Two cases of partial trisomy 21 (pter‐q22.1) without the major features of Down syndrome

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Abstract
We report two cases of partial trisomy 21 with clinical features distinct from Down syndrome (DS). These patients presented with moderate mental retardation and short stature, but the typical facial appearance of DS was not observed. Each patient had a similarly sized extra chromosome 21. We performed FISH analysis to examine whether deletions of reported ∼5 Mb DS critical region (DSCR) might be associated with unusual clinical features in these cases. The results showed that each of their extra chromosomes 21 contained a distal part of chromosome 3p or 14q at the telomeric region of chromosome 21q. The translocation breakpoint of 21q for each patient was located on the centromeric side of DSCR (DSCR was deleted) and the sizes of partial trisomy 21 in respective patients are ∼34.5 (21pter‐q22.12) and ∼33.0 Mb (21pter‐q22.11). In one patient, the additional region of the short arm of chromosome 3 was 3pter‐p26.1 from maternal origin, measuring approximately 9 Mb in size. The second patient had an extra 14q32.1‐qter of maternal origin, measuring approximately 14 Mb in size. These are one of the shortest partial distal trisomy among reported cases. Taken together, two patients with partial trisomy 21 lack all of DSCR on 21q22, and their distinct clinical features are likely caused by the genes located at 21pter‐q22.1 and the distal part of chromosome 3p or 14q.