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Summary: Pure duplication 21q21.2[arrow right]qter due to a rea(2l) in a Down syndrome girl. Remarks on nomenclature: We report on an 8-year-old girl with a typical Down syndrome phenotype and a 46,XX,rea(21)(qter[arrow right]p12::q2l.2[arrow right] qter).ish rea(2l)(qter[arrow right]p12::q21.2[arrow right]qter)(LSI 21++,AMLt++) karyotype; the mother had normal chromosomes but the father was unavailable. The great resemblance of the patient's rearranged chromosome to lhe rec(21)diip(q) from a parental pericentric inversion suggests that it would be better depicted as a recombinant-like chromosome. Altogether, 13 recombinant-like chromosomes of de novo or unknown (parents no! karyotyped) origin have been described. Although these rearranged chromosomes should formally be described as derivatives because no parental inversion is identified, we underlie that the unofficial term recombinant-like would be more appropriate because no "multiple aberrations within a single chromosome" (as required by the ISCN) have been proved, not to mention that the term derivative usually designates abnormal chromosomes resulting from a translocation between non homologous chromosomes. Accordingly, we prefer to identify such rearrangements of a single chromosome precisely with the more neutral and sanctioned term rea (expanding its use to designate a rearranged chromosome) coupled with the lengthy description of the abnormal chromosome. We assume that the rea(21) chromosomes result from illegitimate recombination between non allelic homologous LCRs located in both the short and long arms.
Key-words: Down syndrome - Duplication 2lq22 - Recombinant-like chromosome.
INTRODUCTION
About 50 patients with a pure duplication mostly involving 21 q22 and associated with a typical or atypical Down syndrome (DS) phenotype have been described (1,15, 22). This imbalance can be inherited or de novo and result from intrachromosomal duplications, recombination within a parental pericentric inversion, or other rearrangements (5, 7, 14, 20, 22). Among these patients, 12 (9 de novo and 3 of unknown origin) cases have presented the extra 21q distal segment attached to 21 p (Table I). This 2 Ip+ chromosome has been described mostly as a derivative of a 2 1 ;2 1 translocation or an add(21); yet, its great resemblance to the rec(21)dup(q) from a parental pericentric inversion has prompted the designation of recombinant in two instances (15, 19). We describe here a girl with a rather typical DS phenotype due to a 21q distal duplication in another recombinant-like chromosome.
CASE REPORT
The...