Francesca Persichetti

Genome-wide analysis of mammalian promoter architecture and evolution

Mammalian promoters can be separated into two classes, conserved TATA box–enriched promoters, which initiate at a well-defined site, and more plastic, broad and evolvable CpG-rich promoters. We have sequenced tags corresponding to several hundred thousand transcription start sites (TSSs) in the mouse and human genomes, allowing precise analysis of the sequence architecture and evolution of distinct promoter classes. Different tissues and families of genes differentially use distinct types of promoters. Our tagging methods allow quantitative analysis of promoter usage in different tissues and show that differentially regulated alternative TSSs are a common feature in protein-coding genes and commonly generate alternative N termini. Among the TSSs, we identified new start sites associated with the majority of exons and with 3′ UTRs. These data permit genome-scale identification of tissue-specific promoters and analysis of the cis-acting elements associated with them.

Heterogeneous Topographic and Cellular Distribution of Huntingtin Expression in the Normal Human Neostriatum

A striking heterogeneous distribution of topographic and cellular huntingtin immunoreactivity was observed within the human neostriatum using three distinct huntingtin antibodies. Patchy areas of low huntingtin immunoreactivity were present in both the caudate nucleus and putamen, surrounded by an intervening area of greater immunoreactivity. Comparison of huntingtin immunoreactivity with contiguous serial sections stained for enkephalin and calbindin D28k immunoreactivities showed that the topographic heterogeneity of huntingtin immunostaining corresponded to the patch (striosome) and matrix compartments within the striatum. Huntingtin immunoreactivity was confined primarily to neurons and neuropil within the matrix compartment, whereas little or no neuronal or neuropil huntingtin immunostaining was observed within the patch compartment. There was marked variability in the intensity of huntingtin immunolabel among medium-sized striatal neurons, whereas a majority of large striatal neurons were only faintly positive or without any immunoreactivity. Combined techniques for NADPH-diaphorase enzyme histochemistry and huntingtin immunocytochemistry, as well as double immunofluorescence for either nitric oxide synthase or calbindin D28k in comparison with huntingtin expression, revealed a striking correspondence between calbindin D28k and huntingtin immunoreactivities, with little or no colocalization between NADPH-diaphorase or nitric oxide synthase neurons and huntingtin expression. These observations suggest that the selective vulnerability of spiny striatal neurons and the matrix compartment observed in Huntington’s disease is associated with higher levels of huntingtin expression, whereas the relative resistance of large and medium-sized aspiny neurons and the patch compartments to degeneration is associated with low levels of huntingtin expression.

Rrs1 Is Involved in Endoplasmic Reticulum Stress Response in Huntington Disease

The induction of Rrs1 expression is one of the earliest events detected in a presymptomatic knock-in mouse model of Huntington disease (HD). Rrs1 up-regulation fulfills the HD criteria of dominance, striatal specificity, and polyglutamine dependence. Here we show that mammalian Rrs1 is localized both in the nucleolus as well as in the endoplasmic reticulum (ER) of neurons. This dual localization is shared with its newly identified molecular partner 3D3/lyric. We then show that both genes are induced by ER stress in neurons. Interestingly, we demonstrate that ER stress is an early event in a presymptomatic HD mouse model that persists throughout the life span of the rodent. We further show that ER stress also occurs in postmortem brains of HD patients.

Differential expression of normal and mutant Huntington's disease gene alleles.

Huntingtin expression was examined by Western blot and immunoprecipitation studies of lymphoblastoid cell lines from Huntingtons disease (HD) homozygotes, heterozygotes, and a phenotypically normal individual with a t(4p16.3;12p13.3) breakpoint in the HD gene. The latter produced a reduced level of normal huntingtin without evidence of an altered protein, indicating that simple loss of huntingtin activity does not cause HD. In juvenile onset HD heterozygotes, NH2- and COOH-terminal antisera revealed reduced relative expression from the mutant allele. Pulse-chase studies indicated that huntingtin is a stable protein whose differential allelic expression is not due to destabilization of the mutant isoform. No stable breakdown products specific to mutant huntingtin were detected in either HD homozygotes or heterozygotes. These data are consistent with HD involving either a gain of function or a dominant negative loss of function that operates within severe constraints and suggest that in either case the pathogenic process is usually saturated by the amount of abnormal huntingtin produced from a single mutant allele.

The human Y chromosome shows a low level of DNA polymorphism

Six new Y‐specific probes have been isolated and are reported. Along with another six already described they have been used in a systematic search for male specific RFLPs. An overall number of 46515 nucleotides have been screened with 12 enzymes and no polymorphic pattern observed. Our data reveal a greatly reduced level of polymorphism compared with other chromosomes.

Factors associated with HD CAG repeat instability in Huntington disease

Background: The Huntington disease (HD) CAG repeat exhibits dramatic instability when transmitted to subsequent generations. The instability of the HD disease allele in male intergenerational transmissions is reflected in the variability of the CAG repeat in DNA from the sperm of male carriers of the HD gene. Results: In this study, we used a collection of 112 sperm DNAs from male HD gene-positive members of a large Venezuelan cohort to investigate the factors associated with repeat instability. We confirm previous observations that CAG repeat length is the strongest predictor of repeat-length variability in sperm, but we did not find any correlation between CAG repeat instability and either age at the time of sperm donation or affectedness status. We also investigated transmission instability for 184 father–offspring and 311 mother–offspring pairs in this Venezuelan pedigree. Repeat-length changes were dependent upon the sex of the transmitting parent and parental CAG repeat length but not parental age or birth order. Unexpectedly, in maternal transmissions, repeat-length changes were also dependent upon the sex of the offspring, with a tendency for expansion in male offspring and contraction in female offspring. Conclusion: Significant sibling–sibling correlation for repeat instability suggests that genetic factors play a role in intergenerational CAG repeat instability.

