title: A missense mutation in the human cytochrome b5 gene causes 46,XY disorder of sex development due to true isolated 17,20 lyase deficiency.
abstract: BACKGROUND
  Isolated 17,20 lyase deficiency is commonly defined by apparently normal 17α-hydroxylase activity but severely reduced 17,20 lyase activity of the bifunctional enzyme cytochrome P450 (CYP) enzyme 17A1 (CYP17A1), resulting in sex steroid deficiency but normal glucocorticoid and mineralocorticoid reserve. Cytochrome b5 (CYB5A) is thought to selectively enhance 17,20 lyase activity by facilitating the allosteric interaction of CYP17A1 with its electron donor P450 oxidoreductase (POR).
  
  
  OBJECTIVE
  We investigated a large consanguineous family including three siblings with 46,XY disorder of sex development (DSD) presenting with isolated 17,20 lyase deficiency.
  
  
  METHODS
  We investigated the clinical and biochemical phenotype, conducted genetic analyses, and functionally characterized the identified CYB5A mutation in cell-based CYP17A1 coexpression assays.
  
  
  RESULTS
  All three siblings presented with 46,XY DSD, sex steroid deficiency, normal mineralocorticoids and glucocorticoids, and a urine steroid metabolome suggestive of isolated 17,20 lyase deficiency. CYP17A1 and POR sequences were normal, but we detected a homozygous CYB5A missense mutation (g.28,400A→T; p.H44L). Functional in vitro analysis revealed normal CYP17A1 17α-hydroxylase activity but severely impaired 17,20 lyase activity. In silico analysis suggested the disruption of CYB5A heme binding by p.H44L.
  
  
  CONCLUSIONS
  We have identified the first human CYB5A missense mutation as the cause of isolated 17,20 lyase deficiency in three individuals with 46,XY DSD. Detailed review of previously reported cases with apparently isolated 17,20 lyase deficiency due to mutant CYP17A1 and POR reveals impaired 17α-hydroxylase activity as assessed by steroid metabolome analysis and short cosyntropin testing. This suggests that truly isolated 17,20 lyase deficiency is observed only in individuals with inactivating CYB5A mutations.
authors.affiliation: Centre for Endocrinology, Diabetes, and Metabolism, School of Clinical and Experimental Medicine, University of Birmingham, Birmingham B15 2TT, United Kingdom.
authors.forename: Jan
authors.initials: J
authors.lastname: Idkowiak
authors.forename: Tabitha
authors.initials: T
authors.lastname: Randell
authors.forename: Vivek
authors.initials: V
authors.lastname: Dhir
authors.forename: Pushpa
authors.initials: P
authors.lastname: Patel
authors.forename: Cedric H L
authors.initials: CH
authors.lastname: Shackleton
authors.forename: Norman F
authors.initials: NF
authors.lastname: Taylor
authors.forename: Nils
authors.initials: N
authors.lastname: Krone
authors.forename: Wiebke
authors.initials: W
authors.lastname: Arlt
mesh_terms: D000293:Adolescent; D000312:Adrenal Hyperplasia, Congenital / Q000201:enzymology / Q000235:genetics*; D002648:Child; D002675:Child, Preschool; D015786:Cytochromes b5 / Q000235:genetics* / Q000378:metabolism; D058490:Disorder of Sex Development, 46,XY / Q000201:enzymology / Q000235:genetics*; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D020125:Mutation, Missense; D013254:Steroid 17-alpha-Hydroxylase / Q000378:metabolism
chemical_list: D015786:Cytochromes b5; C501892:CYP17A1 protein, human; D013254:Steroid 17-alpha-Hydroxylase