Rune Andreassen 1 Jan Lundsted 2 Bjørnar Olaisen 1 1 Institute of Forensic Medicine, University of Oslo, the National Hospital, Oslo, Norway 2 Department of Forensic Genetics, Institute of Forensic Medicine, University of Copenhagen, Copenhagen, Denmark Mutation at minisatellite locus DYF155S1: Allele length mutation rate is affected by age of progenitor A father/son material consisting of 1071 pairs was screened for de novo allele length mutation in locus DYF155S1. Six hundred of these pairs were also analyzed in locus DYF155S1 to detect de novo mutations in the minisatellite variant repeat (MVR)-code not resulting in a length change (“boundary switch” mutations). A modified MVR-poly- merase chain reaction (PCR) method was used for this purpose. Twenty-seven de novo allele length mutations and eight “boundary switch” mutations were detected indicat- ing mutation frequencies of approximately 2.5% and 1.3%, respectively. The com- bined mutation rate for MVR-code mutation is approximately 3.8%. There is a signifi- cant increase in mutation rate with paternal age (p = 0.049) in allele length mutations. In the present material, the mutation rate in the oldest age group is three times that of the youngest age group. A similar age relationship is not observed in “boundary switch” mutations. A comparison between progenitors and the other fathers in the material revealed no obvious association between mutation rate and allele length or modular structure (variation in repeat sequence). More than 75% of the length muta- tions involved the gain or loss of one repeat only. This finding as well as the observed paternal age influence on mutation rate, suggests replication slippage to be the major mutation mechanism in length mutations. However, in one particular case, an allele length mutant revealed rearrangements with direct duplication of repeats at distant sites within the repeat array, and with both loss and gain of repeats. Such complex structural changes could indicate that some of the mutants might arise from sister chromatide exchange. The mutation rate of “boundary switch” mutations is by far higher than would be expected if these mutations are two independent one-step allele length mutations. A different age distribution of “boundary switch” mutations than of allele length mutations also argue against such a hypothesis. Together this could indi- cate that “boundary switches” are products of another mutation mechanism than the one-step allele length mutations. Keywords: Age / Minisatellite / Minisatellite variant repeat-polymerase chain reaction / MSY-1 / Mutation EL 4998 1 Introduction The human minisatellite DYF155S1 (MSY-1) is hypervari- able and located in the nonrecombining region on the Y-chromosome [1, 2]. The extensive allelic variation observed in this minisatellite is presumably caused by a high mutation rate [1]. DYF155S1 is classified as a mini- satellite due to the length of the repeats (25 bp), but, unlike most other known hypervariable minisatellites [3– 5], it is AT-rich. The repeat array has a predicted ability to adapt secondary hairpin-like structures [1], a property DYF155S1 shares with other known AT-rich minisatellites [6–9]. Many microsatellite loci are involved in the develop- ment of hereditary diseases (reviewed in Cummings and Zoghbi [10]). Recently, it has been reported that also expanded AT-rich minisatellites might create fragile sites [9] or be involved in the development of hereditary dis- eases [11]. An age dependency in the mutation rate has been shown in a study of microsatellites [12], but so far not reported in any minisatellite. In contrast to the situationfor the autosomal AT-rich min- isatellites, the haploid state of DYF155S1 excludes inter- allelic mechanisms as a source for evolution. Thus, the investigation of mutation processes in this minisatellite provides an opportunity to obtain specific information about intra-allelic instability in AT-rich minisatellites. Correspondence: Rune Andreassen, Institute of Forensic Medicine, University of Oslo, The National Hospital, N-0027 Oslo, Norway E-mail: rune.andreassen@hf.hio.no Fax: 147-23071260 Abbreviations: CI, confidence interval; MVR, minisatellite variant repeat Electrophoresis 2002, 23, 2377–2383 2377  WILEY-VCH Verlag GmbH, 69451 Weinheim, 2002 0173-0835/02/1508–2377 $17.501.50/0 Nucleic acids