“Pseudomosaicism” for 4p - in amniotic fluid cell culture proven to be true mosaicism after birth

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Abstract
Pseudomosaicism is noted in approximately 1% of amniotic fluid cell studies. Some represent numerical abnormalities, but pseudomosaicism for structural chromosomal abnormalities is also seen. Pseudomosaicism is not usually considered clinically significant. Recently, we evaluated a 13-month-old girl with developmental delay and minor anomalies suggestive of 4p- syndrome. In 5 of 100 peripheral lymphocytes, she had a deletion 46,XX,del(4)(p15). Review of a prenatal amniocentesis study performed on the mother of our patient disclosed that one colony of 18 examined from 2 in situ cultures had the same abnormality, whereas none of the 27 cells from a flask culture showed the abnormality. Results of this study had originally been reported as showing pseudomosaicism. To our knowledge, amniotic fluid pseudomosaicism of a structural abnormality has not previously been shown to reflect true mosaicism in fetal tissue or liveborn children. The actual incidence of this phenomenon is unknown, but it may be present in unexamined children with minimal clinical findings. Apparently only one previous case of mosaic 4p- in a liveborn individual has been reported.