A disorder called primary oxalosis or hyperoxaluria (259900) is caused by abnormal location of enzyme in the endoplasmic reticulum, causing increased production of oxalic acid with consequent accumulation in kidney (urinary stones) and joints (arthritis). A potential cure for the disorder was pioneered by altering the mutant oxalate enzyme so its normal cytosolic location was restored. This alteration most likely consisted of which of the following?
1.Changing its protein processing to produce a smaller peptide
2.Ablating its amino terminal signal recognition sequence
3.Cleaving its carboxy terminal segment
4.Changing its RNA splicing to include an extra exon
5.Proteolytic cleavage within the cytosol
Solution
By using recombinant DNA techniques, mRNAs can be produced that yield chimeric proteins. When DNA encoding an amino terminal signal sequence is inserted upstream of the α-globin gene, the usual cytosolic α-globin becomes a secretory protein and is translocated into the lumen of endoplasmic reticulum. The signal sequence thus contains all the information needed to direct the translocation of protein across endoplasmic reticulum. For diseases with abnormal targeting to or abnormal storage of proteins in the endoplasmic reticulum, cleavage of the signal sequene can alter the targeting or lessen the accumulation. To be effective as therapy, the factors that modify protein structure in vitro must be introduced into the appropriate tissues in a sustaining manner. So far, such protein or enzyme therapy has only been effective when specific targeting mechanisms exist (like targeting of enzymes to lysosomes by adding mannose-6-phosphate signal residues) and when tissues are accessible (i.e., not protected by the blood-brain barrier).