Peter J. Detloff

Ectopically Expressed CAG Repeats Cause Intranuclear Inclusions and a Progressive Late Onset Neurological Phenotype in the Mouse

The mutations responsible for several human neurodegenerative disorders are expansions of translated CAG repeats beyond a normal size range. To address the role of repeat context, we have introduced a 146-unit CAG repeat into the mouse hypoxanthine phosphoribosyltransferase gene (Hprt). Mutant mice express a form of the HPRT protein that contains a long polyglutamine repeat. These mice develop a phenotype similar to the human translated CAG repeat disorders. Repeat containing mice show a late onset neurological phenotype that progresses to premature death. Neuronal intranuclear inclusions are present in affected mice. Our results show that CAG repeats do not need to be located within one of the classic repeat disorder genes to have a neurotoxic effect.

Intraflagellar transport is essential for endochondral bone formation

While cilia are present on most cells in the mammalian body, their functional importance has only recently been discovered. Cilia formation requires intraflagellar transport (IFT), and mutations disrupting the IFT process result in loss of cilia and mid-gestation lethality with developmental defects that include polydactyly and abnormal neural tube patterning. The early lethality in IFT mutants has hindered research efforts to study the role of this organelle at later developmental stages. Thus, to investigate the role of cilia during limb development, we generated a conditional allele of the IFT protein Ift88 (polaris). Using the Cre-lox system, we disrupted cilia on different cell populations within the developing limb. While deleting cilia in regions of the limb ectoderm had no overt effect on patterning, disruption in the mesenchyme resulted in extensive polydactyly with loss of anteroposterior digit patterning and shortening of the proximodistal axis. The digit patterning abnormalities were associated with aberrant Shh pathway activity, whereas defects in limb outgrowth were due in part to disruption of Ihh signaling during endochondral bone formation. In addition, the limbs of mesenchymal cilia mutants have ectopic domains of cells that resemble chondrocytes derived from the perichondrium, which is not typical of Indian hedgehog mutants. Overall these data provide evidence that IFT is essential for normal formation of the appendicular skeleton through disruption of multiple signaling pathways.

Aberrant splicing of HTT generates the pathogenic exon 1 protein in Huntington disease

Huntington disease (HD) is a devastating, late-onset, inherited neurodegenerative disorder that manifests with personality changes, movement disorders, and cognitive decline. It is caused by a CAG repeat expansion in exon 1 of the HTT gene that translates to a polyglutamine tract in the huntingtin protein (HTT). The formation of HTT fragments has been implicated as an essential step in the molecular pathogenesis of HD and several proteases that cleave HTT have been identified. However, the importance of smaller N-terminal fragments has been highlighted by their presence in HD postmortem brains and by the fact that nuclear inclusions are only detected by antibodies to the N terminus of HTT. Despite an intense research effort, the precise length of these fragments and the mechanism by which they are generated remains unknown. Here we show that CAG repeat length–dependent aberrant splicing of exon 1 HTT results in a short polyadenylated mRNA that is translated into an exon 1 HTT protein. Given that mutant exon 1 HTT proteins have consistently been shown to be highly pathogenic in HD mouse models, the aberrant splicing of HTT mRNA provides a mechanistic basis for the molecular pathogenesis of HD. RNA-targeted therapeutic strategies designed to lower the levels of HTT are under development. Many of these approaches would not prevent the production of exon 1 HTT and should be reviewed in light of our findings.

DNA instability in postmitotic neurons

Huntingtons disease (HD) is caused by a CAG repeat expansion that is unstable upon germ-line transmission and exhibits mosaicism in somatic tissues. We show that region-specific CAG repeat mosaicism profiles are conserved between several mouse models of HD and therefore develop in a predetermined manner. Furthermore, we demonstrate that these synchronous, radical changes in CAG repeat size occur in terminally differentiated neurons. In HD this ongoing mutation of the repeat continuously generates genetically distinct neuronal populations in the adult brain of mouse models and HD patients. The neuronal population of the striatum is particularly distinguished by a high rate of CAG repeat allele instability and expression driving the repeat upwards and would be expected to enhance its toxicity. In both mice and humans, neurons are distinguished from nonneuronal cells by expression of MSH3, which provides a permissive environment for genetic instability independent of pathology. The neuronal mutations described here accumulate to generate genetically discrete populations of cells in the absence of selection. This is in contrast to the traditional view in which genetically discrete cellular populations are generated by the sequence of random variation, selection, and clonal proliferation. We are unaware of any previous demonstration that mutations can occur in terminally differentiated neurons and provide a proof of principle that, dependent on a specific set of conditions, functional DNA polymorphisms can be produced in adult neurons.

Rodent genetic models of Huntington disease

Huntington disease (HD) is a dominantly inherited human neurodegenerative disorder characterized by motor deficits, cognitive impairment, and psychiatric symptoms leading to inexorable decline and death. Since the identification of the huntingtin gene and the characteristic expanded CAG repeat/polyglutamine mutation, multiple murine genetic models and one rat genetic model have been generated. These models fall into two general categories: transgenic models with ectopic expression of the characteristic expanded CAG codon mutation, and knock-in models with expression of mutant huntingtin under control of endogenous regulatory elements. Rodent genetic models are valuable tools for studying mechanisms of pathogenesis in HD and for preclinical evaluation of possible therapies. In this mini-review, we provide a concise comparative summary of rodent genetic models of HD.

