Magnetic resonance imaging of Huntington's disease: preparing for clinical trials

The known genetic mutation causing Huntington's disease (HD) makes this disease an important model to study links between gene and brain function. An autosomal dominant family history and the availability of a sensitive and specific genetic test allow pre-clinical diagnosis many years before the onset of any typical clinical signs. This review summarizes recent magnetic resonance imaging (MRI)–based findings in HD with a focus on the requirements if imaging is to be used in treatment trials. Despite its monogenetic cause, HD presents with a range of clinical manifestations, not explained by variation in the number of CAG repeats in the affected population. Neuroimaging studies have revealed a complex pattern of structural and functional changes affecting widespread cortical and subcortical regions far beyond the confines of the striatal degeneration that characterizes this disorder. Besides striatal dysfunction, functional imaging studies have reported a variable pattern of increased and decreased activation in cortical regions in both pre-clinical and clinically manifest HD-gene mutation carriers. Beyond regional brain activation changes, evidence from functional and diffusion-weighted MRI further suggests disrupted connectivity between corticocortical and corticostriatal areas. However, substantial inconsistencies with respect to structural and functional changes have been reported in a number of studies. Possible explanations include methodological factors and differences in study samples. There may also be biological explanations but these are poorly characterized and understood at present. Additional insights into this phenotypic variability derived from study of mouse models are presented to explore this phenomenon.

Stereochemical aspects of the interaction between steroidal diamines and DNA

A complete series of stereoisomeric quaternised diaminoandrostanes, differing in their stereochemistry at the 3,5 and 17 positions, has been examined for effects on the thermal denaturation of calf thymus DNA and for the ability to remove and reverse the supercoiling of closed circular duplex PM2 DNA. In both types of test the eight isomers rank in the same order of effectiveness. The preferred stereochemistry for the quaternary substituents at positions 3 and 17 is beta; of these the orientation of the 17- substituent is more critical. Folding of the steroid skeleton between the A and B rings, as in 5beta-androstanes, diminishes effectiveness but does not necessarily abolish the effect on supercoiling. The over-riding importance of the C-D ring end of the steroid nucleus bearing a 17beta-amino substituent is confirmed by a comparison of the effects of five mon-amino androstanes. Relative helix=unwinding angles per bound steroidal diamine molecule have been determined for four of the isomers; for three 17beta compounds the estimated values are similar to that previously reported for irehdiamine A. For a fourth isomer the angle is 0.22 times that of ethidium, the lowest yet determined for any DNA-binding drug. The results lend further support to the argument that intercalation can be reled out, and alternative models for the binding mechanism are discussed.