The STAG1 gene (OMIM 604358) is located on chromosome 3q22.3 and encodes cohesin subunit SA-1, a core component of the cohesin complex together with SMC1A, SMC3, and RAD21. Cohesin is crucial for sister chromatid cohesion, DNA repair, and higher-order chromatin organisation, and it plays an important role in the regulation of gene expression during development.
Variant spectrum
Pathogenic STAG1variants reported to date include:
• Heterozygous intragenic deletions (single- or multi-exon)
• Nonsense and frameshift variants predicted to result in premature termination codons
• Essential splice site variants
• Missense variants (usually de novo) affecting conserved residues
• At least one larger 3q22.3 deletion encompassing STAG1
Across the published cohort, the majority of clearly pathogenic variants are predicted loss-of-function (LoF) changes (nonsense, frameshift, canonical splice variants, or intragenic deletions), consistent with STAG1being haploinsufficient.
Recent functional work on a novel frameshift variant (c.500dup; p.Gly168TrpfsTer13) demonstrated reduced STAG1mRNA abundance and markedly decreased STAG1 protein levels in vitro due to nonsense-mediated mRNA decay, providing direct evidence that haploinsufficiency is a key pathogenic mechanism for MRD47.
ClinGen currently considers STAG1 to have sufficient evidence for dosage sensitivity, with haploinsufficiency implicated in autosomal dominant intellectual developmental disorder 47; triplosensitivity is not established.
Missense variants are less common and likely act through LoF or dominant-negative effects on cohesin complex assembly or chromatin binding, although detailed mechanistic data are still limited.
Pathophysiology
Disruption of STAG1-containing cohesin complexes is thought to lead to altered chromatin looping and dysregulated transcription of developmental genes, rather than a primary defect in chromosome segregation. This model is consistent with other cohesinopathies (e.g. Cornelia de Lange syndrome and RAD21-related disorder), where transcriptional dysregulation underlies multi-system developmental anomalies.
Diagnostic testing
• STAG1analysis is typically performed as part of:
o Clinical exome or genome sequencing
o Targeted neurodevelopmental disorder / intellectual disability panels
o Cohesinopathy / Cornelia de Lange–spectrum panels
• Copy-number analysis (e.g. exome- or genome-based CNV calling, chromosomal microarray) is important to detect intragenic deletions or larger 3q22.3 rearrangements involving STAG1.
• Variant interpretation should follow ACMG/AMP guidelines, integrating segregation data, in silico prediction, population frequency, and, when available, RNA or protein functional studies.
At present there is no validated biochemical biomarker for STAG1-related disease; diagnosis relies on molecular genetic testing in the appropriate clinical context.
STAG1