Title: Design and Synthesis of Novel Cleavable Fluorescent Nucleotide Reversible Terminators Using Disulfide Linkers for DNA Sequencing by Synthesis
Description: High-throughput DNA sequencing technology has advanced rapidly in the past few decades and is the driving force for personalized precision medicine. In this Thesis, a set of novel disulfide linker-based nucleotide reversible terminators (NRTs) has been designed and synthesized for application in DNA sequencing by synthesis (SBS), which is the dominant sequencing platform. The design and synthesis principles are outlined as follows. Four nucleotides (A, C, G, T) are modified as NRTs for the DNA extension reaction catalyzed by polymerase by attaching a cleavable fluorophore to a specific location on the base and blocking the 3′-OH group with a small chemically-reversible moiety so that the resulting molecules are still recognized by DNA polymerase as substrates. In these fluorescent NRTs, the fluorophores are attached through a disulfide (-SS-) cleavable linker to the 5-position of cytosine and thymine, and to the 7-position of deaza-adenine and deaza-guanine, and a small disulfide moiety is used to cap the 3'-OH group of the deoxyribose.

The resulting fluorescent NRTs (3′-O-tert-butyldithiomethyl-dNTP-SS-fluorophores) are shown to be good substrates in DNA polymerase catalyzed reactions. The fluorophore and the 3′-O-tert-butyldithiomethyl group on a DNA extension product, which is generated by incorporating the 3′-O-tert-butyldithiomethyl-dNTP-SS-fluorophore in a polymerase reaction, are removed simultaneously and rapidly by treatment with a reducing agent, tris (3-hydroxypropyl) phosphine, in aqueous buffer solution. This one-step dual-cleavage reaction thus allows the reinitiation of the polymerase reaction and increases the SBS efficiency. DNA templates consisting of homopolymer regions were accurately sequenced by using this class of fluorescent nucleotide analogues on a DNA chip and a four-color fluorescent scanner. Compared with existing fluorescent NRTs, the unique disulfide linkers used to synthesize the NRTs described in this thesis are cleaved efficiently under DNA compatible conditions, leading to shorter scars on the DNA extension strand to further improvement of the DNA SBS technology.