DNA
For many years, DNA extracted from the cell line K562 was supplied as a control sample from Promega Corporation with their GenePrint STR typing kits. Back in the RFLP days of DNA testing (see Chapter 3), K562 served as a de facto control material in forensic laboratories. However, as can be seen from the peak profiles below, some of the STR loci exhibit imbalanced heterozygous alleles and/or multiple peaks that would make the sample appear to come from more than one source of DNA. In this particular case, these extra peaks or peak imbalances are the result of an abnormal number of chromosomes present in the sample rather than a problem with the DNA typing system. K562 cells are derived from a female human subject with a diagnosis of chronic myelogenous leukemia. Because mutant cells are present with chromosomes that possess somatic mutations that affect the number of repeats in various STR markers, the K562 cell line results do not possess the normal balance of chromosomal material seen in healthy individuals. Thus, balanced heterozygous allele peaks are not always seen. Shown below are six STR markers amplified from K562 genomic DNA that possess a significant variation in the balance of the STR allele peak heights.
DNA