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A supernumerary marker chromosome 15 tetrasomic for the Prader-Willi/Angelman syndrome critical region in a patient with a severe phenotype

The proximal region of chromosome 15q is predisposed to a wide range of structural rearrangements. Deletions of this region, spanning approximately 4 Mb, can be paternally or maternally derived and result in Prader-Willi syndrome and Angelman syndrome, respectively.1 Additional copies of the Prader-Willi/Angelman syndrome critical region (PWACR) can occur as interstitial duplications and triplications or as supernumerary marker chromosomes (SMCs).2–6

SMC(15), often referred to as inv dup(15), is the most common marker chromosome in humans, representing 50% of all markers.7,8 SMC(15) can be classified into two major groups, depending on the presence or absence of the PWACR. Small SMC(15)s do not contain the PWACR, can be either paternal or maternal in origin, and seem to have no phenotypic effect.6,9 Large SMC(15)s usually contain two extra copies of the q11–13 region, are exclusively derived from the maternal chromosome 15 homologues, and are associated with various abnormal phenotypes, including mental or growth retardation, seizures, behavioural problems, and dysmorphisms.6,10,11

Mapping studies have shown that the small SMCs share the same two common proximal breakpoints, BP1 and BP2,12 as standard PWS/AS deletions, whereas the large SMCs have at least three breakpoints, one similar to the distal breakpoint (BP3) of the PWS/AS deletions.13,14

Other rare variants of the inv dup(15) that do not follow the common pattern have also been described. Mignon et al 15 and Robinson et al 10 each reported a patient with an asymmetrical SMC(15), containing only one copy of the PWACR. The patient described by Mignon et al 15 showed several of the classic signs of PWS. The coexistence of SMC(15) with other chromosome 15 derived rearrangements has also been reported including an interstitial duplication16 and an interstitial triplication.17 The presence of two extra marker chromosomes …