Prenatal diagnosis of trisomy 21 without the Down syndrome phenotype.

OBJECTIVE

To report a patient with the prenatal diagnosis of trisomy 21 without the clinical Down syndrome (DS) phenotype secondary to the absence of the Down syndrome chromosomal region (DSCR) in a derivative chromosome 21.

CASE REPORT AND

METHODS

A newborn patient with prenatal diagnosis of duodenal atresia. Cytogenetic studies revealed a regular trisomy 21. At birth, she did not present the clinical features of DS. FISH analysis was performed in the patient with the LSI spectrum probe for the DSCR and in the mother with FISH multicolor analysis using painting probes for chromosomes 20 and 21.

RESULTS

FISH analysis in the patient showed two hybridization signals suggesting that the third chromosome 21 did not have the DSCR region explaining the absence of the DS phenotype. FISH multicolor analysis in the mother showed three hybridization signals for chromosomes 20 and 21, concluding a maternal karyotype, 46,XX,t(20;21)(p11.2;q22.1).

CONCLUSIONS

The patient was found to have a derivative chromosome 21 secondary to a nondisjunction error in meiosis II without the DS critical region and the phenotype was mostly secondary to the combination of the two partial trisomies.