title: Multiple forms of atypical rearrangements generating supernumerary derivative chromosome 15.
abstract: BACKGROUND
  Maternally-derived duplications that include the imprinted region on the proximal long arm of chromosome 15 underlie a complex neurobehavioral disorder characterized by cognitive impairment, seizures and a substantial risk for autism spectrum disorders1. The duplications most often take the form of a supernumerary pseudodicentric derivative chromosome 15 [der(15)] that has been called inverted duplication 15 or isodicentric 15 [idic(15)], although interstitial rearrangements also occur. Similar to the deletions found in most cases of Angelman and Prader Willi syndrome, the duplications appear to be mediated by unequal homologous recombination involving low copy repeats (LCR) that are found clustered in the region. Five recurrent breakpoints have been described in most cases of segmental aneuploidy of chromosome 15q11-q13 and previous studies have shown that most idic(15) chromosomes arise through BP3:BP3 or BP4:BP5 recombination events.
  
  
  RESULTS
  Here we describe four duplication chromosomes that show evidence of atypical recombination events that involve regions outside the common breakpoints. Additionally, in one patient with a mosaic complex der(15), we examined homologous pairing of chromosome 15q11-q13 alleles by FISH in a region of frontal cortex, which identified mosaicism in this tissue and also demonstrated pairing of the signals from the der(15) and the normal homologues.
  
  
  CONCLUSIONS
  Involvement of atypical BP in the generation of idic(15) chromosomes can lead to considerable structural heterogeneity.
authors.affiliation: Nemours Biomedical Research, Alfred I, duPont Hospital for Children, Wilmington, Delaware, 19803, USA. NJWang@lbl.gov
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Wang
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Parokonny
authors.forename: Karen N
authors.initials: KN
authors.lastname: Thatcher
authors.forename: Jennette
authors.initials: J
authors.lastname: Driscoll
authors.forename: Barbara M
authors.initials: BM
authors.lastname: Malone
authors.forename: Naghmeh
authors.initials: N
authors.lastname: Dorrani
authors.forename: Marian
authors.initials: M
authors.lastname: Sigman
authors.forename: Janine M
authors.initials: JM
authors.lastname: LaSalle
authors.forename: N Carolyn
authors.initials: NC
authors.lastname: Schanen
mesh_terms: D017204:Angelman Syndrome / Q000235:genetics; D015139:Blotting, Southern; D001921:Brain / Q000648:ultrastructure; D002460:Cell Line; D002869:Chromosome Aberrations; D022202:Chromosomes, Artificial, Bacterial; D002884:Chromosomes, Human, Pair 15 / Q000235:genetics*; D019175:DNA Methylation; D005260:Female; D020440:Gene Duplication; D005838:Genotype; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D018404:Isochromosomes / Q000235:genetics*; D007621:Karyotyping; D008297:Male; D011218:Prader-Willi Syndrome / Q000235:genetics