Also known as Somatotropin, this hormone is secreted by the anterior part of the pituitary and is made up of 190 amino acid residues. The deficiency of this hormones results in dwarfism. For the conventional treatment of dwarfism
HGH is injected into the patients. But the extraction of hormone from the pituitary glands cannot meet the ever increasing demand for this hormone. Genetic engineering procedures have successfully produced somatotrophin.
Double stranded cDNAs were obtained from the mRNA precursors of HGH and then incorporated into bacterial cells where they expressed in non precursor form. The bacteria were unable to convert the peptides of HGH into biological active forms.
Image Source: pnas.org
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A recombinant plasmid containing a full length of HGH cDNA is cleaved with restriction enzymes that release fragments containing the complete HGH coding sequence after codon 24.
Several overlapping complimentary oligo nucleotides were joined to form a synthetic strand of a small DNA fragment which contains the coding sequence for the first 24 aminoacids of mature HGH.
The synthetic DNA and cDNA molecules are joined together to produce a new fragment which has the complete coding sequence of HGH.
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This is then ligated to a restriction site a little below the lac promoter operator gene cloned on a plasmid. These plasmids are cloned in E.coli. Synthesis of HGH is induced by an inducer (IPTG) of lac operon. The HGH thus obtained is later purified by chemical methods.
About 1 lakh molecules of the hormone per cell of E.coli have been produced by genetic engineering techniques.
One of the problems in this is the attachment of an extra methionine to the molecule. This has to be removed to make the molecule biologically active.