Telomere Length Distribution and Southern Blot Analysis

Abstract

Southern blot analysis of terminal restriction fragments (TRFs) is the standard method for quantitative examination of telomere length distributions. Since TRFs contain a subtelomeric component, central parameters of the TRF distributionn(L) such as the arithmetic mean (M) or the median (Me) cannot be derived directly from Southern blot data, i.e. from the optical density distributionOD(L). Several estimates have been applied instead; the seeming arithmetic meanA, the “center of mass”C, and the positions of maximal (P) and half-maximal optical density (P12). We show thatC>A>Mfor any non-truncated distributionsn(L), andP>M>P1/2 for any symmetrical unimodaln(L).Symmetric appearance on a Southern blot, however, suggests positive skewness ofn(L). Thus, alognormalform ofn(L) may be considered. Then,C>A>M>P=Me>P12. Alternatively, aWeibulldistribution may be assumed. The latter is compatible with negative feedback-regulation of the telomere lengths. Using the maximum likelihood method we compare these distributions with FISH-data on telomere lengths in different cell types. The fit of the lognormal distribution is clearly superior. Lognormal genesis may relate to telomere breakage and recombination.Truncation of the upper end of the TRF distribution is possible due to Southern blot artifacts. Thereby, the order of the estimates may change toP>C>A. Having minimal sensitivity to truncation,Pseems to be the optimal choice. however, the variability ofPis high since peakedness ofOD(L) and DNA length resolution are inversely related.