Huntington's disease (HD) is a dominant neurodegenerative disorder caused by expansion of a CAG repeat in the gene encoding huntingtin, a protein of unknown function. To distinguish between "loss of function" and "gain of function" models of HD, the murine HD homolog Hdh was inactivated by gene targeting. Mice heterozygous for Hdh inactivation were phenotypically normal, whereas homozygosity resulted in embryonic death. Homozygotes displayed abnormal gastrulation at embryonic day 7.5 and were resorbing by day 8.5. Thus, huntingtin is critical early in embryonic development, before the emergence of the nervous system. That Hdh inactivation does not mimic adult HD neuropathology suggests that the human disease involves a gain of function.

The AML͞CBF␣ runt transcription factors are key regulators of hematopoietic and bone tissue-specific gene expression. These factors contain a 31-amino acid nuclear matrix targeting signal that supports association with the nuclear matrix. We determined that the AML͞CBF␣ factors must bind to the nuclear matrix to exert control of transcription. Fusing the nuclear matrix targeting signal to the GAL4 DNA binding domain transactivates a genomically integrated GAL4 responsive reporter gene. These data suggest that AML͞CBF␣ must associate with the nuclear matrix to effect transcription. We used f luorescence labeling of epitopetagged AML-1B (CBFA2) to show it colocalizes with a subset of hyperphosphorylated RNA polymerase II molecules concentrated in foci and linked to the nuclear matrix. This association of AML-1B with RNA polymerase II requires active transcription and a functional DNA binding domain. The nuclear matrix domains that contain AML-1B are distinct from SC35 RNA processing domains. Our results suggest two of the requirements for AML-dependent transcription initiation by RNA polymerase II are association of AML-1B with the nuclear matrix together with specific binding of AML to gene promoters.Many observations suggest a linkage between nuclear architecture and the regulation of gene expression. These include drastic changes in nuclear morphology often seen in differentiation and in transformation to malignancy. The filamentous ribonucleoprotein network known as the nuclear matrix is a major component of nuclear structure (1-5). It serves to localize gene sequences and may maintain the distribution of regulatory factors throughout the nuclear space (6-19).The AML͞CBF␣ transcription factors are key regulators of hematopoietic and bone-tissue-specific gene expression (20)(21)(22)(23). We have recently shown that the transcriptionally active AML factors contain a specific sequence that targets them to the nuclear matrix (24). In contrast, several inactive forms of AML that lack this targeting signal do not associate with the matrix (24). The targeting sequence resides in a 31-aa segment (nuclear matrix targeting signal, NMTS; aa 351-381) within the C-terminal domain. This NMTS is physically distinct from the nuclear localization signal and functions autonomously to direct AML-1B (CBFA2) to the nuclear matrix (24). These findings suggest that association with nuclear structure may be an important aspect of the mechanisms of gene regulation.In this report we begin to examine the significance of the AML-1B transcription factor binding to the nuclear matrix. We show that AML-1B localizes to sites where there is also active transcription. We demonstrate that the NMTS, which directs AML-1B to the nuclear matrix, functions as a transactivation domain when interacting with an appropriate promoter. Furthermore, we show that the AML-1B transcription factor is targeted to discrete sites on the nuclear matrix. These sites also contain the hyperphosphorylated active form of RNA polymerase II. Colocalization of AML...

Significant sibling-sibling correlation for repeat instability suggests that genetic factors play a role in intergenerational CAG repeat instability.

Huntington's disease (HD) is an autosomal dominant disorder characterized by involuntary movements, dementia, and progressive, global, but regionally accentuated, brain atrophy. The disease affects the striatum most severely. An expansion of a trinucleotide repeat on chromosome 4p16.3 within the coding region of a gene termed IT15 has been identified as the mutation causing HD. The normal function of IT15 and the mechanisms by which the presence of the mutation causes HD are unknown. Although IT15 expression has been detected in the brain, as well as in other organ tissues, by Northern blot and in situ hybridization, it is not known whether a preferential regional or cellular expression of IT15 exists within the central nervous system of normal, affected, and presymptomatic individuals. Using quantitative in situ hybridization methods, we examined extensively the regional and cellular expression of IT15. In controls, IT15 expression was observed in all brain regions examined with the highest levels seen in cerebellum, hippocampus, cerebral cortex, substantia nigra pars compacta, and pontine nuclei. Expression in the striatum was intermediate and expression in the globus pallidus was low. IT15 was expressed predominantly in neurons; a low but significant level of expression was seen in glial cells. Analysis of grain counts per square micrometer in neurons showed that the regional differences in the level of mRNA expression were related to density and size of neurons in a given region and not primarily to differences in levels of mRNA expression in individual cells after correction for cell size. Neurons susceptible to degeneration in HD did not selectively express high levels of IT15 mRNA. In HD brains (grades 2-4), the distribution and levels of IT15 mRNA were comparable with controls in all areas except in neostriatum where the intensity of labeling was significantly reduced. Presymptomatic HD brains had a striatal expression similar to controls and surviving striatal neurons in more advanced HD had an expression of IT15 within normal limits. It is apparent from these results that the presence of expanded trinucleotide repeats in HD does not result in the absence of IT15 mRNA expression or in altered patterns or levels of expression. The lack of correlation between the levels of IT15 mRNA expression and susceptibility to degeneration in HD strongly suggests that the mutant gene acts in concert with other factors to cause the distinctive pattern of neurodegeneration in HD.