29 May 2009

Efforts to design tauopathy drugs that target its RNA may get some wind from a study in today’s issue of Chemistry & Biology. Structural analyses by Gabriele Varani of the University of Washington, Seattle, and colleagues elsewhere offer insight into how a cancer drug stabilizes a tau pre-messenger RNA (mRNA) structure to curb production of a tau splice form that leads to dementia when made in excess. The structural data provides a basis for tweaking this and other RNA-binding molecules for future therapeutic application.

The normal human brain has about equal amounts of the 3R and 4R tau isoforms, which contain three or four microtubule-binding repeats, respectively. Churning out too much of the 4R variant appears enough to send a person toward dementia. The number of repeats that get incorporated into the protein depends on alternative splicing of exon 10—inclusion of the exon produces the 4R isoform (Goode and Feinstein, 1994; Gustke et al., 1994). Curiously, several tau mutations that cause familial frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) cluster within a tau pre-mRNA regulatory element that partially masks an exon 10 splice site. The FTDP-17 mutations destabilize the stem-loop structure of the regulatory element (Hutton et al., 1998; Varani et al., 1999), presumably leading to increased splicing of exon 10 and, in turn, overproduction of the 4R isoform relative to the 3R isoform. Conversely, a team led by Michael Wolfe of Brigham and Women’s Hospital, Boston, who are co-authors in the new work, has shown that mutations that stabilize the splicing regulatory element in vitro create the opposite scenario—they decreased exon 10 splicing, causing a relative drop in 4R isoform levels (Donahue et al., 2006 and ARF related conference story).

These data fueled a search for compounds that bind to and stabilize this pivotal regulatory structure in tau pre-mRNA. Christine Donahue and other Wolfe lab members developed a high-throughput fluorescent binding assay and used it to screen a ~110,000-molecule library for promising candidates (Donahue et al., 2007). In the present paper, Donahue, first author Suxin Zheng of Varani’s group, and colleagues describe key structural features of the interaction between the tau pre-mRNA regulatory element and one such compound—the cancer drug mitoxantrone. This compound is used to treat breast cancer, acute leukemia, non-Hodgkin’s lymphoma, and some forms of multiple sclerosis, but not tauopathies at present.

From nuclear magnetic resonance and other structural biology studies, the researchers determined that mitoxantrone probably binds the tau RNA stem-loop structure in two places. The stronger interaction occurs near the bulge region and likely confers stability to the regulatory structure; the other happens in a minor groove and does not induce conformation changes in the RNA. They also characterized various side chain interactions that could be optimized in future drug development—for example, made more rigid to increase specificity for the tau splicing regulatory element.—Esther Landhuis