BLM helicase facilitates RNA polymerase I-mediated ribosomal RNA transcription

Abstract

Bloom's syndrome (BS) is an autosomal recessive disorder that is invariably characterized by severe growth retardation and cancer predisposition. The Bloom's syndrome helicase (BLM), mutations of which lead to BS, localizes to promyelocytic leukemia protein bodies and to the nucleolus of the cell, the site of RNA polymerase I-mediated ribosomal RNA (rRNA) transcription. rRNA transcription is fundamental for ribosome biogenesis and therefore protein synthesis, cellular growth and proliferation; its inhibition limits cellular growth and proliferation as well as bodily growth. We report that nucleolar BLM facilitates RNA polymerase I-mediated rRNA transcription. Immunofluorescence studies demonstrate the dependance of BLM nucleolar localization upon ongoing RNA polymerase I-mediated rRNA transcription. In vivo protein co-immunoprecipitation demonstrates that BLM interacts with RPA194, a subunit of RNA polymerase I. (3)H-uridine pulse-chase assays demonstrate that BLM expression is required for efficient rRNA transcription. In vitro helicase assays demonstrate that BLM unwinds GC-rich rDNA-like substrates that form in the nucleolus and normally inhibit progression of the RNA polymerase I transcription complex. These studies suggest that nucleolar BLM modulates rDNA structures in association with RNA polymerase I to facilitate RNA polymerase I-mediated rRNA transcription. Given the intricate relationship between rDNA metabolism and growth, our data may help in understanding the etiology of proportional dwarfism in BS.

Blinded by the UV light: How the focus on transcription-coupled NER has distracted from understanding the mechanisms of Cockayne syndrome neurologic disease

“…A reduction in RNAPII transcription in permeabilized WS cells has also been reported [145]. In addition to CS, Bloom’s and Werner’s, growth defects are also observed in patients with other “ribosomopathies” as well, further supporting a link between abnormal RNAPI transcription and growth deficiency [144]. …”

“…Thus a defect in transcription of rRNA genes provides a simple explanation for growth defects observed in CS patients, one of the canonical features of the disease. Interestingly, patients with Werner’s syndrome and Bloom’s syndrome have growth defects, and reduced nucleolar transcription has been described in cells from both diseases [143, 144]. A reduction in RNAPII transcription in permeabilized WS cells has also been reported [145].…”

Crosstalk between the nucleolus and the DNA damage response

“…BLM, however, has been localized to the nucleolus by immunofluorescence 70, 71 , and co-immunoprecipitates the RNAPI subunit, RPA194 71 . Functionally, it has been shown that BLM facilitates the transcription of the pre-rRNA though its helicase activity on the rDNA 71 . Pulse-chase analysis for instance in BLM-deficient cells reveals a decrease in the production of the 45S pre-rRNA primary transcript 71 .…”

“…Functionally, it has been shown that BLM facilitates the transcription of the pre-rRNA though its helicase activity on the rDNA 71 . Pulse-chase analysis for instance in BLM-deficient cells reveals a decrease in the production of the 45S pre-rRNA primary transcript 71 . Defects in pre-rRNA transcription can lead to decreases in mature 18S and 28S rRNA species, and/or the triggering of the cell stress response and apoptosis 72 .…”

Bloom's Syndrome: Clinical Spectrum, Molecular Pathogenesis, and Cancer Predisposition

“…In about one-third of cells, BLM localizes to the nucleolus, arranged in a large number of irregularly shaped, closely associated blobs. BLM function in the nucleolus is important for rDNA transcription [Grierson et al, 2012] and interaction between BLM, and topoisomerase I is critical to this function [Grierson et al, 2013]. In cells treated with agents that damage DNA, particularly agents that affect the progression of replication forks, BLM concentrates focally to stalled or damaged replication forks.…”