Partial trisomy and tetrasomy of chromosome 21 without Down Syndrome phenotype and short overview of genotype-phenotype correlation. A case report

Aims. Trisomy of chromosome 21 is associated with Down syndrome (DS) - the commonest genetic cause of mental retardation. We report two unusual cases with partial trisomy of chromosome 21 and tetrasomy of chromosome 21 without DS phenotype. We include a short overview of the genotype-phenotype correlation studies in discussion. Methods. Conventional chromosomal analysis, fluorescent in situ hybridisation (FISH), quantitative fluorescent PCR (QFPCR) and Nimblegen targeted chromosome 21 array were used for deciphering the genotypes. Results. Conventional chromosomal analysis revealed one extra copy of derivative chromosome 21 in peripheral blood lymphocytes of the patients. FISH and QF PCR analyses identified duplicated loci (D21Z1, D21S1414, D21S1435) spanning from the centromere to band 21q21. Nimblegen targeted chromosome 21 array specified the range of duplication from the centromere to the band 21q21.3 (19 Mb) in the first case and the range of duplication and triplication resp from centromere to the bands 21q21.3 (15 Mb) and 21q11.2 (4 Mb) resp. in the second case. Additional material was of maternal origin in both cases. The different mechanisms led to the formation of the particular chromosomal imbalances. Conclusion. These findings confirm the conclusion of nonpresence of DS when bands 21q22.2 and 21q22.3 (Down critical region) are not duplicated. The patients had nonspecific phenotypes although some of their features such as sandal gaps, joint hyperlaxity, hypotonia and brachycephaly are present in patients with DS. Our observation can help to narrow the region responsible for DS and to map the loci accountable for minor features of DS.