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The objective of this study was to review, using Medline, all cases with supernumerary marker chromosomes (SMC) and uniparental disomy (UPD), and to reflect on the aetiology and mechanisms of formation of SMC.
The survey showed only 19 cases, a maternal UPD(1), a paternal UPD(6), a maternal UPD(7), a maternal UPD(9), a maternal UPD(10), a maternal UPD(12), a maternal UPD(20), and a maternal UPD(22), as well as eight cases with maternal UPD(15) and three cases with paternal UPD(15).
Mechanisms of formation assumed are functional rescue by mitotic reduction of the monosomic homologue to a SMC in a trisomic zygote; somatic reduplication of the normal homologue in a 46,XN,mar zygote; completely mitotic formation by non-disjunction and subsequent reduction of the monosomic homologue or vice versa; and fertilisation of a disomic gamete by a gamete already carrying a marker chromosome. Apart from the first, all these mechanisms increase the likelihood for UPD being present in cases with a SMC.
Mechanisms of formation seem to be different in additional inv dup(15), other additional isochromosomes, and SMCs characterised by one or two deletions. The low incidence of SMC associated with UPD, the formation of additional inv dup(15), and other isochromosomes, as well as the results of molecular investigations of trisomy 21 mosaicism are all evidence against an active mechanism creating a SMC in a trisomic zygote.
Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent.1 Mechanisms of formation are trisomy rescue, monosomy rescue, post-fertilisation errors, and gamete complementation. Problems associated with UPD include trisomy mosaicism, genomic imprinting, homozygosity of autosomal recessive mutations, or even a combination of these.2 Supernumerary marker chromosomes (SMCs) are predominant in only a proportion of cells leading to a specific kind of trisomy mosaicism. The presence of a cell line …