General information
The SMAD4gene, located on chromosome 18q21.2, contains 11 exons and encodes the Mothers against decapentaplegic homolog 4 protein. The SMAD4 protein plays a role in the signal transduction pathways of the transforming growth factor-beta (TGFβ1) superfamily and bone morphogenetic proteins (BMP1) by facilitating the transcriptional activation of downstream target genes. Heterozygous variants c.1498A>G (p.Ile500Val) and c.1486C>T (p.Arg496Cys) are the most commonly detected pathogenic gain-of-function variants, with detection rates in patients being 50% and 40%, respectively.
Pathophysiologic mechanism
Some studies indicate enhanced TGFβ signaling due to gain-of-function effects; however, other research supports a dominant-negative mechanism that disrupts normal TGFβ and BMP signaling pathways. The discrepancies in these findings have not yet been fully resolved.
Genotype-phenotype correlation
There is no clear genotype–phenotype correlation. The accumulating data suggest genotype-phenotype correlation: Individuals carrying the c.1498A>G (p.Ile500Val) pathogenic variant tend to present with growth deficiency and severe aortic obstruction more often. Those with the c.1486C>T (p.Arg496Cys) pathogenic variant appear to have a higher likelihood of experiencing seizures and precocious puberty (especially in females).
Genetic testing
Since all reported variants are single nucleotide changes and due to the gain-of-function mechanism, sequencing analysis of the SMAD4gene is sufficient. If clinical suspicion is strong, genetic testing can be planned either as a single-gene test. However, it should be emphad that most patients were diagnosed through whole-exome sequencing.
SMAD4