EBP

EBP (emopamil-binding protein) is an X-linked sterol delta8-delta7 isomerase (EC 5.3.3.5) that catalyzes a step in cholesterol biosynthesis, and its loss of function causes X-linked dominant chondrodysplasia punctata type 2 (CDPX2) (PMID:10942423, PMID:11038443). Heterologous expression confirms that EBP possesses both sterol delta8-delta7 isomerase and sigma-ligand-binding activities, with residue Y111 being critical for catalysis: the Y111W substitution reduces activity, while transfer of this residue to the inactive paralog EBPL fails to confer isomerase function (PMID:12760743). EBP can homodimerize (PMID:12760743). Pathogenic nonsense, frameshift, and missense mutations abolish isomerase activity, producing characteristic accumulation of sterol intermediates—dehydrocholesterol and 8(9)-cholestenol—detectable in patient serum (PMID:11038443, PMID:17949453); somatic and gonadal mosaicism for EBP mutations is sufficient to produce the CDPX2 phenotype, including in males (PMID:10942423, PMID:12503102). Beyond the catalytic role of Y111 and the disease-associated sterol defect, no further structural or regulatory mechanism has been characterized in the available corpus.

1. 2000 High

   Established that EBP encodes a sterol delta8-delta7 isomerase whose loss of function is the genetic cause of CDPX2, defining the disease gene and its enzymatic basis.

   Evidence Mutation analysis (nonsense and frameshift variants) of the EBP gene across seven independent families, with demonstration of gonadal and somatic mosaicism

   PMID:10942423

   Open questions at the time

   • Did not directly assay the enzymatic consequence of individual mutations in vitro
   • Genotype-phenotype mechanism for variable severity unresolved
2. 2000 High

   Linked EBP truncating mutations to a biochemical sterol defect, confirming that the disease arises specifically from loss of delta8-delta7 isomerase activity.

   Evidence EBP sequencing in CDP patients combined with sterol profile analysis showing accumulation of dehydrocholesterol and 8(9)-cholestenol

   PMID:11038443

   Open questions at the time

   • Did not establish structural basis of catalysis
   • Missense severity correlation not mechanistically explained
3. 2003 Medium

   Showed that single-gene somatic mosaicism for an EBP mutation is sufficient to produce the CDPX2 phenotype even in males, clarifying how the X-linked dominant condition can manifest in hemizygous individuals.

   Evidence Biochemical sterol analysis and molecular mutation analysis in a single male patient

   PMID:12503102

   Open questions at the time

   • Single case without independent replication
   • Extent of tissue mosaicism not quantified
4. 2003 High

   Resolved the catalytic determinant of EBP by identifying residue Y111 as critical for isomerase activity and distinguishing EBP from its inactive paralog EBPL.

   Evidence Heterologous expression in yeast with isomerase and sigma-ligand binding assays, site-directed mutagenesis (Y111W), chimeric constructs, and cross-linking for homodimerization

   PMID:12760743

   Open questions at the time

   • No high-resolution structure of the active site
   • Mechanism of sigma-ligand binding relative to catalysis unclear
   • Functional significance of homodimerization not defined
5. 2007 Medium

   Validated additional patient mutations as pathogenic by demonstrating the in vivo sterol biochemical signature, reinforcing the enzymatic loss-of-function model.

   Evidence EBP sequencing, gas chromatography-mass spectroscopy of serum sterols, and X-inactivation studies in CDPX2 patients

   PMID:17949453

   Open questions at the time

   • Small patient series from a single lab
   • No in vitro enzymatic assay of the new variants

• The structural basis of EBP catalysis and the physiological role of its sigma-ligand binding and homodimerization remain unresolved.
• No atomic structure of EBP
• Functional purpose of sigma-ligand binding within cholesterol biosynthesis unknown
• Role of homodimerization in activity undefined