title: Increased susceptibility of spinal muscular atrophy fibroblasts to camptothecin is p53-independent.
abstract: BACKGROUND
  Deletion or mutation(s) of the survival motor neuron 1 (SMN1) gene causes spinal muscular atrophy (SMA). The SMN protein is known to play a role in RNA metabolism, neurite outgrowth, and cell survival. Yet, it remains unclear how SMN deficiency causes selective motor neuron death and muscle atrophy seen in SMA. Previously, we have shown that skin fibroblasts from SMA patients are more sensitive to the DNA topoisomerase I inhibitor camptothecin, supporting a role for SMN in cell survival. Here, we examine the potential mechanism of camptothecin sensitivity in SMA fibroblasts.
  
  
  RESULTS
  Camptothecin treatment reduced the DNA relaxation activity of DNA topoisomerase I in human fibroblasts. In contrast, kinase activity of DNA topoisomerase I was not affected by camptothecin, because levels of phosphorylated SR proteins were not decreased. Upon camptothecin treatment, levels of p53 were markedly increased. To determine if p53 plays a role in the increased sensitivity of SMA fibroblasts to camptothecin, we analyzed the sensitivity of SMA fibroblasts to another DNA topoisomerase I inhibitor, beta-lapachone. This compound is known to induce death via a p53-independent pathway in several cancer cell lines. We found that beta-lapachone did not induce p53 activation in human fibroblasts. In addition, SMA and control fibroblasts showed essentially identical sensitivity to this compound. By immunofluorescence staining, SMN and p53 co-localized in gems within the nucleus, and this co-localization was overall reduced in SMA fibroblasts. However, depletion of p53 by siRNA did not lessen the camptothecin sensitivity in SMA fibroblasts.
  
  
  CONCLUSIONS
  Even though p53 and SMN are associated, the increased sensitivity of SMA fibroblasts to camptothecin does not occur through a p53-dependent mechanism.
authors.affiliation: Nemours Biomedical Research, Alfred I, duPont Hospital for Children, Wilmington, DE, USA. wcy@udel.edu
authors.forename: Chia-Yen
authors.initials: CY
authors.lastname: Wu
authors.forename: Ilsa
authors.initials: I
authors.lastname: Gómez-Curet
authors.forename: Vicky L
authors.initials: VL
authors.lastname: Funanage
authors.forename: Mena
authors.initials: M
authors.lastname: Scavina
authors.forename: Wenlan
authors.initials: W
authors.lastname: Wang
mesh_terms: D017209:Apoptosis; D002166:Camptothecin / Q000494:pharmacology*; D002478:Cells, Cultured; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism*; D006801:Humans; D009134:Muscular Atrophy, Spinal / Q000378:metabolism*; D009285:Naphthoquinones / Q000494:pharmacology; D034741:RNA, Small Interfering / Q000378:metabolism; D055533:Survival of Motor Neuron 1 Protein / Q000378:metabolism; D059004:Topoisomerase I Inhibitors; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*
chemical_list: D009285:Naphthoquinones; D034741:RNA, Small Interfering; D055533:Survival of Motor Neuron 1 Protein; D059004:Topoisomerase I Inhibitors; D016159:Tumor Suppressor Protein p53; C014638:beta-lapachone; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin