huntington 's disease ( hd ) is a progressive , autosomal dominantly inherited neurodegenerative disorder , characterized by impaired motor control , cognitive decline , and occasional psychiatric illness .
hd results from the expansion of a cag repeat in exon 1 of the huntingtin ( htt ) gene , yielding a protein with a polyglutamine ( polyq ) expansion tract near its amino terminus .
this polyq - expanded huntingtin protein has a propensity to misfold , making it resistant to proteasomal and autophagy - mediated degradation .
earlier age of disease onset and more rapid disease progression correlate with increasing cag repeat length , with hd manifesting in patients carrying 40 or more cag repeats .
hd is the most common polyq repeat disease , with a prevalence of at least 1 in 10,000 in the usa and europe . in hd
, polyq - expanded huntingtin protein is expressed throughout the brain , with the most severe degeneration occurring in striatal medium spiny neurons and cortical projection neurons that extend to the striatum .
currently , hd research is conducted in rodent models , rodent primary neurons , and non - neural human cell lines , but there are questions as to whether such systems yield findings that are truly relevant to the mechanistic basis of human hd pathogenesis .
about six years ago , takahashi and yamanaka   developed a method for reprogramming human somatic cells so that they entered a pluripotent state , allowing for further differentiation into other cell types , including neurons .
this induced pluripotent stem cell ( ipsc ) approach has ushered in a new era of human neurodegenerative disease modeling .
two recent publications , one from the hd ipsc consortium   and another by an et al .
, respectively , reported the generation and characterization of ipsc - derived models for hd and the genetic correction of a disease - causing cag repeat expansion mutation in ipscs from individuals with hd .
together , these two studies provide important insights into the utility and limitations of ipsc modeling of neurodegenerative disease .
one of the greatest challenges facing the ipsc modeling field is the enormous variability of different ipsc lines and the neurons derived from them . as generation and characterization of ipsc lines
can be laborious and costly , one obvious solution to assure the validity of ipsc disease models is to create research teams that work together on a particular disorder , sharing ipsc lines and derived neurons .
this very strategy was applied in the hd ipsc consortium study , in which eight different groups produced ipscs from three individuals with hd and three control individuals and used them to assay a wide range of phenotypes .
these included gene expression , cell adhesion , bioenergetics , glutamate toxicity , cell death , calcium flux , and trophic factor withdrawal .
these phenotypes were assessed in neural stem cells ( nscs ) grown in spherical aggregates under defined growth factor conditions , as well as in differentiated striatal - like neurons obtained using both long and short differentiation protocols , which differed in the growth factors that were added .
this comprehensive inventory of cellular and molecular assays revealed that certain phenotypes correlate with hd disease cag repeat length , whereas other phenotypes do not .
the common results that were recapitulated across the various laboratories highlight the integrity of the developed ipsc lines , making the hd ipsc consortium study an impressive blueprint that is worthy of emulation .
the study by an et al .   had quite a different goal : to perform genetic correction in an hd - ipsc line , thereby creating an isogenic revertant ipsc line that would carry two normal - length htt alleles .
these ' corrected ' controls had a genetic background that was identical to that of the ' uncorrected ' ipsc line , so any phenotypic or expression differences between the two could be solely attributed to the cag expansion .
the ' corrected ' line was created successfully using a standard recombination approach , allowing the comparison of a number of cellular and molecular characteristics between the mutant progenitor line and the normalized derivative line .
this study also demonstrated that the genetically corrected hd - ipsc line could be used to produce nscs using a protocol described previously by this group .
furthermore , these nscs could be successfully transplanted into the striatum of hd mice , were able to populate this crucial brain region , and then underwent proper differentiation into neurons and glia .
thus , the stage is set for the potential future application of this strategy as a therapeutic intervention in hd .
in tandem , these papers catalogue a variety of phenotypes that were observed in hd - ipsc - derived neural progenitors and in medium spiny neurons . both groups performed gene expression analysis to compare hd - ipscs and their neuronal derivatives . the hd ipsc consortium findings implicated genes that are involved in cell signaling , cell cycling , axon guidance and neuronal development as dysregulated in nscs ( figure 1 ) .
importantly , medium cag repeat expansions and longer cag repeat expansions expressed most of these genes differently . an and co - workers performed transcript expression analysis , although they did this in ipscs rather than in ipsc - derived nscs . they observed that , in comparison with corrected controls , hd - ipscs showed increased expression of genes that are involved in tgf- signaling and decreased expression of genes involved in cadherin signaling ( figure 1 ) .
notably , the expression differences between the hd - ipscs and corrected hd - ipscs were of an order of magnitude less than those between non - related control ipscs and hd - ipscs .
hd - ipscs , corrected ipscs , and ipsc - derived neurons reveal key disease - associated phenotypes .
an et al .   also generated corrected hd - ipscs by homologous recombination and showed that , when compared to corrected hd - ipscs , hd - ipscs have increased expression of genes that are induced by the tgf- signaling pathway and decreased expression of cadherin pathway genes .
hd - ipsc lines developed by the hd ipsc consortium were further differentiated to form npcs and darpp-32-positive striatal - like neurons .
