The CAG repeats (a polyglutamime) are measured by the HD genetic test.
There is a new player in the HD puzzle. Expression of the gene product CBP blocks the toxic effects of the expanded HD gene in the lab cultured cells. It could be that cell deposits are protective and CBP is required to form deposits.
The following paper shows meaningful progress from HD research. The complete paper is available online. --Jerry 21Mar2001, updated 22Mar2001
From: Human Molecular Genetics, 2000, Vol. 9, No. 14 2197-2202, McCampbell A et al.
| Finding that CBP binds to huntingtin-positive inclusions, together with our evidence that it binds to AR- and ataxin-3-positive inclusions, indicates that CBP sequestration may be a general phenomenon of polyglutamine expansion diseases. |
Spinal and bulbar muscular atrophy (SBMA) is one of eight inherited neurodegenerative diseases known to be caused by CAG repeat expansion.
The expansion results in an expanded polyglutamine tract, which likely confers a novel, toxic function to the affected protein. Cell culture and transgenic mouse studies have implicated the nucleus as a site for pathogenesis, suggesting that a critical nuclear factor or process is disrupted by the polyglutamine expansion.
In this report we present evidence that CREB-binding protein (CBP), a transcriptional co-activator that orchestrates nuclear response to a variety of cell signaling cascades, is incorporated into nuclear inclusions formed by polyglutamine-containing proteins in cultured cells, transgenic mice and tissue from patients with SBMA. We also show CBP incorporation into nuclear inclusions formed in a cell culture model of another polyglutamine disease, spinocerebellar ataxia type 3. We present evidence that soluble levels of CBP are reduced in cells expressing expanded polyglutamine despite increased levels of CBP mRNA.
Finally, we demonstrate that over-expression of CBP rescues cells from polyglutamine-mediated toxicity in neuronal cell culture. These data support a CBP-sequestration model of polyglutamine expansion disease.
Lastly, to determine whether the sequestration of CBP was responsible for cellular toxicity, we measured the effect of CBP over-expression in cells transfected with polyglutamine-containing AR constructs. As reported previously (3), cells expressing expanded polyglutamine died over 48–72 h (Fig. 4b). Cells over-expressing exogenous CBP showed enhanced viability, and the toxic effect of the expanded polyglutamine was blocked (Fig. 4c). CBP failed to rescue cells co-transfected with constructs for the pro-apoptotic factor bax (data not shown), indicating that the CBP was not simply having a non-specific effect on cellular toxicity.
We have found that CBP, normally present in a diffuse nuclear pattern, is sequestered in nuclear inclusions formed in cells expressing expanded polyglutamine proteins. This sequestration is found not only in cultured cells, but also in transgenic mice and patient tissue, and is common to at least two of the polyglutamine disorders. The ability of over-expressed CBP to block polyglutamine-induced toxicity in cell culture suggests that this sequestration may be an important step in the disease pathogenesis.
CBP as a general target in polyglutamine disease CBP is a particularly good candidate gene for disruption, as it is present at functionally limiting levels in the cell. Mice lacking one allele of the CBP gene have pronounced developmental defects, despite the presence of the second allele and two copies of the gene for p300, a protein thought to be largely redundant with CBP. Furthermore, ample evidence indicates that different cell signaling pathways compete for use of CBP in driving different transcriptional programs that influence cell proliferation and survival. Our finding that over-expression of CBP blocks polyglutamine-induced toxicity in these cells suggests that the toxicity is due to CBP depletion.
Recently, CBP was shown to co-localize with mutant huntingtin in cells co-transfected with plasmids for both genes (14). The recruitment of CBP to the inclusions in this study was dependent on the polyglutamine domain of CBP. This domain is in the C-terminal region of the protein and consists of 18 glutamines in humans and 15 in mice. Furthermore, CBP has been found in nuclear inclusions formed in mice transgenic for exon 1 of huntingtin (16). Importantly, the CBP antibodies used in both studies were raised against an epitope that does not include the polyglutamine repeat. Finding that CBP binds to huntingtin-positive inclusions, together with our evidence that it binds to AR- and ataxin-3-positive inclusions, indicates that CBP sequestration may be a general phenomenon of polyglutamine expansion diseases.
Another site listed CBP as CREB binding protein. That accronym stands for Calcium Response Element Binding (protein) Jim's article reffers to it as a "small regulatory molecule" I was hoping it was so small that it could cross the blood brain barrier. The bad news is the molecule has a molecular weight of more that 300,000 That's a huge molecule. It's made up of more than 300 amino acids. No way that's going to cross the blood brain barrier.
CREB-binding protein or CBP (in Drosophila, the protein termed Nejire) is a transcriptional coactivator that interacts
with a large number of developmentally important transcription factors. CBP and p300 are highly related proteins:
mammalian CBP was originally identified by its interaction with CREB (cAMP response-element-binding protein) and
p300 was originally identified as a target of the adenoviral E1A oncoprotein. CBP is recruited to DNA by several
transcription factors, including CREB (Drosophila homolog: CrebB-17A) and cFos (Drosophila homolog: Fos-related
antigen/Kayak). Transcriptional coactivators are considered to be accessory proteins that interact with transcription
factors and are required for proper transcription factor function.