Isochromosome 5p mosaicism at prenatal diagnosis: Observations and outcomes in six cases at chorionic villus sampling and one at amniocentesis

We present six cases of 47, + i(5p)/46 mosaicism diagnosed at chorionic villus sampling (CVS), this being the first prospective series to be reported. The clinical indication in each was advanced maternal age. Further prenatal studies in four (amniocentesis, plus fetal blood sampling in one) did not show the isochromosome. In one case, subsequent amniocentesis showed 1/48 in situ colonies with the isochromosome, but fetal blood was karyotypically normal. These five pregnancies resulted in phenotypically normal livebirths; further normal follow-up reports (from age 4 months through 4 years) are noted in four of these. Analysis of placental tissue in one case confirmed the presence of the i(5p) mosaicism. In the remaining case, in which 100% of CVS cultured cells had the i(5p), the pregnancy was terminated. Fetal skin fibroblasts did not show the i(5p). Thus, in none of these six cases was true fetal mosaicism detected, nor an abnormal phenotype noted. We suggest that a 47, + i(5p)/46 karyotype, detected at CVS, may frequently reflect confined placental mosaicism. In addition, we report a case of the primary diagnosis of 47, + i(5p)/46 mosaicism at amniocentesis. The infant appeared normal at birth, but a brain malformation was subsequently identified.