Early alterations of brain cellular energy homeostasis in Huntington disease models

Brain energy deficit has been a suggested cause of Huntington disease (HD), but ATP depletion has not reliably been shown in preclinical models, possibly because of the immediate post-mortem changes in cellular energy metabolism. To examine a potential role of a low energy state in HD, we measured, for the first time in a neurodegenerative model, brain levels of high energy phosphates using microwave fixation, which instantaneously inactivates brain enzymatic activities and preserves in vivo levels of analytes. We studied HD transgenic R6/2 mice at ages 4, 8, and 12 weeks. We found significantly increased creatine and phosphocreatine, present as early as 4 weeks for phosphocreatine, preceding motor system deficits and decreased ATP levels in striatum, hippocampus, and frontal cortex of R6/2 mice. ATP and phosphocreatine concentrations were inversely correlated with the number of CAG repeats. Conversely, in mice injected with 3-nitroproprionic acid, an acute model of brain energy deficit, both ATP and phosphocreatine were significantly reduced. Increased creatine and phosphocreatine in R6/2 mice was associated with decreased guanidinoacetate N-methyltransferase and creatine kinase, both at the protein and RNA levels, and increased phosphorylated AMP-dependent protein kinase (pAMPK) over AMPK ratio. In addition, in 4-month-old knock-in Hdh(Q111/+) mice, the earliest metabolic alterations consisted of increased phosphocreatine in the frontal cortex and increased the pAMPK/AMPK ratio. Altogether, this study provides the first direct evidence of chronic alteration in homeostasis of high energy phosphates in HD models in the earliest stages of the disease, indicating possible reduced utilization of the brain phosphocreatine pool.     

A recurrent expansion of a maternal allele with 36 CAG repeats causes Huntington disease in two sisters

Large intergenerational repeat expansions of the CAG trinucleotide repeat in the HD gene have been well documented for the male germline. We describe a recurrent large expansion of a maternal allele with 36 CAG repeats (to 66 and 57 repeats, respectively, in two daughters) associated with onset of Huntington disease (HD) in the second and third decade in a family without history of HD. Our findings give evidence of a gonadal mosaicism in the unaffected mother. We hypothesize that large expansions also occur in the female germline and that a negative selection of oocytes with long repeats might explain the different instability behavior of the male and the female germlines.     

Evidence for maternal inheritance of female height in a British national sample

Analyses of the height variation in a large contemporary British sample found evidence for maternal inheritance of female height, because the mother-daughter correlation coefficient was significantly higher than the father-daughter value. Further evidence for a maternal effect came from the comparison of the daughter's stature with the difference in parental heights. The maternal effect was estimated to be 1%.     

Homozygote for Huntington disease.

Four offspring of three different Huntington disease (HD) affected x affected matings were assessed by genetic linkage analysis for possible homozygosity. One individual was found to have a 95% likelihood of being an HD homozygote. The homozygote individual had an age at onset and symptoms which were similar to those of affected HD heterozygote relatives, including some with younger onset. This confirms the observation of Wexler et al. that in HD the homozygote is not more severely afflicted than the heterozygote.     

Ovary-autonomous maternal inheritance at a developmental locus in Drosophila

The sex-linked temperature-sensitive mutation shibire^ts of Drosophila melanogaster shows a maternal effect causing embryonic lethality at 29°C. The maternal influence is due to gene action autonomous to the ovary. Embryos carrying the paternally derived wild-type gene can survive at 29°C but only if heat pulses are begun at least 9 hr after oviposition. The paternal rescue is presumably due to zygotic gene action at this locus beginning part way through embryogenesis. A maternal wild-type genome, however, can produce shi embryos that have sufficient shi⁺ product to support embryogenesis up to the hatching stage even at 29°C.     

