Ring Chromosome 18: Clinical, Cytogenetic and Molecular Genetic Studies on Four Patients

ABSTRACT Ring chromosomes are a rare chromosomal aberration but have meanwhile been reported for nearly all human chromosomes. We describe four de novo carriers (1 boy and 3 girls) of ring chromosome 18 (r(18)): while three patients had a non-mosaic 46,r(18) karyotype, the fourth was a mosaic: mos 46,XX,r(18)/46,XX,der(18). Phenotypically, the boy showed only minor anomalies, but the female probands presented several clinical features, among them microcephaly, a moderate to severe muscular hypotonia, psychomotoric retardation and short stature. Major malformations were heart defects, cleft lip and palate and atresia of the external auditory canal. In one girl with very short stature, we found a hypothalamic growth hormone deficiency. By investigating the children over 2,5 years it could be demonstrated that the ring chromosomes were passed regularly through mitosis. The parental origin of the ring was determined in three cases indicating a postzygotic mitotic error