Mixoploidy in humans: two surviving cases of diploid-tetraploid mixoploidy and comparison with diploid-triploid mixoploidy.

We report on 2 cases of diploid/tetraploid (2n/4n) mixoploidy in surviving females, 11 and 21 years old. Both individuals manifested severe mental retardation, reduced peripheral limb muscle bulk, asymmetric growth deficiency, seizure disorder, and skin pigmentary dysplasia. Previous lymphocyte karyotypes had been normal on 2 occasions, but when skin fibroblast karyotypes were done, 33% of the cells were tetraploid on the younger woman, and over 60% were tetraploid in the older woman (on 2 separate occasions). In both individuals, the distal limbs and digits were long and thin, with reduced small muscle bulk. The similarity in distal limb findings prompted reexamination of the younger woman's chromosomal constitution in skin fibroblasts. We concluded that the clinical findings in these cases are unique and similar, and we caution clinicians about uniformly dismissing tetraploidy as artifactual in amniocytes from normal patients, especially since this phenotype would be very difficult to detect, even with directed prenatal ultrasonography. We compare the 2n/4n phenotype with that in diploid/triploid (2n/3n) mixoploidy and note subtle differences which might be detected postnatally. These findings should be useful in guiding clinicians on when to request skin fibroblast karyotypes in mentally-deficient individuals with asymmetric growth deficiency and pigmentary skin variation.