Huntington disease phenocopy is a familial prion disease

Huntington disease (HD) is a common autosomal dominant neurodegenerative disease with early adult-onset motor abnormalities and dementia. Many studies of HD show that huntingtin (CAG)n repeat-expansion length is a sensitive and specific marker for HD. However, there are a significant number of examples of HD in the absence of a huntingtin (CAG)n expansion, suggesting that mutations in other genes can provoke HD-like disorders. The identification of genes responsible for these "phenocopies" may greatly improve the reliability of genetic screens for HD and may provide further insight into neurodegenerative disease. We have examined an HD phenocopy pedigree with linkage to chromosome 20p12 for mutations in the prion protein (PrP) gene (PRNP). This reveals that affected individuals are heterozygous for a 192-nucleotide (nt) insertion within the PrP coding region, which encodes an expanded PrP with eight extra octapeptide repeats. This reveals that this HD phenocopy is, in fact, a familial prion disease and that PrP repeat-expansion mutations can provoke an HD "genocopy." PrP repeat expansions are well characterized and provoke early-onset, slowly progressive atypical prion diseases with an autosomal dominant pattern of inheritance and a remarkable range of clinical features, many of which overlap with those of HD. This observation raises the possibility that an unknown number of HD phenocopies are, in fact, familial prion diseases and argues that clinicians should consider screening for PrP mutations in individuals with HD-like diseases in which the characteristic HD (CAG)n repeat expansions are absent.     

Evidence for maternal inheritance of mitochondrial DNA in allotetraploid.

The complete mitochondrial DNA (mtDNA) sequences of the allotetraploid and red crucian carp were determined in this paper. We compared the complete mtDNA sequences between the allotetraploid and its female parent red crucian carp, and between the allotetraploid and its male parent common carp. The results indicated that the complete mtDNA nucleotide identity (99.7%) between the allotetraploid and its female parent red crucian carp was higher than that (89.0%) between the allotetraploid and its male parent common carp. Moreover, the analysis on the start and stop codons, overlaps and spacers, and phylogeny of the mt genomes indicated the genetic relationship between the allotetraploid and its female parent red crucian carp was closer than that between the allotetraploid and its male parent common carp. Our results indicated that the allotetraploid mt genome was strictly maternally inherited. Through maternal inheritance, the mt genome in the F(11) allotetraploid displayed extremely high similarity to that in the female parent red crucian carp after 11 generations (from F(1) to F(11) hybrids). Such results indicated that the F(11) allotetraploid possessed the stable inheritance characteristic. Thus the tetraploid stocks possessed the good base to form a new tetraploid species in the future. Since the establishment of the new tetraploid stocks has the great significance in analyzing evolutionary theory of vertebrate and in improving aquaculture industry, analysis of the mt genome and the elucidation of the variation of the mt genome in the allotetraploid and its parents proved that it was a useful genetic marker to monitor the variations in the progeny of the crosses.     

Huntington’s disease and mitochondrial alterations: emphasis on experimental models

Huntington’s disease (HD) is an inheritable neurological disorder coursing with degeneration of basal ganglia and producing chorea and dementia. One common factor accounting for neurodegeneration in this disorder is mitochondrial deterioration at both morphologic and functional levels. The development of experimental models in animals or cell preparations to resemble pathologic and pathogenic conditions of this disorder has served for more than four decades to describe part of the mechanistic alterations that could be occurring in mitochondria of HD patients, and the subsequent design of therapeutic alternatives where mitochondrial alterations are the primary target. In this miniriview we describe some of the most relevant studies at the experimental level, giving support to the hypothesis that mitochondria play a central role in HD pathogenesis.     

Protein oxidation in Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disorder caused by expansion of CAG repeats in the huntingtin gene, affecting initially the striatum and progressively the cortex. Oxidative stress, and consequent protein oxidation, has been described as important to disease progression. This review focuses on recent advances in the field, with a particular emphasis on the identified target proteins and the role that their oxidation has or might have in the pathophysiology of HD. Oxidation and the resulting inactivation and/or degradation of important proteins can explain the impairment of several metabolic pathways in HD. Oxidation of enzymes involved in ATP synthesis can account for the energy deficiency observed. Impairment of protein folding and degradation can be due to oxidation of several heat shock proteins and Valosin-containing protein. Oxidation of two enzymes involved in the vitamin B6 metabolism could result in decreased availability of pyridoxal phosphate, which is a necessary cofactor in transaminations, the kynurenine pathway and the synthesis of glutathione, GABA, dopamine and serotonin, all of which have a key role in HD pathology. In addition, protein oxidation often contributes to oxidative stress, aggravating the molecular damage inside the cell.     

