title: Evidence for an hMSH3 defect in familial hamartomatous polyps.
abstract: BACKGROUND
  Patients with hamartomatous polyposis syndromes have increased risk for colorectal cancer (CRC). Although progression of polyps to carcinoma is observed, pathogenic mechanisms remain unknown. The authors examined whether familial hamartomatous polyps harbor defects in DNA mismatch repair (MMR), and assayed for somatic mutation of PTEN, a gene inactivated in the germline of some hamartomatous polyposis syndrome patients.
  
  
  METHODS
  Ten hamartomatous polyposis syndrome patients were genotyped for germline mutations. Epithelial and nonepithelial polyp DNA were assayed for microsatellite instability (MSI) and PTEN frameshift mutation. DNA MMR and PTEN protein expression were assessed in all polyps by immunohistochemistry. In addition, 99 MSI-high sporadic CRCs and 50 each of hMLH1(-/-) and hMSH3(-/-) cell clones were examined for PTEN frameshifts.
  
  
  RESULTS
  Twenty-five (58%) of 43 hamartomatous polyposis syndrome polyps demonstrated dinucleotide or greater MSI in polyp epithelium, consistent with hMSH3 deficiency. MSI domains lost hMSH3 expression, and PTEN expression was lost in polyps from germline PTEN patients; sporadic hamartomatous polyps did not show any of these findings. PTEN analysis revealed wild type exon 7 and 8 sequences suggestive of nonexistent or rare events for PTEN frameshifts; however, MSI-high sporadic CRC showed 11 (11%) of 99 frameshifts within PTEN, with 4 tumors having complete loss of PTEN expression. Subcloning hMLH1(-/-) and hMSH3(-/-) cells revealed somatic PTEN frameshifts in 4% and 12% of clones, respectively.
  
  
  CONCLUSIONS
  Nondysplastic epithelium from hamartomatous polyposis syndrome polyps harbors hMSH3 defects, which may prime neoplastic transformation. Polyps from PTEN(+/-) patients lose PTEN expression, but loss is not a universal early feature of all hamartomatous polyposis syndrome. However, PTEN frameshifts can occur in hMSH3-deficient cells, suggesting that hMSH3 deficiency could drive hamartomatous polyposis syndrome tumorigenesis.
authors.affiliation: Department of Pediatrics, University of California at San Diego, San Diego, California, USA.
authors.forename: Sherry C
authors.initials: SC
authors.lastname: Huang
authors.forename: Jeffrey K
authors.initials: JK
authors.lastname: Lee
authors.forename: E Julieta
authors.initials: EJ
authors.lastname: Smith
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Doctolero
authors.forename: Akihiro
authors.initials: A
authors.lastname: Tajima
authors.forename: Stayce E
authors.initials: SE
authors.lastname: Beck
authors.forename: Noel
authors.initials: N
authors.lastname: Weidner
authors.forename: John M
authors.initials: JM
authors.lastname: Carethers
mesh_terms: D000293:Adolescent; D000328:Adult; D045744:Cell Line, Tumor; D002648:Child; D002675:Child, Preschool; D015179:Colorectal Neoplasms / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics*; D017353:Gene Deletion; D042822:Genomic Instability; D006801:Humans; D000074081:MutS Homolog 3 Protein; D051059:PTEN Phosphohydrolase / Q000235:genetics*; D010580:Peutz-Jeghers Syndrome / Q000235:genetics*
chemical_list: D004268:DNA-Binding Proteins; C078444:MSH3 protein, human; D000074081:MutS Homolog 3 Protein; D051059:PTEN Phosphohydrolase; C494929:PTEN protein, human