Topoisomerase I in Human Disease Pathogenesis and Treatments

Abstract

Mammalian topoisomerase 1 (TOP1) is an essential enzyme for normal development. TOP1 relaxes supercoiled DNA to remove helical constraints that can otherwise hinder DNA replication and transcription and thus block cell growth. Unfortunately, this exact activity can covalently trap TOP1 on the DNA that could lead to cell death or mutagenesis, a precursor for tumorigenesis. It is therefore important for cells to find a proper balance between the utilization of the TOP1 catalytic activity to maintain DNA topology and the risk of accumulating the toxic DNA damages due to TOP1 trapping that prevents normal cell growth. In an apparent contradiction to the negative attribute of the TOP1 activity to genome stability, the detrimental effect of the TOP1-induced DNA lesions on cell survival has made this enzyme a prime target for cancer therapies to kill fast-growing cancer cells. In addition, cumulative evidence supports a direct role of TOP1 in promoting transcriptional progression independent of its topoisomerase activity. The involvement of TOP1 in transcriptional regulation has recently become a focus in developing potential new treatments for a subtype of autism spectrum disorders. Clearly, the impact of TOP1 on human health is multifold. In this review, we will summarize our current understandings on how TOP1 contributes to human diseases and how its activity is targeted for disease treatments.

Keywords: Autism; Cancer; DNA replication; Scleroderma; Topoisomerase 1; Transcription.

Figure 1

Illustration of TOP1 DNA cleavage reaction The TOP1 DNA cleavage reaction is initiated by the binding and DNA nicking (purple line) of TOP1 to form a TOP1cc complex that covalently links TOP1 to the DNA. The intact DNA strand (blue line) passes through the DNA nick before the nick is religated, followed by the release of TOP1 from the DNA. TOP1 DNA cleavage next to a misincorporated ribonucleotide U or an aborted TOP1cc reaction can lead to mutations and cell death. TOP1 is shown in yellow and the two DNA strands are shown in purple and blue, respectively. TOP1, topoisomerase 1; TOP1cc, TOP1–DNA cleavage complex; U, uridine.

Figure 2

The positive and negative attributes of TOP1 action to human health Summary diagram showing the functions of human TOP1, the regulation of these functions by SUMOylation and its potential link to human diseases (indicated in red) and therapies using TOP1 poisons (indicated in green). TOP1, topoisomerase 1; RNAPII, RNA polymerase II; AS, Angelman syndrome.