title: Characterization of a prenatally assessed de novo supernumerary minute ring chromosome 20 in a phenotypically normal male.
abstract: BACKGROUND
  The heterogeneous group of small supernumerary marker chromosomes (sSMCs) presents serious counseling problems, especially if they are present de novo and diagnosed prenatally. The incidence has been estimated at 1 in 1000 prenatal samples. We present a case of mosaic sSMC diagnosed prenatally after amniocentesis. The sSMC was characterized by various molecular cytogenetic techniques and determined to be a r(20) chromosome. After genetic counseling, the parents decided to continue the pregnancy, and a boy with minor phenotypic variants was born after 39 weeks of pregnancy. The case is compared with four other cases of prenatally detected r(20) mosaicism.
  
  
  RESULTS
  Here we describe a 3 months old male child with normal pre- and postnatal development and with a de novo ring supernumerary marker chromosome in amniocytes cultures. Using new fluorescence in situ hybridization (FISH) techniques, three distinguishable sSMCs (cryptic mosaicism), all derived from chromosome 20, were observed, including ring and minute chromosomes. This heterogeneity was impossible to detect by the conventional G-banding technique or conventional FISH technique that were used before the application of new FISH techniques (subcentromere-specific multicolor-FISH [subcenM-FISH]) and a probe, specific for the 20p12.2 band. The sSMC present in 25% of the cells was present as r(20)(::p12.2~12.3->q11.1::)5/r(20;20)(::p12.1->q11.1::q11.1 >p12.1::)2/min(20;20)(:p12.1->q11.1::q11.1->p12.1:)1. The final karyotype was 47,XY,+r(20)[25%]/46,XY[75%].
  
  
  CONCLUSIONS
  We emphasize the importance of application of molecular cytogenetics in a prenatally diagnostic laboratory and description of more cases to enable a better genetic counseling and risk evaluation.
authors.affiliation: Bioiatriki S,A, Kifisias Av, 132 and Papada, GR-115 26 Athens, Greece. emanolakos@bioiatriki.gr.
authors.forename: Sofia
authors.initials: S
authors.lastname: Kitsiou-Tzeli
authors.forename: Emmanouil
authors.initials: E
authors.lastname: Manolakos
authors.forename: Magdalini
authors.initials: M
authors.lastname: Lagou
authors.forename: Maria
authors.initials: M
authors.lastname: Kontodiou
authors.forename: Nadezda
authors.initials: N
authors.lastname: Kosyakova
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Ewers
authors.forename: Anja
authors.initials: A
authors.lastname: Weise
authors.forename: Antonios
authors.initials: A
authors.lastname: Garas
authors.forename: Sandro
authors.initials: S
authors.lastname: Orru
authors.forename: Thomas
authors.initials: T
authors.lastname: Liehr
authors.forename: Aikaterini
authors.initials: A
authors.lastname: Metaxotou