Balkan Journal of Medical Genetics

HUMAN RING CHROMOSOMES – NEW INSIGHTS FOR THEIR CLINICAL SIGNIFICANCE
Guilherme RS1,2, Klein E1, Hamid AB1, Bhatt S1, Volleth M3, Polityko A4, Kulpanovich A5, Dufke A6, Albrecht B7, Morlot S8, Brecevic L9, Petersen MB10, Manolakos E10, Kosyakova N1, Liehr T1*
*Corresponding Author: Thomas Liehr, Institut für Humangenetik, Kollegiengasse 10, Postfach D-07740 Jena, Germany; Tel.: +49-3641-935533; Fax: +49-3641-935582; E-mail: i8lith@mti.uni-jena.de
page: 13

INTRODUCTION

It is common sense that ring chromosomes result from two terminal breaks on both chromosome arms followed by fusion of the broken ends, leading to the loss of genetic material. Alternatively, they can be formed by telomere-telomere fusion without deletion [1] or the so-called McClintock mechanism [2]. Also more complex mechanisms of ring chromosome formation have been proposed [3,4]. Ring chromosomes are also observed as small supernumerary marker chromosomes (sSMC) [5,6], however, their formation seems to be completely different from that of ring chromosomes in a numerically normal karyotype [7,8]. Phenotypes associated with ring chromosomes can be highly variable, since in addition to the primary deletion associated with ring formation, secondary loss or gain of material may occur due to ring chromosome instability. It has also been reported that the phenotype of ring chromosome patients can overlap that of the deletion of both ends of the respective chromosome syndromes without ring formation. Moreover, there have also been numerous reports on ring chromosomes without clinical consequences, apart from possible infertility, if no relevant genetic material was lost due to ring chromosome formation [1]. Here we report ring chromosomes observed in 29 patients with (severe) clinical problems, and/or solely infertility, evaluated by cytogenetic and molecular techniques.

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