2020同步年報

042 ACTIVITY REPORT 2020 Small Molecule Targeting T:T Mismatches in CTG Trinucleotide Repeat DNA Induces a Unique Structure The current study describes new structural features of higher-order non-canonical DNA confor- mations induced by a small molecule ligand and its selective supramolecular interactions. T he abnormal expansion of repetitive DNA in a ge- nome results in dysfunctional cellular processes such as replication, repair and recombination that ultimately lead to altered genetic processes. DNA repeat expansion is associated mainly with neurological diseases that might have severe consequences for human health. For example, myotonic dystrophy type 1 (DM1) is associated with the abnormal expansion of cytosine-thymine-guanine (CTG) trinucleotide repeats (TNRs) DNA located on the non- coding region of the myotonic dystrophy protein kinase gene. 1 Expansion of the aberrant TNRs is typically correlated with genetic inheritability, age of disease onset, and sever- ity. Studies have shown that the formation of atypical DNA structures containing mismatches flanked between canon- ical Watson−Crick base-pairs in the genome is common in disease pathobiology. 2 As the formation of non-canonical structures has been identified in many neurodegenerative diseases, small mole- cules targeting these structures can play a significant role in the diagnosis and treatment of these diseases. Triaminotriazine- acridine conjugate (Z1), a drug originally developed by Steven C. Zimmerman (University of Illinois, USA) and his team, that targets mismatches in trinucleotide repeat DNA has the potential to cure DM1 disease ( Fig. 1(a) ), 3 but its binding mechanism remains unclear. A collaborative team led by Ming-Hon Hou (National Chung Hsing University) and Zimmerman used X-ray crystallography and biophysical methods to understand the structural basis for recognition of disease-associated DNA by Z1 ( Figs. 1(b) and 1(c) ). The extraordinarily complex structure solved by Hou and his team showed many unprecedented features that not only throw light on structural details about drug-DNA complex- es but also enhance our understanding of the supramolec- ular chemistry of a chemical compound that causes non- canonical DNA superstructure formation. 4 The elucidation of a complex crystal structure required the collection of high-resolution X-ray data that was conducted using TPS 05A and TLS 15A1 at NSRRC. The team solved a complex structure of drug Z1 and CTG repeat-associated DNA containing three homopyrimidine T:T mismatches with the SAD method using a brominated oligonucleotide [br5U]TCTGCTGCTGAA. Their crystallographic observations Fig. 1 : (a) Chemical structure of triaminotriazine-acridine conjugate (ligand Z1). (b,c) Overall structure and schematic representation of d[(5BrU)T(CTG) 3 AA] complexed to Z1 in an asymmetric unit. (d) Core of the complex formed by a crossing of all four chains in the G5-C6-T7-G8-C9 penta-sequence. (e) Two central stacked G:C pairings form two GC tetrads labeled GC-I and GC-II. The additional stacked G:C pairings formed by adjacent G8pC9 bases of chains A and C are denoted GC-III and GC-IV, respectively. [Reproduced from Ref. 4]