Finished Projects on the Improvement of DNA-Microarray Technology

For the production of DNA-microarrays from PCR-products, the purification of the DNA-fragments prior to spotting is a major expense in cost and time. Also, a considerable amount of material is lost during this process and contamination might occur.

We developed a protocol that permits the manufacturing of microarrays from unpurified PCR-products. The presence of primer molecules in the PCR-sample does not increase unspecific signal upon hybridisation. Overall, signal intensity on arrays made of unpurified PCR-products is 94% of the intensity obtained with the respective purified molecules. This slight loss in signal, however, is offset by a reduced variation in the amount of DNA present at the individual spot positions across an array, apart from the considerable savings in time and cost. In addition, a larger number of arrays that can be made from one batch of amplification products.

By using betaine as an additive to the spotting solution, both the binding efficiency of spotted PCR-products and the homogeneity of the DNA-spots is improved significantly on aminated surfaces such as glass slides coated poly-L-lysine or aminosilane. In addition, unspecific background signal is markedly diminished. Concomitantly, the betaine reduces evaporation from the microtitre dish wells during the arraying procedure.

Subsequent blocking of the chip surface with succinic anhydride was improved in presence of the unpolar, non-aqueous solvent 1,2-dichloroethane and the acylating catalyst N-methylimidazole. This procedure prevents overall background signal that occurs with the frequently applied aqueous solvent 1-methyl-2-pyrrolidone in borate buffer because of DNA that re-dissolves from spots during the blocking process, only to bind again across the entire glass surface.

Figure. The amount of DNA present before (left bar) and after purification (right bar) of 24 randomly chosen PCR products. On average, 49% of the material was lost during purification. For more details see NAR 30, e79. [PDF]

For DNA-chip analyses, oligonucleotide quality has immense consequences to accuracy, sensitivity and dynamic range. Quality of chips produced by photolithographic in situ synthesis depends critically on the efficiency of photo-deprotection. By means of base-assisted enhancement of this process using 5'-[2-(2-nitrophenyl)-propyloxycarbonyl]-2'-deoxynucleoside phosphoramidites, synthesis yields improved by at least 12% per condensation compared to current chemistries. Thus, the eventual total yield of full-length oligonucleotide is increased more than tenfold in case of 20-mers. Furthermore, the quality of every individual array position was checked quantitatively after synthesis. Subsequently, the very, quality-tested chips were used in successive hybridisation experiments.

Photolabile 3'-O-[2-(2-nitrophenyl)propoxycarbonyl]-protected 5'-phosphoramidites were synthesised for an alternative mode of light-directed production of oligonucleotide arrays. Because of the characteristics of these monomeric building blocks, photolithographic in situ DNA-synthesis occurs in 5'-3' direction, conform to the orientation of enzymatic synthesis. The production of such oligonucleotide chips adds new procedural avenues to the growing number of applications of DNA-microarrays.