The Down syndrome critical region.

Area of Science:

• Genetics
• Human Biology
• Developmental Biology

Background:

• For decades, research has investigated specific chromosome 21 segments responsible for Down syndrome (DS) phenotypes.
• Early studies focused on the distal long arm (21q22) as the critical region for DS.
• Conflicting definitions of "minimal" or "critical" regions complicated earlier research.

Purpose of the Study:

• To evaluate the evidence for specific chromosomal segments causing Down syndrome.
• To determine if individual loci on chromosome 21 are solely responsible for DS phenotypic abnormalities.
• To understand how aneuploidy influences the expression of multifactorial traits in DS.

Main Methods:

• Review of existing studies on partial trisomy of chromosome 21 and associated clinical diagnoses.
• Analysis of reported cases linking Down syndrome to partial aneuploidy of various chromosome 21 segments.
• Synthesis of evidence regarding the contribution of different chromosome 21 regions to the DS phenotype.

Main Results:

• Down syndrome diagnoses have been reported with partial aneuploidy across nearly all segments of chromosome 21.
• Evidence does not support the hypothesis that individual loci on chromosome 21 are singularly responsible for specific DS abnormalities.
• Each clinical finding associated with Down syndrome is a multifactorial trait.

Conclusions:

• The clinical condition of Down syndrome is not attributable to a single critical region or specific loci on chromosome 21.
• The DS phenotype results from the complex interplay of multiple genetic factors and aneuploidy.
• Analysis of DS traits should adopt a multifactorial approach, similar to studying these traits in the general population.