these npcs yielded evidence for the altered expression in hd of genes that are involved in cell signaling , cell cycling , axon guidance , and neurodevelopmental pathways . as delineated here
, future studies might compare these two datasets and thus could evaluate the expression of the target genes of those transcription factors known to interact with huntingtin , examine mitochondrial dysfunction in hd and investigate the exact identity of the dying cells in hd .
npc , neural progenitor cell ; ros , reactive oxygen species ; tf , transcription factor .
comparison of the two studies also highlights the shared relevance of certain cellular and molecular phenotypes , such as cell death and mitochondrial dysfunction ( figure 1 ) .
initial studies had previously indicated that hd - ipsc - derived nscs display increased caspase activity , and that both hd - ipscs and derived neurons exhibit increased lysosomal activity .
an important finding that is shared by the two groups relates to defects in energy metabolism .
mitochondrial dysfunction , resulting in impaired cellular bioenergetics , is an emerging hallmark of hd .
the hd ipsc consortium reported decreased intracellular atp levels in hd neural progenitor cells ( npcs ) , and an et al .
both of these phenotypes revealed defects in energy metabolism in the hd models , but the importance of mitochondrial dysfunction requires further investigation .
another hd - associated phenotype that was observed by both groups is increased cell death .
the hd ipsc consortium used a variety of assays , including longitudinal survival tracking , nuclear condensation assays , and caspase 3/7 activation , to detect increased cell death in hd - nscs and striatal - like derivatives . an and colleagues also observed cag repeat - length - dependent cell death in similar assays .
both groups demonstrated a more severe cell death phenotype upon withdrawal of brain - derived neurotrophic factor ( bdnf ) , adding to the mounting evidence to support a role for bdnf - associated toxicity in hd .
bdnf is a secreted neurotrophin , which undergoes transcriptional repression in certain hd models , and there are indications that over - expression of bdnf can ameliorate the hd phenotype .
cell death in the striatum and cortex are hallmarks of hd ; hence , this shared observation is significant , as it recapitulates in vitro what is seen in vivo .
an et al .   have taken an initial step toward the application of corrected hd - ipsc - derived neurons in cell replacement therapy .
they showed that differentiated striatal - like neurons can survive transplantation into mouse brain , but they did not report whether transplanted r6/2 mice exhibited any recovery in phenotype . despite this uncertainty , it is quite clear that the newly derived hd - ipsc models hold great potential for use in drug screening . as illustrated in figure 1 , future studies should examine effects on htt interactor target genes , and compare the results so obtained with the data sets from these two studies . the observations from both studies regarding gene expression and mitochondrial dysfunction support findings generated in other models .
the huntingtin protein is known to interact with a number of transcription factors and co - activators , including creb - binding protein ( cbp ) , sp1 , tafii130 , and ppar co - activator 1 ( pgc-1 ) .
follow - up analysis of the expression of the target genes that encode these already - implicated transcription factors might be worthwhile ( figure 1 ) .
diminished pgc-1 function appears to be a major contributor to hd pathogenesis , so the hd - ipsc model could be used to evaluate the mitochondrial phenotypes seen in hd mice , including decreased mitochondrial complex activities , decreased mitochondrial number , and increased oxidative stress .
pgc-1 overexpression can rescue hd - associated phenotypes in mice by inducing transcription factor eb ( tfeb ) , a master regulator of autophagy .
validation of the role of pgc-1 and tfeb dysfunction in hd - ipsc models would be worthwhile and could facilitate the development of new therapies . finally , a missing piece of the puzzle is the exact identity of the dying cells in hd .
the hd ipsc consortium used a method for tracking live cells through longitudinal studies , which allowed them to identify the dying cells as being morphologically similar to neurons , although the specific neural type was unspecified .
as hd patients with longer cag repeats suffer more widespread neuron cell death , it will be important to determine if the vulnerability of ipsc - derived neurons differs between neural cell types ( figure 1 ) .
amelioration of cell death in a relevant ipsc - derived neural lineage could be a useful indicator of potential drug efficacy .
once the hd field homes in on viable lead compounds , hd - ipsc models could serve as invaluable tools for validation and should accelerate therapeutic development .
bdnf : brain - derived neurotrophic factor ; hd : huntington 's disease ; htt : huntingtin gene ; ipsc : induced pluripotent stem cell ; nsc : neural stem cell .
the authors acknowledge funding support from the nih ( r01 ns065874 ) for their huntington 's disease research .
 deconstructing the mechanistic basis of neurodegenerative disorders , such as huntington 's disease ( hd ) , has been a particularly challenging undertaking , relying mostly on post - mortem tissue samples , non - neural cell lines from affected individuals , and model organisms . 
 two articles recently published in cell stem cell report first the generation and characterization of induced pluripotent stem cell ( ipsc)-derived models for hd , and second , the genetic correction of a disease - causing cag expansion mutation in ipscs from individuals with hd . taken together 
 , these two studies provide a framework for the production and validation of ipsc materials for human neurodegenerative disease research and yield crucial tools for investigating future therapies . 
Application of stem cell modeling to Huntington's disease: the time is now
A plethora of high-quality and high-quantity HD-iPSC models unveiled
Candidate pathways revealed by gene expression profiling and by cellular and molecular assays
HD-iPSC models offer opportunities for finding new therapies
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