Longitudinal Evaluation of the Hdh (CAG)150 Knock-In Murine Model of Huntington's Disease

Several murine genetic models of Huntingtons disease (HD) have been developed. Murine genetic models are crucial for identifying mechanisms of neurodegeneration in HD and for preclinical evaluation of possible therapies for HD. Longitudinal analysis of mutant phenotypes is necessary to validate models and to identify appropriate periods for analysis of early events in the pathogenesis of neurodegeneration. Here we report longitudinal characterization of the murine Hdh (CAG)150 knock-in model of HD. A series of behavioral tests at five different time points (20, 40, 50, 70, and 100 weeks) demonstrates an age-dependent, late-onset behavioral phenotype with significant motor abnormalities at 70 and 100 weeks of age. Pathological analysis demonstrated loss of striatal dopamine D1 and D2 receptor binding sites at 70 and 100 weeks of age, and stereological analysis showed significant loss of striatal neuron number at 100 weeks. Late-onset behavioral abnormalities, decrease in striatal dopamine receptors, and diminished striatal neuron number observed in this mouse model recapitulate key features of HD. The Hdh (CAG)150 knock-in mouse is a valid model to evaluate early events in the pathogenesis of neurodegeneration in HD.

Early autophagic response in a novel knock-in model of Huntington disease

The aggregation of mutant polyglutamine (polyQ) proteins has sparked interest in the role of protein quality-control pathways in Huntingtons disease (HD) and related polyQ disorders. Employing a novel knock-in HD mouse model, we provide in vivo evidence of early, sustained alterations of autophagy in response to mutant huntingtin (mhtt). The HdhQ200 knock-in model, derived from the selective breeding of HdhQ150 knock-in mice, manifests an accelerated and more robust phenotype than the parent line. Heterozygous HdhQ200 mice accumulate htt aggregates as cytoplasmic aggregation foci (AF) as early as 9 weeks of age and striatal neuronal intranuclear inclusions (NIIs) by 20 weeks. By 40 weeks, striatal AF are perinuclear and immunoreactive for ubiquitin and the autophagosome marker LC3. Striatal NIIs accumulate earlier in HdhQ200 mice than in HdhQ150 mice. The earlier appearance of aggregate pathology in HdhQ200 mice is paralleled by earlier and more rapidly progressive motor deficits: progressive imbalance and decreased motor coordination by 50 weeks, gait deficits by 60 weeks and gross motor impairment by 80 weeks of age. At 80 weeks, heterozygous HdhQ200 mice exhibit striatal and cortical astrogliosis and a approximately 50% reduction in striatal dopamine receptor binding. Increased LC3-II protein expression, which is noted early and sustained throughout the disease course, is paralleled by increased expression of the autophagy-related protein, p62. Early and sustained expression of autophagy-related proteins in this genetically precise mouse model of HD suggests that the alteration of autophagic flux is an important and early component of the neuronal response to mhtt.

In Vivo Evidence for NMDA Receptor-Mediated Excitotoxicity in a Murine Genetic Model of Huntington Disease

N-methyl-d-aspartate receptor (NMDAR)-mediated excitotoxicity is implicated as a proximate cause of neurodegeneration in Huntington Disease (HD). This hypothesis has not been tested rigorously in vivo. NMDAR–NR2B subunits are a major NR2 subunit expressed by striatal medium spiny neurons that degenerate in HD. To test the excitotoxic hypothesis, we crossed a well validated murine genetic model of HD (Hdh(CAG)150) with a transgenic line overexpressing NMDAR–NR2B subunits. In the resulting double-mutant line, we show exacerbation of selective striatal neuron degeneration. This is the first direct in vivo evidence of NR2B–NMDAR-mediated excitotoxicity in the context of HD. Our results are consistent with previous suggestions that direct and/or indirect interactions of mutant huntingtin with NMDARs are a proximate cause of neurodegeneration in HD.

A promoter deletion reduces the rate of mitotic, but not meiotic, recombination at the HIS4 locus in yeast

SummarySeveral investigators have reported that transcription stimulates some types of mitotic recombination in the yeast Saccharomyces cerevisiae. We find that mutations that reduce the rate of trancription of the yeast HIS4 gene in vegetative cells reduce the frequency of mitotic, but not meiotic, recombination events.

Repair of specific base pair mismatches formed during meiotic recombination in the yeast Saccharomyces cerevisiae.

Heteroduplexes formed between DNA strands derived from different homologous chromosomes are an intermediate in meiotic crossing over in the yeast Saccharomyces cerevisiae and other eucaryotes. A heteroduplex formed between wild-type and mutant genes will contain a base pair mismatch; failure to repair this mismatch will lead to postmeiotic segregation (PMS). By analyzing the frequency of PMS for various mutant alleles in the yeast HIS4 gene, we showed that C/C mismatches were inefficiently repaired relative to all other point mismatches. These other mismatches (G/G, G/A, T/T, A/A, T/C, C/A, A/A, and T/G) were repaired with approximately the same efficiency. We found that in spores with unrepaired mismatches in heteroduplexes, the nontranscribed strand of the HIS4 gene was more frequently donated than the transcribed strand. In addition, the direction of repair for certain mismatches was nonrandom.