Partial hexasomy 15pter→15q13 including SNRPN and D15S10: first molecular cytogenetically proven case report: Figure 1

Isodicentric chromosomes 15 (idic(15) or inv dup(15) chromosomes) are the most common supernumerary marker chromosome (SMC) in humans.1 Buckton et al 2 found that 0.24% of newborns had a SMC, of which 50% could be described as idic(15). The presence of such chromosomes is usually associated with mental retardation, developmental delay, epilepsy, and behavioural problems like autism.3 It is possible that the clinical severity depends on the size of the SMC and its euchromatic material, for example, almost no clinical signs were detectable in a case with idic(15)(pter→q12),4 while a severe phenotype including all typical clinical signs was present in a patient with the karyotype 47,+idic(15)(pter→q13).3,5,6 However, there are also reports which show an inconsistent relationship between marker size, gene dosage, and severity of the phenotype.7,8 In the study of Mignon et al ,8 in all 16 cases studied the derivative chromosomes 15 were of maternal origin with an identical methylation profile, and neither imprinting nor methylation could explain the phenotypic variability. Partial trisomy of 15q11-q14 resulting from intrachromosomal duplication of chromosome 15q11-q14 has also been reported.9,10 ### Key points