Modeling Huntington disease in Drosophila

Huntington disease (HD) is an inherited neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in the huntingtin (Htt) gene. Despite years of research, there is no treatment that extends life for patients with the disorder. Similarly, little is known about which cellular pathways that are altered by pathogenic Huntingtin (Htt) protein expression are correlated with neuronal loss. As part of a longstanding effort to gain insights into HD pathology, we have been studying the protein in the context of the fruitfly Drosophila melanogaster. We generated transgenic HD models in Drosophila by engineering flies that carry a 12-exon fragment of the human Htt gene with or without the toxic trinucleotide repeat expansion. We also created variants with a monomeric red fluorescent protein (mRFP) tag fused to Htt that allows in vivo imaging of Htt protein localization and aggregation. While wild-type Htt remains diffuse throughout the cytoplasm of cells, pathogenic Htt forms insoluble aggregates that accumulate in neuronal soma and axons. Aggregates can physically block transport of numerous organelles along the axon. We have also observed that aggregates are formed quickly, within just a few hours of mutant Htt expression. To explore mechanisms of neurodegeneration in our HD model, we performed in vivo and in vitro screens to search for modifiers of viability and pathogenic Htt aggregation. Our results identified several novel candidates for HD therapeutics that can now be tested in mammalian models of HD. Furthermore, these experiments have highlighted the complex relationship between aggregates and toxicity that exists in HD.     

Paternal inheritance of chloroplast DNA and maternal inheritance of mitochondrial DNA in the genus Actinidia

PCR amplification of four chloroplast DNA (cpDNA) and two mitochondrial DNA (mtDNA) regions followed by restriction of the amplified products was used to identify restriction fragment length polymorphisms in 21 Actinidia taxa. Subsequently, the mode of organelle inheritance was investigated in both interspecific and intraspecific controlled crosses made between genotypes showing different cpDNA and/or mtDNA haplotypes. Fifty-six seedlings produced from three interspecific crosses, including in one case the pseudo reciprocal (different genotypes of the same species used as opposite parents), were checked for cpDNA inheritance, and 102 seedlings from the same interspecific crosses and 32 seedlings from two intraspecific crosses within the species A. deliciosa were checked for mtDNA inheritance. In all cases, cpDNA was inherited from the father and mtDNA was inherited from the mother. Maternal inheritance of mtDNA was expected, being the rule in plants, but A. deliciosa is the first genus in angiosperms for which a widespread and strictly paternal inheritance of cpDNA has been reported. Transmission of chloroplastic and mitochondrial genomes through opposite parents provides an exceptional opportunity for studying the paternal and maternal genetic lineages of species in the genus Actinidia.     

Selection response in traits with maternal inheritance

Maternal inheritance is the non-Mendelian transmission of traits from mothers to their offspring. Despite its presence in virtually all organisms, acting through a variety of mechanisms, the evolutionary consequences of maternal inheritance are not well understood. Here we review and extend a model of the inheritance and evolution of multiple quantitative characters with complex pathways of maternal effects. Extensions of the earlier model include common family environmental effects not associated with maternal phenotype, sexual dimorphism, and paternal effects (non-Mendelian influence of the father on offspring traits). We find that, in contrast to simple Mendelian inheritance, maternal inheritance produces qualitatively different evolutionary dynamics for two reasons: (1) the response to selection on a set of characters depends not only on their additive genetic variances and covariances, but also on maternal characters that influence them, and (2) time lags in the response to selection create a form of evolutionary momentum. These results have important implications for evolution in natural populations and practical applications in the economic improvement of domesticated species. We derive selection indices that maximize either the economic improvement in a single generation of artificial selection or the asymptotic rate of improvement in long-term selection programmes, based on individual merit or a combination of individual and family merit. Numerical examples show that accounting for maternal inheritance can lead to considerable increases in the efficiency of artificial selection.     

Covariate-dependent age-at-onset distributions for Huntington disease.

A combined logistic regression and life-table analysis is presented on age-at-onset data for Huntington disease. Covariates included in the analysis were sex of the at-risk individual, parental age at onset, and sex of transmitting parent. Parental age at onset and parental sex were found to be significant covariates for age at onset in the offspring, and the appropriate logistic regression functions are calculated by maximum likelihood methods. These regression functions permit a more precise evaluation of carrier risks and likelihoods than hitherto was possible by simple computational means. We further introduce a novel method to account for sibship correlations in the significance assessment, using log-likelihood differences between different models.