title: Cellular defects caused by hypomorphic variants of the Bloom syndrome helicase gene BLM.
abstract: BACKGROUND
  Bloom syndrome is an autosomal recessive disorder characterized by extraordinary cancer incidence early in life and an average life expectancy of ~27 years. Premature stop codons in BLM, which encodes a DNA helicase that functions in DNA double-strand-break repair, make up the vast majority of Bloom syndrome mutations, with only 13 single amino acid changes identified in the syndrome. Sequencing projects have identified nearly one hundred single nucleotide variants in BLM that cause amino acid changes of uncertain significance.
  
  
  RESULTS
  Here, in addition to identifying five BLM variants incapable of complementing certain defects of Bloom syndrome cells, making them candidates for new Bloom syndrome causing mutations, we characterize a new class of BLM variants that cause some, but not all, cellular defects of Bloom syndrome. We find elevated sister-chromatid exchanges, a delayed DNA damage response and inefficient DNA repair. Conversely, hydroxyurea sensitivity and quadriradial chromosome accumulation, both characteristic of Bloom syndrome cells, are absent. These intermediate variants affect sites in BLM that function in ATP hydrolysis and in contacting double-stranded DNA.
  
  
  CONCLUSIONS
  Allele frequency and cellular defects suggest candidates for new Bloom syndrome causing mutations, and intermediate BLM variants that are hypomorphic which, instead of causing Bloom syndrome, may increase a person's risk for cancer or possibly other Bloom-syndrome-associated disorders, such as type-2 diabetes.
authors.affiliation: Department of Cell Biology, Microbiology and Molecular BiologyUniversity of South FloridaTampaFlorida33620; Graduate Program in Cell and Molecular BiologyUniversity of South FloridaTampaFlorida33620.
authors.forename: Vivek M
authors.initials: VM
authors.lastname: Shastri
authors.affiliation: Department of Cell Biology, Microbiology and Molecular BiologyUniversity of South FloridaTampaFlorida33620; Cancer Biology and Evolution ProgramH. Lee Moffitt Cancer Center and Research InstituteTampaFlorida33612.
authors.forename: Kristina H
authors.initials: KH
authors.lastname: Schmidt
keywords: BLM variants; DNA helicase; bloom syndrome; genome instability; hypomorphic