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ScienceWeek
APPLIED CHEMISTRY: MINIATURE ANALYTICS FOR MEDICAL DIAGNOSTICS
The following points are made by David R. Walt (Science 2005 308:217):
1) A current growing trend has brought medical diagnostic devices closer to the patient. Point-of-care devices for measuring electrolytes, cardiac markers, and several small molecules now reside in nursing stations, surgical suites, emergency rooms, and even at patient bedsides. Police carry breathalyzers for alcohol screening, while diabetics conduct routine glucose tests using credit card- or pen-sized devices.
2) In addition to ions and small molecules, proteins, genes (DNA), and gene transcripts (mRNA) can now be measured routinely. In some cases, specific analytes or DNA sequences are diagnostic for a particular disease state, while in other cases, an overall profile correlates to a disease [for example, elevated LDL (low-density lipoprotein), cholesterol, and C-reactive protein are characteristic of cardiovascular disease (CVD)].
3) The discovery of clinically important analytes often involves large, sophisticated, and expensive analytical instrumentation such as mass spectrometers. The challenge then is to develop smaller, more focused instruments to monitor these analytes. Advances in surface and materials chemistry, engineering, and the availability of new electronic components, such as light sources, memory chips, detectors, and integrated electronic components, have all contributed to advances in sensors. A focus has been on diabetes monitoring. One innovation is the GlucoWatch [1], an electrochemical glucose biosensor incorporated into a wristwatch-like device. When the patient wears the watch, electrical stimulation causes fluid to pass through the skin in a process called iontophoresis. A glucose sensor on the back of the watch analyzes the fluid, thereby providing a relatively continuous readout of glucose concentration that is reasonably accurate. A second new glucose biosensor [Therasense, Abbott [2]], requires a submicroliter blood sample that can be acquired virtually painlessly from many areas of the body, such as the forearm.
4) For acquiring parallel measurements of many analytes, arrays are unequaled [3-5]. Most arrays contain hundreds to tens of thousands of micrometer-sized features. Two strategies exist for performing array analysis -- measuring multiple specific analytes simultaneously, or measuring patterns or profiles. Most arrays fall in the first category, with signals generated upon analyte binding to specific molecular receptors attached to each array element. Signals are typically either electronic, such as a change in current, or optical, such as a change in fluorescence. For example, DNA microarrays are now commercially available that contain all the known genes in the human genome. Although these arrays are primarily confined to research laboratories, the decreasing feature sizes of the array elements and the simplification in the readout instrumentation should enable the technology to become part of the clinical setting.
References (abridged):
1. www.glucowatch.com/
2. www.therasense.com/
3. L. Bodrossy, A. Sessitsch, Curr. Opin. Microbiol. 7, 245 (2004)
4. C. J. Campbell, P. Ghazal, J. Appl. Microbiol. 96, 18 (2004)
5. K. K. Mantripragada, P. G. Buckley, T. Diaz de Ståhl, J. P. Dumanski, Trends Genet. 20, 87 (2004)
Science http://www.sciencemag.org
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MEDICAL BIOLOGY: ON CYTOGENETICS
The following points are made by Thomas Ried (New Engl. J. Med. 2004 350:1597):
1) Cytogenetics is the genetic analysis of cells, a discipline that has flourished since the chromosome-banding techniques introduced in 1969 by Torbjoern Caspersson and Lore Zech first provided a simple and inexpensive way to gauge the number and assess the structural integrity of chromosomes. Chromosome banding is probably the most commonly performed genetic test: it is used about 500,000 times each year in North America (most laboratories use G-banding, named after the German chemist Gustav Giemsa). It is used prenatally and postnatally to screen for the presence of constitutional chromosomal aberrations associated with birth defects, as well as for the diagnosis and differential diagnosis of cancer -- hematologic cancers in particular.
2) The application of cytogenetics to the screening of the genomes of patients has led to the linking of specific chromosomal abnormalities (and consequently genetic alterations) with specific diseases; such findings carry enormous implications for medicine. The first such specific correlation was made when Jerome Lejeune, in Paris, detected extra copies of a small chromosome in patients with Down's syndrome. Other descriptions of constitutional abnormalities, involving both autosomes and the sex chromosomes, followed in rapid sequence.
