Microsatellite Stability in STR Analysis Depends on Number of Repeat Units.

More than a decade ago a short tandem repeat-based typing method was developed for the fungus . This STR assay is based on the analysis of nine short tandem repeat markers. Interpretation of this STR assay is complicated when there are only one or two differences in tandem repeat markers between isolates, as the stability of these markers is unknown. To determine the stability of these nine markers, a STR assay was performed on 73-100 successive generations of five clonally expanded isolates. In a total of 473 generations we found five times an increase of one tandem repeat unit. Three changes were found in the trinucleotide repeat marker STR 3A, while the other two were found in the trinucleotide repeat marker STR 3C. The di- or tetranucleotide repeats were not altered. The altered STR markers 3A and 3C demonstrated the highest number of repeat units (≥50) as compared to the other markers (≤26). Altogether, we demonstrated that 7 of 9 STR markers remain stable for 473 generations and that the frequency of alterations in tandem repeats is positively correlated with the number of repeats. The potential low level instability of STR markers 3A and 3C should be taken into account when interpreting STR data during an outbreak.