by Kevin C Zebari

A rare autosomal recessive disorder of genetic (XY) males leads to ambiguous external genitalia and often to an initial misdetermination of sex. This disorder is classified as Psuedohermaphroditism and it can have significant biological and social impact on the individual. It is characterized by a clitoris-like phallus, bifid scrotum, and urogenital sinus with a blind vaginal pouch (Davis, 1992, p.1216). The testes are undescended pre-pubescently, and are usually located in the abdomen, inguinal canals, or scrotal folds. The position of the urethra on these males is located near the location of the normal female urethra (Herdt, 1990, p.436). Psuedohermaphroditism has a high incidence in the Dominican Republic, the focus of many studies to understand and characterize this disease. Implicated in this disorder is a deficiency in the activity of the steroid enzyme, 5-alpha reductase, which converts testosterone to dihydrotestosterone (DHT), an androgen responsible for external male development (Behrman, 1992, p.1468). At fault in this deficiency is a single base-pair substitution in the DNA encoding this enzyme. Internally, affected males have internal male development; testes, wolffian ducts, vas deferens, epididymis and seminal vesicles are all present. At puberty, further male development occurs including enlargement of the penis, descension of the testicles, and a deeper voice (Diamond, 1992, p.76).

The high incidence of the condition in the Dominican Republic kindred, 47 affected males over seven generations, has made them the target of research to further characterize this disorder. The formation of an accurate pedigree to analyze the inheritance pattern of the disease posed difficulty due to its non-expression in females. The fact that all carriers could be traced back to a single female carrier accounts for this conditions high incidence in this small, isolated population (Davis, 1992, p.1217). Phenotypically, affected males were found to have less facial and body hair than normal males, and less acne as well. Two isotypes of the gene encoding the 5-alpha reductase enzyme have been located. The first type is found on chromosome 5 and is usually not present in very significant concentrations in androgen-sensitive tissues. The second isotype is present in androgen-sensitive tissues at higher concentrations than the first type. The second isotype was the target of the substitution and ultimately, the deficiency. Electrophoretic DNA migration patterns of heterozygous parents and their homozygous sons revealed patterns of migration that differed between heterozygous and homozygous (Davis, 1992, p.1218). Various social aspects of the kindred contributed to the prevalence of the disease. The people are socially isolated and marriage between relatives, which serve to amplify expression of rare recessive conditions, are not uncommon (Herdt, 1990, p.435).

The locus of this mutation is on exon 5 of chromosome 2. Using PCR and site-directed mutagenesis, it was determined that the mutation is a substituion of thymidine in place of cytosine that correlates to the production of tryptophan, a bulky, hydrophobic amino acid, instead of arginine, a highly basic amino acid . This results in the deficient activity of the enzyme. Further testing revealed that this mutation was indeed responsible and present in all cases. It appears that the single base substitution of cytosine-guanosine dinucleotides are not uncommon to the human genome. It is usually the result of methylation-induced deamination of 5-methylated cytosine residues. The 246th position of exon 5 encoding the 5-alpha reductase may be a site of high frequency of mutation (Davis, 1992, p.1218-19).

The effects of this base change were determined with biochemical characterization and comparison of the mutants and the normal, wild-type strain. It was determined that the mutant enzyme functioned optimally at a more alkaline pH (5.3 - 5.5) than the wild-type (4.8 - 4.9). The mutant enzyme also failed to reach the velocity of the wild-type enzyme. An important co-factor in the ability of this enzyme, NADPH, is utilized by the normal enzyme at relatively low concentrations (Km = 8 - 13 umol/L) while the mutant type enzyme requires much greater concentrations (Km = 600 - 650 umol/L). The Km values for both normal and mutant enzymes for testosterone were the same (Davis, 1992, p.1219). This data implicated that the deficiency in this enzyme is in itís affinity for itís substrate, which results in the decreased ability to perform.

Studies based on these incidences of Psuedohermaphroditism have offered insights into normal male development, and which androgens and enzymes are responsible for internal and external differentiation and development. The inability of the enzyme to produce significant amounts of DHT from testosterone is the cause of the underdevelopment of external male genitalia, implicating DHTís importance in external development (Behrman, 1992, p.1468). In the absence of DHT, tissue follow the default pathway which leads to female external development. Testosterone levels are usually elevated so injection of testosterone would not alleviate the situation, but DHT supplements during critical periods prenatally could affect the development of the adelphia.

Davis, D., Gautier, T.,Imperato-McGinley, J., Russel, D., & Thigpen, A. (1992, October 22). The Molecular basis of Steroid 5-Alpha Deficiency in a large Dominican Kindred. New England Journal of Medicine, 327:17 pp. 1216 - 1219.

Diamond, J. Turning a Man. Discover, June 1992, volume 13, number 6. pp.70-77.

Herdt, G. Mistaken Gender: 5-Alpha reductase hermaphroditism and biological reductionism in sexual identity reconsidered. American Anthropologist, June 1990, volume 92, pp.433 - 446.

Behrman, R.E. (1992). Psuedohermaphroditism has a high incidence in the Dominican Republic. Nelson Textbook of Pediatrics. pp. 1455-1468. W.B. Saunders Company, Phil.

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