title: Molecular characterization of an analphoid supernumerary marker chromosome derived from 18q22.1➔qter in prenatal diagnosis: a case report.
abstract: BACKGROUND
  Small supernumerary marker chromosomes (sSMC) occur in 0.072% of unselected cases of prenatal diagnoses, and their molecular cytogenetic characterization is required to establish a reliable karyotype-phenotype correlation. A small group of sSMC are C-band-negative and devoid of alpha-satellite DNA. We report the molecular cytogenetic characterization of a de novo analphoid sSMC derived from 18q22.1→qter in cultured amniocytes.
  
  
  RESULTS
  We identified an analphoid sSMC in cultured amniocytes during a prenatal diagnosis performed because of advanced maternal age. GTG-banding revealed an sSMC in all metaphases. FISH experiments with a probe specific for the chromosome 18 centromere, and C-banding revealed neither alphoid sequences nor C-banding-positive satellite DNA thereby suggesting the presence of a neocentromere. To characterize the marker in greater detail, we carried out additional FISH experiments with a set of appropriate BAC clones. The pattern of the FISH signals indicated a symmetrical organization of the marker, the breakpoint likely representing the centromere of an inverted duplicated chromosome that results in tetrasomy of 18q22.1→qter. The karyotype after molecular cytogenetic investigations was interpreted as follows: 47,XY,+inv dup(18)(qter→q22.1::q22.1→neo→qter).
  
  
  CONCLUSIONS
  Our case is the first report, in the prenatal diagnosis setting, of a de novo analphoid marker chromosome originating from the long arm of chromosome 18, and the second report of a neocentromere formation at 18q22.1.
authors.affiliation: Department of Genetics, ASL Napoli 1 Centro, Napoli, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Altieri
authors.affiliation: Department of Biology, University of Bari, Bari, Italy.
authors.forename: Oronzo
authors.initials: O
authors.lastname: Capozzi
authors.affiliation: Diagnostic Service Srl, Casoria, Napoli Italy.
authors.forename: Maria Cristina
authors.initials: MC
authors.lastname: Marzano
authors.affiliation: MeriGen Research Srl., Napoli, Italy.
authors.forename: Oriana
authors.initials: O
authors.lastname: Catapano
authors.affiliation: MeriGen Research Srl., Napoli, Italy.
authors.forename: Immacolata
authors.initials: I
authors.lastname: Di Biase
authors.affiliation: Department of Biology, University of Bari, Bari, Italy.
authors.forename: Mariano
authors.initials: M
authors.lastname: Rocchi
authors.affiliation: Department of Genetics, ASL Napoli 1 Centro, Napoli, Italy.
authors.forename: Giuliana
authors.initials: G
authors.lastname: De Tollis
keywords: FISH analysis; Neocentromere; Prenatal diagnosis; Small supernumerary marker chromosomes