Methods for DNA testing are abundant, however, there is still a need for lower cost, higher-throughput genotyping. Much emphasis has been placed on short tandem repeat polymorphisms (STRPs) and single nucleotide polymorphisms (SNPs). Another important polymorphism is the diallelic short insertion/deletion (indel). Our laboratory is using a continuous reel of 384 well arrays on polypropylene tape to genotype indel polymorphisms. The reel of arrays allows low cost automation, and an opportunity to decrease reaction volumes. The diallelic indel is typed by tagging allele-specific PCR primers with a FAM or JOE molecular beacon uniprimer. Our most recent array pattern contains wells that hold a maximum of 1.1 microliters with reaction volumes of 800 nanoliters. Because micro array tape (microtape) is unique, commercial equipment is not yet available. A series of instruments were developed in-house to handle the tape. A pipetting instrument was developed to deliver the DNA samples or other reagents. A solenoid micro-valve aspirating and jetting unit was developed for dispensing a common reaction mix. The arrays are sealed with commercially available microtiter plate seal material prior to polymerase chain reaction (PCR) within a custom built waterbath thermal cycler. The arrays are scanned using an epi-fluorescence detection unit designed to read FAM and JOE fluorescent dyes. The detector uses an argon ion laser for excitation and two photomultiplier tubes (PMTs) for detection. The resulting images are processed using custom software.
A new scanning fluorescence detector (SCAFUD) was developed for high-throughput genotyping of short tandem repeat polymorphisms (STRPs). Fluorescent dyes are incorporated into relatively short DNA fragments via polymerase chain reaction (PCR) and are separated by electrophoresis in short, wide polyacrylamide gels (144 lanes with well to read distances of 14 cm). Excitation light from an argon laser with primary lines at 488 and 514 nm is introduced into the gel through a fiber optic cable, dichroic mirror, and 40X microscope objective. Emitted fluorescent light is collected confocally through a second fiber. The confocal head is translated across the bottom of the gel at 0.5 Hz. The detection unit utilizes dichroic mirrors and band pass filters to direct light with 10 - 20 nm bandwidths to four photomultiplier tubes (PMTs). PMT signals are independently amplified with variable gain and then sampled at a rate of 2500 points per scan using a computer based A/D board. LabView software (National Instruments) is used for instrument operation. Currently, three fluorescent dyes (Fam, Hex and Rox) are simultaneously detected with peak detection wavelengths of 543, 567, and 613 nm, respectively. The detection limit for fluorescein-labeled primers is about 100 attomoles. Planned SCAFUD upgrades include rearrangement of laser head geometry, use of additional excitation lasers for simultaneous detection of more dyes, and the use of detector arrays instead of individual PMTs. Extensive software has been written for automatic analysis of SCAFUD images. The software enables background subtraction, band identification, multiple- dye signal resolution, lane finding, band sizing and allele calling. Whole genome screens are currently underway to search for loci influencing such complex diseases as diabetes, asthma, and hypertension. Seven production SCAFUDs are currently in operation. Genotyping output for the coming year is projected to be about one million total genotypes (DNA samples X polymorphic markers) at a total cost of
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