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An extra idic(21)(q22.1) in a child with some features of Down's syndrome
M Gütiérrez-Angulo1, A L Ramos, N Dávalos
1Centro de Investigación Biomédica de Occidente, Instituto Mexicano del Seguro Social, Guadalajara, Jal., Mexico.
Insights
A rare genetic condition involving an extra isodicentric chromosome 21 (idic(21)) in a young boy resulted in some Down syndrome features. This case highlights how duplications in specific chromosome 21 regions can cause certain Down syndrome characteristics.
Area of Science:
- Genetics
- Human Biology
- Chromosomal Abnormalities
Background:
- Down syndrome (DS) is typically caused by trisomy 21.
- The critical region for DS is located on chromosome 21q22.2-q22.3.
- Genetic variations can lead to atypical presentations of DS.
Observation:
- A 30-month-old boy presented with hypotonia, joint hyperlaxity, and other features, alongside a 47,XY,+psu idic(21)(q22.1) karyotype.
- The patient exhibited some, but not all, typical Down syndrome features.
- Phenotypic comparison with two other patients with similar idic(21) imbalances revealed discrepancies.
Findings:
- The patient's phenotype is likely due to disomy for the 21q22.2-q22.3 region, supporting the role of proximal 21q duplications in causing certain DS features.
- Variability in clinical presentation among patients with similar chromosomal imbalances suggests inherent biological variability.
- Notably, none of the three patients with 21q proximal tetrasomy had cardiac defects or multiple DS features linked to distal 21q22 genes.
Implications:
- This case expands the understanding of genotype-phenotype correlations in Down syndrome.
- It underscores that specific chromosomal duplications, not just trisomy, can manifest DS characteristics.
- Further research is needed to delineate the precise genetic contributions to the DS phenotype and its variability.
Abstract:
A 30-month-old boy with mental retardation, hypotonia, joint hyperlaxity, Brushfield spots, open mouth, distal axial triradius t", and ulnar loops on both forefingers was found to have a 47,XY, + psu idic(21)(q22.1).ish psu idic(21)(q22.1)(D13Z1/D21Z1 + + ,ETS2-) karyotype. The patient's phenotype, with only some Down's syndrome (DS) features, is probably related to his disomy for most or all of the critical region 21q22.2 q22.3 and agrees with the current notion that certain DS features may also result from 21q proximal duplications. The phenotypical comparison with 2 other patients with a similar extra idic(21) reveals some discrepancies, which may be related to the inherent clinical variability of similar imbalances: yet, a real difference between the tetrasomic segments cannot be excluded. Noticeably, all 3 patients with 21q proximal tetrasomy did not have cardiac defect and exhibited none or just one out of the five other DS phenotypic features attributed to a single gene or cluster on distal 21q22.