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Stable non-Robertsonian dicentric chromosomes: four new cases and a review
  1. EMMANUELLE LEMYRE*,†,
  2. VAZKEN M DER KALOUSTIAN†‡,
  3. ALESSANDRA M V DUNCAN*,†
  1. *Division of Cytogenetics, Department of Pathology, Montreal Children's Hospital and McGill University, Montreal, Canada
  2. †Department of Human Genetics, McGill University, Montreal, Quebec, Canada.
  3. ‡F Clarke Fraser Clinical Genetics Unit, Department of Pediatrics, Montreal Children's Hospital and McGill University, Montreal, Canada
  1. Dr Duncan, Cytogenetics Laboratory, Montreal Children's Hospital, 2300 Tupper Street, Montreal, Quebec, Canada H3H 1P3, alessandra.duncan{at}muhc.mcgill.ca

https://doi.org/10.1136/jmg.38.1.76

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    Editor—Dicentric autosomes are rarely encountered as stable constitutional chromosomes in humans, with the exception of Robertsonian translocations. The presence of two alpha satellite sequences on the same chromosome leads to a high risk of attachment of the same chromatid to the mitotic spindle from opposite poles and to the formation of anaphase bridge during cell division. Therefore, breakage of the dicentric can occur with subsequent cell death.1 Stability can be achieved when the centromeres are very close together and form only one heterochromatin block, or when one of them is inactivated. One report by Vianna-Morgante and Rosenberg2 shows a rarer mechanism of stabilisation, the deletion of one centromere. Several mechanisms can lead to dicentric chromosomes: meiotic recombination within a paracentric inversion loop, isochromatid break with U shaped rejoining, mitotic crossing over, Robertsonian translocation, and non-homologous non-Robertsonian translocation. Non-homologous dicentric autosomes are expected to be formed by the latter. We have found 22 reported cases of cytogenetically recognisable, non-homologous, non-Robertsonian dicentric autosomes.1-19 We present four new cases of non-homologous, non-Robertsonian dicentric autosomes with centromeres distinguishable by standard cytogenetic techniques; two were inherited from asymptomatic carriers and two occurred de novo in children with deletion 18p syndrome phenotype.

    Metaphase chromosomes were obtained from peripheral blood or amniocytes harvested according to standard protocols. GTG banding was performed on all cases. C banding using Ba(OH)2 was used. Fluorescence in situ hybridisation (FISH) of alpha satellite sequences was performed on metaphase chromosomes according to the protocols provided by ONCOR. FISH with a 16q subtelomeric sequence was performed according to the protocol provided by AL Technology.

    Case 1 was ascertained through amniocentesis at 14 weeks' gestation for advanced maternal age. The mother was a 41 year old gravida 3, para 2, aborta 0. The couple's family history was unremarkable. The …

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