A handy definition of "MicroArray", from:

From: RogerEkins <r.ekins@u...> Date: Fri May 12, 2000 3:29 am Subject: MicroArray definition

(This note is being resent, the original (transmitted yesterday) having apparently not been received. I have very slightly expanded the penultimate paragraph, to make clearer the key feature that I believe distinguishes and defines a "microarray".)

Further to my colleague FredChu's posting on the 9th, another paper that gives a broad overview of the development of the ligand assay field, and the PhysicoChemical concepts that gave rise to MicroArray Technology, is the "special" presentation I gave (as inaugural recipient of the AACC Edwin F Ullman Award) at the 1998 OakRidgeConference (see Ekins R.P. (1998) Ligand assays: from electrophoresis to miniaturized microarrays. Clin Chem. 44. 201, 2015-2030.)

As indicated in the article cited by FredChu, I and my colleagues have described the concepts underlying the technology at international meetings (and publications) in Europe and the US since 1988. The first in the US was an invited presentation at the 23rd Oak Ridge Conference at St Louis in April 1991 (see Ekins RP and Chu FW (1991) MultianalyteMicrospotImmunoassay - micro analytical "compact disk" of the future. Clin Chem 37, 1955-1967), followed shortly by invited presentations at a meeting of the CanadianChemicalSociety, at a GordonConference in California, at a meeting at Beckman and subsequently by other invited presentations at (e.g.) meetings of the AmericanChemicalSociety. In all such presentations, our use of a LaserScanningConfocalMicroscope was described "permitting 'microspots' of 100 sq 탆 or less to be analysed , implying in principle that arrays comprising 1,000,000 MicroSpot s could be accommodated on an area of 1 sq cm" (see abstract for 1991 Oak Ridge Conf, submitted 4.Oct 1990).

These presentations related primarily to the ImmunoAssay field - essentially because this was the area of specific interest to meeting organisers, but it was obvious to us from the outset that the technology was equally applicable to DNA/RNA analysis, and meeting participants often made this point following the presentations. Note that the patents referred to by FredChu cover all microarray-based LigandAssay's. Not only these patents, but certain presentations at international meetings describing the technology and its basic mathematical theory preceded the priority date of EdSouthern's patents in this field.

Having demonstrated the technology's feasibility, we commenced collaborative work on its industrial development with BoehringerMannheimGmbH (which licensed the patents) in 1992 (Note: following Roche's takeover of BM in 1998, Roche currently holds the license.)

MicroArray technology is clearly applicable both to DNA/RNA analysis and to ImmunoDiagnostic's - a thought that seems to have only recently occurred to MolecularBiologists. A key difference is that ImmunoDiagnostic applications demand very high sensitivity. This was the focal point of our own studies in the mid-late 80's, and guided by our theoretical analysis of microarray design, we achieved sensitivities permitting detection of 1 analyte molecule/15 sq 탆 of MicroSpot surface area. With purpose-designed confocal scanning equipment, BM researchers claimed (at the 1996 OakRidgeConference) the ability to detect 1 molecule/100 sq 탆. (This parameter is a key determinant of the sensitivity of the MicroArray to analyte concentrations in the test fluid.) Such sensitivity is orders of magnitude higher than anything so far reported in the US and elsewhere and - in the case of ImmunoAssay - higher than achieved using any other assay format.

With specific reference to what constitutes a "microarray", this was an issue that arose in our minds 15 years ago when applying for grants to develop the technology. It is evident that arrays of various dimensions have been used in an analytical context for many years. It was therefore necessary to identify a specific and novel feature that distinguishes a microarray from other arrays. Our definition was/is based on the finding (which many continue to find surprising) that, as the amount of a binder located in the form of a spot on a surface is reduced, one reaches a situation where the "fractional occupancy" (i.e. the fraction of binding sites occupied by target molecules) becomes independent a. of sample volume, and b. of the amount of binder within the spot.

In this circumstance, fractional occupancy of binder sites in the microspot is dependant only on the concentration of target molecules in the test solution to which the spot is exposed. (Note: a distinguished reviewer of our work - a molecular biologist and NobelLaureate who had specifically considered this issue - was sceptical of this proposition, and demanded experimental proof. This we readily provided.)

A corollary of these ideas is that only a small proportion (generally <10%; ideally < 1%) of the target substance present in the test fluid is bound to the spot. This implies in turn that - in principle at least - one can measure a target polynucleotide concentration with as few as 100 or even 10 'capture" OligoMolecule's located within the spot, (though one gets into statistical problems when the number of binder molecules - and hence the microspots - are made too small). Nevertheless it turns out (also counter-intuitively) that microarrays comprising spots operating under these "ambient analyte assay" conditions can yield greater sensitivity, and faster results, than any other ligand assay design. It was these new insights that led (in 1985) to the concept that one could measure thousands of different molecular species within a few 탅s of a test fluid, using a "microarray" comprising very large numbers of individually-targeted "microspots".

Accept my apologies for the length of this overview, but it seems that most people working in this area are unaware of its genesis, of some of the science, and of the patent situation.

RogerEkins MolecularEndocrinology UniversityCollegeLondonMedicalSchool London UK

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