3) Such associations were also established in cancer cells. The first report on the so-called Philadelphia chromosome (described by Peter Nowell and David Hungerford in 1960 and characterized by Janet Rowley in 1973), created by an exchange of genetic material between chromosomes 9 and 22, launched the highly prolific field of cancer cytogenetics. (The Philadelphia chromosome reflects the specific target of imatinib mesylate in the treatment of chronic myeloid leukemia.) This work not only confirmed the relevance of cytogenetic abnormalities as disease-initiating events, but also led to the molecular cloning of numerous cancer-associated genes, many of which sit at the junctions of chromosomal translocations.
4) Cytogeneticists then adapted and combined molecular cloning and hybridization and so arrived at fluorescence in situ hybridization, which has had profound effects on cytogenetic diagnostic testing and research. Specific fluorescent-tagged DNA probes could now be used to visualize, with dramatically increased resolution, small deletions and other genetic aberrations, as well as to map the chromosomal locations of genes. Complex clone libraries were developed for the "painting" of entire chromosomes or chromosome arms, a technique that is useful not only for the confirmation of suspected aberrations in clinical diagnosis, but also for basic research efforts aimed at elucidating the evolution and structure of genomes, relationships between structure and function, mechanisms of DNA repair, and chromosome segregation.
5) Conceptual and technical developments in molecular cytogenetics now permit the hybridization-based screening of genomes for both numerical aberrations (aneuploidy and genomic imbalances) and structural aberrations. The first such screening technique, called comparative genomic hybridization, changed our perception of genomic instability in cancer genomes. This technique compares the DNA content of cancer cells with that of normal "reference" genomes and thus reveals a blueprint of genomic imbalances. It does so through the differential labeling of the cancer genome (e.g., with the use of a green fluorochrome) and a reference genome (with the use of a red fluorochrome).
New Engl. J. Med. http://www.nejm.org
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BIOTECHNOLOGY: DNA MICROARRAYS IN MEDICINE
The following points are made by M. Diehn et al (J. Am. Med. Assoc. 2000 283:2298):
1) All of the genes in a genome can be arrayed in an area no larger than a standard microscope slide. At present, the largest DNA microarrays contain elements representing almost 40,000 genes, roughly half the predicted number of genes in the human genome. In the near future, when the complete human genome is determined, DNA microarrays will allow a complete profile of all expressed genes in a particular cell type.
2) The important consideration is that gene expression patterns reflect the internal state and microenvironment of a cell, creating a molecular picture of the state of the cell. DNA microarrays can be used to capture these molecular pictures and thus to deduce the condition of cells. Also, systematic microarray studies of global gene expression can provide detailed clues to the functions of specific genes. This is an important advance, since we currently know the functions of fewer than 5 percent of the genes in the human genome.
3) DNA microarrays have already had various applications: messenger RNA expression profiling for improved disease classification; genotyping of *polymorphisms affecting disease susceptibility; identification of genetic lesions within malignancies; design and discovery of new therapeutic drugs; sequencing of DNA. In general, the ability to use a DNA sequence directly as a reagent for detecting and assaying copies of that sequence in a biological sample provides a valuable route to assays for the protein products of every gene.
4) Most disease processes are accompanied not only by characteristic macroscopic or histological changes, but also by systematic changes in gene expression patterns. For some pathological processes such as cancer, inappropriate gene expression is a fundamental aspect of pathogenesis. For other pathological processes, the gene expression programs both in cells directly affect by a disease and in healthy cells responding to the local and systemic effects of a disease can provide a detailed molecular picture of the pathogenic process.
5) The authors suggest that the detailed molecular pictures provided by genomic expression analysis will revolutionize molecular medicine just as high-resolution radiographic imaging methods have revolutionized diagnosis and treatment at the gross anatomic level.
J. Am. Med. Assoc. http://www.jama.com
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Notes by ScienceWeek:
polymorphisms: A genetic polymorphism is a naturally occurring variation in the normal nucleotide sequence of the genome within individuals in a population. Variations are denoted as polymorphisms only if they cannot be accounted for by recurrent mutation and occur with a frequency of at least about 1 percent.
ScienceWeek http://scienceweek.com
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