Microsatellite Repeat

Abstract: Microsatellite instability has been reported to be an important feature of tumors from hereditary nonpolyposis colorectal carcinoma (HNPCC) patients. The recent discovery of genetic instability in small cell lung carcinoma, a neoplasm not associated with HNPCC, led us to investigate the possible presence of microsatellite alterations in other tumor types. We examined 52 microsatellite repeat sequences in the DNA of normal and tumor pairs from 100 head and neck, bladder, and lung cancer patients by the polymerase chain reaction. Although alterations were rare in dinucleotide repeats, larger (tri- or tetranucleotide) repeats were found to be more prone to expansion or deletion. We screened 100 tumors with a panel of nine tri- and tetranucleotide repeat markers and identified 26 (26%) that displayed alterations in at least one locus. This observation prompted us to examine the possibility of using microsatellite alterations as markers to detect clonal tumor-derived cell populations in pathologic samples. The identical microsatellite alterations detected in the primary tumors were successfully identified in corresponding urine, sputum, and surgical margins from affected patients. This study demonstrates that appropriately selected microsatellite loci are commonly altered in many cancers and can serve as clonal markers for their detection.

Abstract: We describe an efficient method for the construction of small-insert genomic libraries enriched for highly polymorphic, simple sequence repeats. With this approach, libraries in which 40-50% of the members contain (CA)n repeats are produced, representing an approximately 50-fold enrichment over conventional small-insert genomic DNA libraries. Briefly, a genomic library with an average insert size of less than 500 base pairs was constructed in a phagemid vector. Amplification of this library in a dut ung strain of Escherichia coli allowed the recovery of the library as closed circular single-stranded DNA with uracil frequently incorporated in place of thymine. This DNA was used as a template for second-strand DNA synthesis, primed with (CA)n or (TG)n oligonucleotides, at elevated temperatures by a thermostable DNA polymerase. Transformation of this mixture into wild-type E. coli strains resulted in the recovery of primer-extended products as a consequence of the strong genetic selection against single-stranded uracil-containing DNA molecules. In this manner, a library highly enriched for the targeted microsatellite-containing clones was recovered. This approach is widely applicable and can be used to generate marker-selected libraries bearing any simple sequence repeat from cDNAs, whole genomes, single chromosomes, or more restricted chromosomal regions of interest.

Abstract: Spinocerebellar ataxia type 31 (SCA31) is an adult-onset autosomal-dominant neurodegenerative disorder showing progressive cerebellar ataxia mainly affecting Purkinje cells. The SCA31 critical region was tracked down to a 900 kb interval in chromosome 16q22.1, where the disease shows a strong founder effect. By performing comprehensive Southern blot analysis and BAC- and fosmid-based sequencing, we isolated two genetic changes segregating with SCA31. One was a single-nucleotide change in an intron of the thymidine kinase 2 gene (TK2). However, this did not appear to affect splicing or expression patterns. The other was an insertion, from 2.5–3.8 kb long, consisting of complex penta-nucleotide repeats including a long (TGGAA)n stretch. In controls, shorter (1.5–2.0 kb) insertions lacking (TGGAA)n were found only rarely. The SCA31 repeat insertion's length inversely correlated with patient age of onset, and an expansion was documented in a single family showing anticipation. The repeat insertion was located in introns of TK2 and BEAN (brain expressed, associated with Nedd4) expressed in the brain and formed RNA foci in the nuclei of patients' Purkinje cells. An electrophoretic mobility-shift assay showed that essential splicing factors, serine/arginine-rich splicing factors SFRS1 and SFRS9, bind to (UGGAA)n in vitro. Because (TGGAA)n is a characteristic sequence of paracentromeric heterochromatin, we speculate that the insertion might have originated from heterochromatin. SCA31 is important because it exemplifies human diseases associated with “inserted” microsatellite repeats that can expand through transmission. Our finding suggests that the ectopic microsatellite repeat, when transcribed, might cause a disease involving the essential splicing factors.

Abstract: Analysis of variation at microsatellite DNA loci is widely used in studies of parentage, linkage and evolutionary history. The utility of microsatellites is primarily due to high levels of allelic diversity, believed to reflect mutation rates orders of magnitude higher than base pair substitutions at single-copy genes. For humans, mice, rats and pigs, microsatellite mutation rates have been estimated at 10(-3)-10(-5). However, a recent study comparing microsatellite variation in humans with non-human primates suggests that microsatellite mutation rates may vary considerably across taxa. We measured mutation rates of 24 microsatellite loci in mutation accumulation lines of Drosophila melanogaster. Surprisingly, only a single mutation was detected after screening 157,680 allele-generations, yielding an estimated average mutation rate per locus of 6.3 x 10(-6), a mutation rate considerably lower than reported for various mammals. We propose that the comparatively low mutation rate is primarily a function of short microsatellite repeat lengths in the D. melanogaster genome.