Huntingtin Immunoreactivity in the Rat Neostriatum: Differential Accumulation in Projection and Interneurons

Huntingtons disease is caused by a mutation of the gene encoding the protein huntingtin. Features of the human disease, characterized by selective loss of neurons from the neostriatum, can be replicated in rodents by administration of excitotoxins. In both affected individuals and the rodent model, there is massive loss of striatal projection neurons with selective sparing of interneurons. Furthermore, in the human disease the earliest evidence of striatal injury is found in striosomal regions of the striatum. The mRNA encoding huntingtin is known to be expressed by neurons throughout the brain, a distribution which does not account for the selective patterns of neuronal death which are observed. Using fluorescence immunocytochemistry and confocal microscopy with an antibody to huntingtin, we have observed that in rats a subset of striatal projection neurons contains dense accumulations of huntingtin immunoreactivity (HT-ir), while most neurons in the striatum contain much smaller amounts. The intensely stained neurons are concentrated within the striatal striosomes, as defined by calbindin-D28K staining. In the matrix regions, relatively few neurons contain dense accumulations of HT-ir, and these cells always lack perikaryal staining for calbindin-D28K. Striatal interneurons, identified by the presence of immunoreactivity for choline acetyltransferase, parvalbumin, calretinin, or neuronal nitric oxide synthase, exhibit little or no HT-ir. The paucity of HT-ir in striatal interneurons, as well as the prominence of staining in a subset of striosomal neurons, mirrors the selective vulnerability of these different types of cells in early stages of human Huntingtons disease and in rodent excitotoxic models of the disorder. Our observations suggest that mechanisms which modulate the accumulation of huntingtin may play a central role in the neuronal degeneration of Huntingtons disease.

Mouse Huntington's disease gene homolog (Hdh)

The incurable neurodegenerative disorder, Huntingtons disease (HD), is caused by an expanded, unstable CAG repeat encoding a stretch of polyglutamine in a 4p16.3 gene (HD) of unknown function. Near the CAG repeat is a polyproline-encoding CCG repeat that shows more limited allelic variation. The mouse homologue,Hdh, has been mapped to chromosome 5, in a region devoid of mutations causing any comparable phenotype. We have isolated overlapping cDNAs from theHdh gene and compared their sequences with the human transcript. The consensus mouse coding sequence is 86% identical to the human at the DNA level and 91% identical at the protein level. Despite the overall high level of conservation,Hdh possesses an imperfect CAG repeat encoding only seven consecutive glutamines, compared to the 13–36 residues that are normal in man. Although no evidence for polymorphic variation of the CAG repeat was seen, a nearby CCG repeat differed in length by one unit between several strains of laboratory mouse andMus spretus. The absence of a long CAG repeat in the mouse is consistent with the lack of a spontaneous mouse model of HD. The information presented concerning the sequence of the mouse gene should facilitate attempts to create such a model.

Ser46 phosphorylation and prolyl-isomerase Pin1-mediated isomerization of p53 are key events in p53-dependent apoptosis induced by mutant huntingtin

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene coding for huntingtin protein. Several mechanisms have been proposed by which mutant huntingtin (mHtt) may trigger striatal neurodegeneration, including mitochondrial dysfunction, oxidative stress, and apoptosis. Furthermore, mHtt induces DNA damage and activates a stress response. In this context, p53 plays a crucial role in mediating mHtt toxic effects. Here we have dissected the pathway of p53 activation by mHtt in human neuronal cells and in HD mice, with the aim of highlighting critical nodes that may be pharmacologically manipulated for therapeutic intervention. We demonstrate that expression of mHtt causes increased phosphorylation of p53 on Ser46, leading to its interaction with phosphorylation-dependent prolyl isomerase Pin1 and consequent dissociation from the apoptosis inhibitor iASPP, thereby inducing the expression of apoptotic target genes. Inhibition of Ser46 phosphorylation by targeting homeodomain-interacting protein kinase 2 (HIPK2), PKCδ, or ataxia telangiectasia mutated kinase, as well as inhibition of the prolyl isomerase Pin1, prevents mHtt-dependent apoptosis of neuronal cells. These results provide a rationale for the use of small-molecule inhibitors of stress-responsive protein kinases and Pin1 as a potential therapeutic strategy for HD treatment.

HLA-LINKED SPINOCEREBELLAR ATAXIA - A CLINICAL AND GENETIC-STUDY OF LARGE ITALIAN KINDREDS

Five families with late onset autosomal dominant spinocerebellar ataxia, were studied. Linkage between the disease and HLA loci on the short arm of chromosome 6 was shown in the two largest pedigrees. Clinical study of 26 patients and neuropathological study in one are reported. The disease was characterized by cerebellar and pyramidal involvement variably associated with cranial nerve and peripheral nervous system disorders. A remarkable concordance of the main clinical features was observed in patients with similar disease duration. Comparison with previous reports of HLA‐linked spinocerebellar ataxia kindreds showed differences in clinical phenotypes. Although these might be due to genetic variation, the hypothesis is suggested that the phenotype might appear more homogeneous if disease duration is taken into account.