be involved in these developmental changes.
One well-described effect of mHTT is the dysregulation of Ca2+ signaling. A new drug lead that corrects deficiencies in storeoperated calcium entry (SOCE) in YAC128 medium spiny neurons (MSNs) has been described in a recent publication.3 SOCE was found to be increased specifically in MSNs but not in other neuronal subtypes in this model.
The group identified a tetrahydrocarbazole that attenuated this phenotype in addition to normalizing mitochondrial function. The group postulates that the dual benefit might come from preventing abnormal Ca2+ accumulation in the mitochondria and endoplasmic reticulum.
1HD iPSC Consortium. Developmental alterations in Huntington’s disease neural cells and pharmacological rescue in cells and mice. Nat Neurosci. 2017 Mar 20. doi: 10.1038/nn.4532. [Epub ahead of print].
2Yu MS, Tanese N. Huntingtin Is Required for Neural But Not Cardiac/Pancreatic Progenitor Differentiation of Mouse Embryonic Stem Cells In vitro. Front Cell Neurosci. 2017;11(33).
3Czeredys M, Maciag F, Methner A, Kuznicki J. Tetrahydrocarbazoles decrease elevated SOCE in medium spiny neurons from transgenic YAC128 mice, a model of Huntington’s disease. Biochem Biophys Res Commun. 2017 Feb 19;483(4):1194-1205. 23;116:233-246.
In gene editing…
Allele-specific silencing of mHTT with methods that degrade mHTT mRNA is a promising treatment for HD, but that silencing is neither complete nor permanent. The discovery of the CRISPR/Cas9 nuclease system has created the possibility of altering genomic DNA in affected cells, and permanently silencing mHTT.
Two groups have recently published studies that attempt this in mice.1,2 Both groups utilized the existence of allele-specific single-nucleotide polymorphisms (SNPs) that create or destroy protospacer adjacent motif (PAM) elements in an allele-specific manner in the mHtt promoter. The groups then used a two-guide RNA (gRNA) system, with one gRNA targeted against the SNP in the promoter, and one in a downstream intron, leading to excision of the mHtt promoter and first few exons.
The Davidson group used this technique, excising exon1 of mHtt in an allele-specific manner, in immortalized cells, in human HD fibroblasts, and in vivo in a BACHD mouse model.1 Shin and colleagues excised a 44 kb segment of mHtt, including exons 1–3, and successfully used their platform in induced pluripotent stem cells and neural precursor cells.
2 Together, these data demonstrate the reproducibility and utility of using the CRISPR nuclease system to potentially permanently alter DNA to silence mHTT for therapeutic effect. CRISPR/Cas9 also can be used to create tools for drug screening. The Pouladi group coupled CRISPR/Cas9 technology with piggyback transposon technology to create corrected isogenic HD lines.3 The corrected lines maintained their pluripotency, differentiation potential, and karyotype, with no off-target effects.
The correction restored HD-induced phenotypes, including neural rosette formation deficiencies, apoptotic susceptibility, and energetic defects in the isogenic iPSC line. Performing global differential gene expression analysis on the parental line, the corrected line, and a non-related control line allowed the scientists to determine which genes were differentially expressed because of the HD mutation, and which genes were differentially expressed because of genetic background.
1Monteys AM, Ebanks SA, Keiser MS, Davidson BL. CRISPR/Cas9 Editing of the Mutant Huntingtin Allele In Vitro and In Vivo. Mol Ther. 2017 Jan 4;25(1):12-23. doi: 10.1016/j.ymthe.2016.11.010. Epub 2017 Jan 4. 2Shin JW, Kim K-H, Chao MJ, et al. Permanent inactivation of Huntington’s disease mutation by personalized allele-specific CRISPR/ Cas9. Hum Mol Genet. 2016 Oct 15;25(20):4566-4576. 3Xu X, Tay Y, Sim B, et al. Reversal of Phenotypic Abnormalities by CRISPR/Cas9-Mediated Gene Correction in Huntington Disease Patient- Derived Induced Pluripotent Stem Cells. Stem Cell Reports.2017 Mar 14;8(3):619-633.
Dr. Sarah Tabrizi’s group continues to datamine the extensive human imaging and disease phenotype data available from the two large multi center cohort studies, Track-HD and Track-On HD. In the largest imaging study ever done to assess depressive symptoms in HD, the group examined variation in structural and functional brain networks in relation to symptoms of depression in premanifest HD (preHD) and healthy controls.1 Onset of depression precedes onset of motor symptoms, and neuroimaging studies may reveal the early affected brain networks. The group found that depressive symptoms in preHD are correlated with specific connectivity changes in the default mode network and the basal ganglia.
Another group published its imaging and electrophysiological study in Nature’s Scientific Reports, looking for functional and structural changes that are present prior to clinical diagnosis.2 This study looked at preHD patients who had a very low disease burden score, far from disease onset. All methods used in this study agree that brain function and structure is normal in this cohort, meaning that symptoms are reflective of an ongoing neurodegenerative process. This may have implications for timing of treatment. Finally, another large-scale patient-based collaborative effort aimed to identify patients with extreme HD motor phenotypes with respect to their CAG repeat length and age, in order to search for disease modifiers.3 By looking at more than 10,000 HD patients, the group found that the total motor score (TMS) could vary by up to 30 years in individuals with the same CAG repeat length. They found that a TMS score of more than 13 would predict motor onset regardless of age and CAG length. The identified patients in the upper and lower quantiles can be further examined for biological, medical, or environmental factors that may lead to their extreme motor phenotype, which will inform patient care and prognosis.
1McColgan P, Razi A, Gregory S, et al. Structural and functional brain network correlates of depressive symptoms in premanifest Huntington’s disease. Hum Brain Mapp. 2017 Mar 15. doi: 10.1002/ hbm.23527. [Epub ahead of print]
2Gorges M, Müller H-P, Mayer IMS, et al. Intact sensory-motor network structure and function in far from onset premanifest Huntington’s disease. Sci Rep. 2017 Mar 7;7:43841. 3Braisch U, Hay B, Muche R, et al. Identification of extreme motor phenotypes in Huntington’s disease. Am J Med Genet B Neuropsychiatr Genet. 2017 Apr;174(3):283-294.