MED12 mutations link intellectual disability syndromes with dysregulated GLI3-dependent Sonic Hedgehog signaling

Abstract

Recurrent missense mutations in the RNA polymerase II Mediator subunit MED12 are associated with X-linked intellectual disability (XLID) and multiple congenital anomalies, including craniofacial, musculoskeletal, and behavioral defects in humans with FG (or Opitz-Kaveggia) and Lujan syndromes. However, the molecular mechanism(s) underlying these phenotypes is poorly understood. Here we report that MED12 mutations R961W and N1007S causing FG and Lujan syndromes, respectively, disrupt a Mediator-imposed constraint on GLI3-dependent Sonic Hedgehog (SHH) signaling. We show that the FG/R961W and Lujan/N1007S mutations disrupt the gene-specific association of MED12 with a second Mediator subunit, CDK8, identified herein to be a suppressor of GLI3 transactivation activity. In FG/R961W and Lujan/N1007S patient-derived cells, we document enhanced SHH pathway activation and GLI3-target gene induction coincident with impaired recruitment of CDK8 onto promoters of GLI3-target genes, but not non-GLI3-target genes. Together, these findings suggest that dysregulated GLI3-dependent SHH signaling contributes to phenotypes of individuals with FG and Lujan syndromes and further reveal a basis for the gene-specific manifestation of pathogenic mutations in a global transcriptional coregulator.

Yin and yang of mediator function revealed by human mutants

“…GLI3, a TF whose activity is regulated by SHH signaling, was shown earlier by the same group to bind directly to MED12 (20). Zhou et al (6) realized that several of the phenotypes of the FG and Lujan syndromes, such as corpus callosum defects, are also observed in patients with mutations in GLI3. Consequently, they analyzed the impact of the FG and Lujan MED12 mutations on transcription of genes activated by SHH signaling and on the association of CDK8 with several promoters through ChIP assays.…”

“…Consequently, they analyzed the impact of the FG and Lujan MED12 mutations on transcription of genes activated by SHH signaling and on the association of CDK8 with several promoters through ChIP assays. They find that GLI3 target genes, but not genes regulated by several other signal transduction pathways, are greatly overexpressed in response to SHH signaling in cells from patients with FG and Lujan syndrome (6). ChIP assays showed that CDK8 was depleted at GLI3-regulated promoters, but not at other promoters.…”

“…We can hope that further mapping of human mutations resulting in developmental abnormalities will uncover additional mutations in Mediator subunits, and that careful analysis of the mutant Mediators will be as revealing about unexpected mechanisms of Mediator function as the FG and Lujan syndrome MED12 mutants have been (6,8). References and in-depth discussion are provided by Spaeth et al (19).…”

“…Zhou et al (6) report that the MED12 amino acid substitutions associated with FG and Lujan syndromes also lead to misregulation by the GLI3 TF in response to SHH signaling. MED12 is a component of a four-subunit Mediator "kinase module" including MED12, MED13, CDK8, and cyclin C. CDK8, the protein kinase subunit of the module, interacts with MED12.…”

“…1). In PNAS, Zhou et al (6) report a significant advance in understanding how mutations in one Mediator subunit, MED12, result in two types of X-linked intellectual disability known as FG and Lujan syndromes. Studies of the mutant Mediators reveal how MED12 interactions with different components of the transcriptional machine produce opposing activating and inhibitory influences necessary for the proper level of transcription required for normal development.…”