Clinical, cytogenetic, and molecular findings in a patient with a 46,XX,del(18)(q22)/46,XX,idic(18)(q22) karyotype.

Pseudoisodicentric or asymmetrical dicentric chromosomes 18 are rare findings in clinical cytogenetics. So far, only 8 patients with breakpoints in 18q have been reported and in none of them breakpoints were narrowed down to the molecular level. Here, we describe a 17 months old girl with a perimembranous ventricular septal defect, cleft palate, and minor dysmorphism including hypertelorism, flat nose, frontal bossing and low set ears as well as mosaicism for a cell line with a pseudoisodicentric chromosome 18q and a second cell line with a terminal deletion of 11 Mb in 18q22.2→qter. SNP-array investigation revealed a symmetric breakpoint in 18q22.2 and most likely postzygotic formation from the maternal chromosome 18. Clinical findings in all patients reported so far as well as in the patient presented here were in part overlapping with the clinical phenotypes of trisomy 18 and partial monosomy 18q.

The changing phenotype in diploid/triploid mosaicism may mimic genetic syndromes with aberrant genomic imprinting: follow up in a 14-year-old girl.

Diploid/triploid mosaicism is a rare chromosome aberration characterized by growth and mental retardation, muscular hypotonia, clinodactyly, syndactyly of fingers and toes, asymmetry of the body and the face, truncal obesity, and pigmentary anomalies of the skin. Many patients initially present with severe growth retardation and develop truncal obesity later in life. Variable phenotype expression during development and restriction of triploid cells to certain tissues explain why the diagnosis of diploid/triploid mosaicism is often delayed. Here, we report on a moderately retarded 14-year-old girl with diploid/triploid mosaicism due to inclusion of the second polar body, whose changing phenotype overlaps considerably with different genetic disorders associated with aberrant genomic imprinting. The observation that triploid cells, which in our patient show remarkably variable distribution in different tissues, may also be present in easily accessible tissues such as urinary sediment or buccal smear may contribute to an earlier diagnosis of this rare syndrome.