Re: How did the male pattern baldness gene come to be?

The Male pattern baldness phenotype involves an enzyme called 5-alpha 
reductase(Randall, 1994).  This enzyme is involved in the the conversion of 
Testosterone to 5-dihydrotestosterone(DHT) which subsequently binds to 
Androgen receptors regulating gene expression within the cell.  So 
essentially the male pattern baldness phenotype that you are asking about 
is caused by the activity of the 5-alpha reductase enzyme. The blockage of 
the 5-alpha reductase activity is one of the strategies currently being 
investigated for the treatment of male pattern baldness (see Liao, 1994 for 
review).

The conversion of Testosterone to DHT is catalysed by two different 5-alpha 
reductase isozymes in a tissue specific and development-dependent 
manner(Thigpen et al, 1993).  These isozymes are referred to as Type I and 
Type II.  It is the expression of the Type II 5-alpha reductase expression 
which appears to be responsible for male pattern baldness (Thigpen et al, 
1993).  Mapping of the 5-alpha reductase Type I gene using in situ 
hybridisation has identified the type 1 gene on 5p15 (ie the short arm of 
chromosome 5) (see Morisette et al, 1996).  So this is really the gene that 
you ask for in your question, the Male pattern baldness gene as it were.

Carey et al (1994) have identified another gene, referred to as CYP17, 
which appears to have some role in the development of polycystic 
ovaries(PCO) and the male pattern baldness (MPB)  phenotypes.  The results 
published by Carey et al(1994) are based on a study carried out on 14 
Caucassian families.  The observed phenotype has an autosomal dominant mode 
of inheritance (only one copy of the defective gene is required in order to 
produce the phenotype).  What Carey et al(1994) have found is that there is 
a statistically significant association between a specific mutation in the 
CYP17 gene and the MPB/PCO phenotypes.  I am not sure how much you know 
about genetics but the identified mutation involves a base change that 
produces an additional promoter site and that this increases the expression 
of the CYP17 gene.  Whilst this gene is associated with the MPB/PCO 
phenotypes it is not considered by Carey et al(1994) to be the primary 
genetic determinant.  This conclusion is based on the fact that the 
mutation does not co-segregate with the phenotypes (i.e- it is not present 
in all affected individuals).

I hope that this answers your question.  Do not hesitate to contact me if 
there is anything that is unclear (ie-terminology etc.  I will be most 
happy to answer any questions).


				Best wishes,


					Robert Deyes

Carey AH, Waterworth D, Patel K, White D, Little J, Novelli P, Franks S, 
Williamson R
Polycystic ovaries and premature male pattern baldness are associated with 
one allele
of the steroid metabolism gene CYP17.
Hum Mol Genet 1994 Oct;3(10):1873-1876 

Liao S
Androgen action: molecular mechanism and medical
application.
J Formos Med Assoc 1994 Sep;93(9):741-751

Morissette J, Durocher F, Leblanc JF, Normand T, Labrie F, Simard J
Genetic linkage mapping of the human steroid 5
alpha-reductase type 2 gene (SRD5A2) close to D2S352 on
chromosome region 2p23-->p22.
Cytogenet Cell Genet 1996;73(4):304-307 

Randall VA
Role of 5 alpha-reductase in health and disease.
Baillieres Clin Endocrinol Metab 1994 Apr;8(2):405-431 

Thigpen AE, Silver RI, Guileyardo JM, Casey ML, McConnell JD, Russell DW
Tissue distribution and ontogeny of steroid 5 alpha-reductase isozyme
expression.
J Clin Invest 1993 Aug;92(2):903-910