Harpers

MOLECULAR GENETICS, RECOMBINANT DNA, & GENOMIC TECHNOLOGY / 407Table 40–5. Localization of human genes. 1Gene Chromosome DiseaseInsulin11p15Prolactin6p23-q12Growth hormone 17q21-qter Growth hormone deficiencyα-Globin 16p12-pter α-Thalassemiaβ-Globin 11p12 β-Thalassemia, sickle cellAdenosine deaminase 20q13-qter Adenosine deaminase deficiencyPhenylalanine hydroxylase 12q24 PhenylketonuriaHypoxanthine-guanine Xq26-q27 Lesch-Nyhan syndromephosphoribosyltransferaseDNA segment G8 4p Huntington’s chorea1 This table indicates the chromosomal location of several genes and the diseases associatedwith deficient or abnormal production of the gene products. The chromosomeinvolved is indicated by the first number or letter. The other numbers and letters referto precise localizations, as defined in McKusick VA: Mendelian Inheritance in Man, 6thed. John Hopkins Univ Press, 1983.human genome. Once the defect is localized to a regionof DNA that has the characteristic structure of a gene(Figure 40–1), a synthetic gene can be constructed andexpressed in an appropriate vector and its function canbe assessed—or the putative peptide, deduced from theopen reading frame in the coding region, can be synthesized.Antibodies directed against this peptide can beused to assess whether this peptide is expressed in normalpersons and whether it is absent in those with thegenetic syndrome.Proteins Can Be Producedfor Research & DiagnosisA practical goal of recombinant DNA research is theproduction of materials for biomedical applications.This technology has two distinct merits: (1) It can supplylarge amounts of material that could not be obtainedby conventional purification methods (eg, interferon,tissue plasminogen activating factor). (2) It canprovide human material (eg, insulin, growth hormone).The advantages in both cases are obvious. Although theprimary aim is to supply products—generally proteins—fortreatment (insulin) and diagnosis (AIDStesting) of human and other animal diseases and fordisease prevention (hepatitis B vaccine), there are otherpotential commercial applications, especially in agriculture.An example of the latter is the attempt to engineerplants that are more resistant to drought or temperatureextremes, more efficient at fixing nitrogen, or that produceseeds containing the complete complement of essentialamino acids (rice, wheat, corn, etc).Recombinant DNA Technology Is Usedin the Molecular Analysis of DiseaseA. NORMAL GENE VARIATIONSThere is a normal variation of DNA sequence just as istrue of more obvious aspects of human structure. Variationsof DNA sequence, polymorphisms, occur approximatelyonce in every 500 nucleotides, or about10 7 times per genome. There are without doubt deletionsand insertions of DNA as well as single-base substitutions.In healthy people, these alterations obviouslyoccur in noncoding regions of DNA or at sites thatcause no change in function of the encoded protein.This heritable polymorphism of DNA structure can beassociated with certain diseases within a large kindredand can be used to search for the specific gene involved,as is illustrated below. It can also be used in a variety ofapplications in forensic medicine.B. GENE VARIATIONS CAUSING DISEASEClassic genetics taught that most genetic diseases weredue to point mutations which resulted in an impairedprotein. This may still be true, but if on reading theinitial sections of this chapter one predicted that geneticdisease could result from derangement of any ofthe steps illustrated in Figure 40–1, one would havemade a proper assessment. This point is nicely illustratedby examination of the β-globin gene. This geneis located in a cluster on chromosome 11 (Figure40–8), and an expanded version of the gene is illustratedin Figure 40–9. Defective production of β-globinresults in a variety of diseases and is due to many