Structural abnormalities involving heterochromatic regions of the human genome are difficult to characterize because these segments are G-band negative by GTG technique, a routinely used procedure in clinical cytogenetic laboratories. Chromosome abnormalities of such cases have gone undetected or were incorrectly characterized because these regions are so-called heteromorphisms or variants. Consequently, much anxiety has been aroused by the confusion between a chromosome abnormality and a normal heteromorphic variant. We report the first documented case with a so-called highly unusual h region of chromosome 9 which is not a variation but a structural rearrangement involving a paracentric inversion and a duplication. The major clinical features were psychomotor retardation, microcephaly, narrow palpebral fissures, renal and genital anomalies, vertebral anomalies, protruding tongue, and learning and behavioral problems. A concise review of variable duplicated segments of 9q is also provided.
We found an abnormal 47,XX,+mar karyotype in a patient with developmental delay, hypotonia, microcephaly, failure to thrive, and cognitive delay. When metaphases were hybridized with Prader-Willi and Angelman loci-specific probes by the FISH technique, two sites were noted at opposite positions on the marker chromosome. The alphoid satellite DNA probe documented the isodicentric nature while retention of the p arms on both sides of the marker chromosome was demonstrated by beta satellite probe. The patient does not exhibit manifestations of either syndrome despite the presence of these loci in tetrasomic dose. The present investigation suggests that other marker chromosomes be reevaluated, as their clinical manifestations are quite variable.
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