Meiotic origin of two ring chromosomes 18 in a girl with developmental delay

We report on the cytogenetic, fluorescence in situ hybridization (FISH), and molecular results obtained for a patient with a mild and nonspecific pattern of minor anomalies and developmental delay. In the proband's karyotype one chromosome 18 was replaced by a ring chromosome 18 in all metaphases, with deletion of the terminal regions. Furthermore, 56% of the metaphases contained a supernumerary small ring chromosome. Microdissection followed by FISH analysis demonstrated that the small ring chromosome consisted of material from the pericentromeric region of chromosome 18. The karyotype was defined as 46,XX,r(18)(p11.3q23)[88]/47,XX,r(18)(p11.3q23)+r(18)(p11.22q12.2)[112]. Thus, the patient has a deletion at 18pter and at 18qter, and a mosaic partial trisomy of the pericentromeric region of chromosome 18. We undertook molecular analysis using DNA samples of the patient and her parents in order to clarify the origin and possible mode of formation of the chromosome abnormalities. Our results show a paternal origin of the structurally normal chromosome 18 and a maternal origin for both ring chromosomes 18. Interestingly, the smaller ring chromosome did not arise postzygotically from the larger ring, since the two ring chromosomes contain genetic material derived from the two different maternal chromosomes 18. The abnormalities appear to have arisen during a meiotic division, and it could be speculated that both ring chromosomes 18 arose simultaneously due to complex pairing and recombination events. After fertilization, the small ring chromosome was lost in a subset of cells, thus leading to mosaicism.