Origin and behavior of a bisatellited isodicentric chromosome

Karyotyping was performed on a 5 year old male with moderate mental retardation, severe speech retardation, hyperactivity, and alternating strabismus. Three cell lines were found. 30 cells exhibited a normal karyotype and 54 had an extra E group sized submetacentric chromosome with satellites on both the long and short arms. The remaining 20 cells each had, in addition to the first marker (M1), a second bisatellited chromosome (M2). This was similar to the small extra bisatellited chromosome occasionally seen in phenotypically normal individuals. Since both ends of both M1 and M2 were satellited and participated in acrocentric associations, the markers were derived from one or more acrocentrics. C banding demonstrated that both markers were dicentric. However, M1 had only one primary construction. Therefore one centromere was apparently inactive, analogous to that seen in certain tandem X X translocations and X isochromosomes. The authors suggest that the dicentric M1 was derived from a meiotic breakage and a sister chromatid fusion in the proximal long arm of an acrocentric. This would have produced a symmetrical isodicentric chromosome, plus one or two acentric fragments. M2 then could have resulted from a dicentric bridge break synthesis reunion phenomenon. G, C and Q banding data were consistent with the isodicentrics having originated from a chromosome 15.