Small polydispersed circular DNA (spcDNA) belongs to the extrachromosomal pool of DNA and is composed of heterogeneous DNA circles. Whether spcDNA has a special function is currently unclear but their occurrence was suggested to be linked to genetic instability. In this study we investigated as to whether human lymphocytes from healthy volunteers also harbour spcDNA and whether spcDNA is present in all permanent cell lines from human normal and malignant tissues. Moreover, we were interested to see whether spcDNA contains sequences of mobile genetic elements. Our results show that spcDNA is present in all samples investigated yet the amount is lower in normal lymphocytes when compared to cancer cell lines (5.4 vs. 17.8%). Alu sequences were present in 12/16 cancer cell lines whereas LINE-1 (L1) sequences were present in 15 of them. Six tumor cell lines also contained telomeric sequences. In contrast to that, spcDNA of normal lymphocytes contains Alu and L1 sequences only in 3/16 cases and no telomeric sequences at all. Our findings suggest a direct dependency of the amount of Alu and L1 sequences on that of spcDNA. Beside these repetitive sequences, sequencing of spcDNA revealed in most cases chromosomal sequences of almost all chromosomes without an increased frequency of single regions. We suggest that the whole spcDNA including retrotranspositional elements and telomeric sequences may play a role for chromosomal rearrangements and genomic instability.

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