Assessing genetic stability at the nucleic acid level

Due to the natural genetic instability of any living organisms as well as faulty amino acid incorporation by the translational machinery or by post-processing alteration, a recombinant protein is prone to contamination at various levels by mutated or altered molecules. However, it should be borne in mind that alteration at the nucleic acid level will be amplified and may affect a substantial percentage of the whole population, while alterations introduced at the protein level are not further amplified and will at the most constitute a harmless background. While it is the protein and not the gene that is used as a medicine, it is important to recognize that analytical results obtained at the protein level are better interpreted when they are backed by good knowledge of the genetic system. Direct sequencing of a gene borne by a plasmid or of a reverse transcriptase-polymerase chain reaction (RT PCR) product can provide the sequence of the majority of the molecules. However, it fails to detect mutated molecules corresponding to a low level of contaminant. Therefore, a statistical treatment of the polymorphism of the population or the use of alternative methods such as those based on single-strand conformational polymorphism (SSCP) or chemical cleavage modification (CCM) are recommended.