Insulin genes

Insulin is one of the important pharmaceutical products produced commercially by genetically engineered bactera. Before this development, commercial insulin was isolated from animal pancreatic tissue. Microbial insulin has been available since 1982. The human insulin gene is introduced into a bacterium like E. coli. Two of the major advantages of insulin production by microorganisms are that the resultant insulin is chemically identical to human insulin, and it can be produced in unlimited quantities. 

Fig. 24.4 Splicing of a messenger RNA molecule transcribed from a hypothetical insulin gene containing two introns.

Once a gene is cloned it is necessary to convert the information contained in it into a functional protein. There are a number of steps in gene expression (i) transcription of DNA into mRNA (ii) translation of the mRNA into a protein sequence and (iii) in some instances, post-translational modification of the protein. In discussing these steps in more detail, expression of a cloned insulin gene will be used as an example. 

Fig. 24.5 Insertion of a cloned insulin gene into a vector carrying a bacterial promoter. The arrow indicates the direction of transcription. If we suppose the bacterial promoter is derived from the lactose operon then transcription will be initiated only in the presence of lactose.

Spielman RS, McGinnis RE, Ewens WJ. Transmission test for linkage disequilibrium the insulin gene region and insulin-dependent diabetes mellitus (IDDM). Am J Hum Genet 1993 52[3] 506-516. 

Type 1 diabetes affects approximately 1 in 200 Americans, with a sibling recurrence risk of about 6%. It is an example of an autoimmune disease (see Immunology Lecture Notes), in which self-reactive T cells infiltrate the pancreas to destroy insulin-produdng islet cells. Mutations in the class II major histocompatibility locus (MHC) region are estimated to contribute about one third of the risk of developing type 1 diabetes. Mutations in or near the insulin gene itself are responsible for another 10-15% of the risk. 

Kaneda, Y., Iwai, K. and Uchida, T. (1989) Introduction and expression of the human insulin gene in adult rat hver J. Biol. Chem., 264,12126-12129. 

Chen, R., Meseck, M.L. and Woo, S.L.C. (2001) Auto-regulated hepatic insulin gene expres-sion in type 1 diabetic rats. Mol. Ther., 3, 584-590. 

Efrat, S., Surana, M. and Fleischer, N. (1991) Glucose induces insulin gene transcription in a murine pancreatic /3-cell line.J. Biol. Chem., 266, 11141-11143. 

German, M.S., Moss, L.G. and Rutter, W.J. (1990) Regulation of insulin gene expression by glucose and calcium in transfected primary islet cultures. J. Biol. Chem., 265, 22063-22066. 

Lu, D., Tamemoto, El., Shibata, El., Saito, I. and Takeuchi, T. (1998) Regulatable production of insulin from primary-cultured hepatocytes Insulin production is up-regulated by glucagon and cAMP and down-regulated by insulin. Gene Ther., 5, 888-895. 

Redmon, J.B., Towle, H.C. and Robertson, R.P. (1994) Regulation of human insulin gene transcription by glucose, epinephrine, and somatostatin. Diabetes, 43, 546-551. 

Sharma, A. and Stein, R. (1994) Glucose-induced transcription of the insulin gene is mediated by factors required for P -cell-type specific expression. Mol. Cell. Biol., 14, 871-879. 

Furthermore, the neo-Darwinian hypothesis says that the insulin gene had been duplicated long ago and that the left-over copy has mutated into a relaxin.6 Once the astronomical number of mutations had led to an active relaxin any further mutation that would hit the invariant positions would have killed or severely handicapped the owner of that hormone, and therefore all constant residues are important. My colleague Dr. Erika Biillesbach has developed an ingenious as well as practical way to synthesize relaxin and insulin for our NIH-funded research.3 These derivatives allowed us to experimentally test some of the postulated mechanisms in the Darwinian model of molecular evolution. 

I., Seijffers, R., Kopolovic, J., Kaiser, N. and Karasik, A. (2000). Pancreatic and duodenal homeobox gene 1 induces expression of insulin genes in liver and ameliorates streptozotocin-induced hyperglycemia. Nat. Med. 6, 568-572. 

Thule, P. M. and Liu, J. M. (2000). Regulated hepatic insulin gene therapy of STZ-diabetic rats. Gene Ther. 7, 1744—1752. 

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