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5-alpha-reductase deficiency
This condition primarily impacts individuals with a Y chromosome, leading to differences in how their external genitalia develop. It is caused by genetic changes that disrupt the production of a key hormone involved in male sex differentiation.
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Overview
5-alpha-reductase deficiency is a rare inherited condition that influences sex development, specifically affecting individuals who are chromosomally male (XY). This condition is characterised by the body's inability to properly convert testosterone into dihydrotestosterone (DHT) [PMID:33678070]. DHT is a powerful male hormone (androgen) that plays a crucial role in forming external male genitalia during foetal development and in driving male secondary sexual characteristics during puberty. Because of this inability, individuals can be born with external genitalia that are ambiguous or appear more typically female. The internal reproductive organs, such as the testes, usually develop normally.
Symptoms & clinical features
The symptoms of 5-alpha-reductase deficiency vary widely, even within the same family [PMID:24647359]. At birth, individuals often have external genitalia that range from a typical female appearance with a blind-ending vaginal pouch and a small, clitoris-like phallus, to ambiguous genitalia, or, in some rarer cases, even a more typically male appearance with an underdeveloped penis. Internally, testes are usually present in the abdomen or groin. At puberty, the testes typically produce enough testosterone to trigger some masculinisation, such as deepening of the voice, muscle development, and often enlargement of the phallus. However, features like receding hairline or significant body hair growth are usually less pronounced due to the continued DHT deficiency [PMID:33678070]. Fertility is generally impaired.
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Affected organs
The primary organs affected are those involved in sex development, particularly the external genitalia. This includes the penis, scrotum, and urethra. While internal male reproductive organs, such as the testes, usually develop without issue, their normal function can lead to hormonal changes at puberty. The pituitary gland and hypothalamus are also involved in the broader endocrine system regulating sex hormones, but they are not directly damaged by this condition.
Risks & severity
The severity of 5-alpha-reductase deficiency can vary significantly, from individuals with almost typical male external genitalia to those with external genitalia initially assigned as female. Individuals are typically infertile. There are no direct life-threatening risks associated with the condition itself, but psychological and social challenges related to gender identity and sex assignment can be substantial. The condition is usually identified at birth due to atypical genitalia or later at puberty if masculinisation occurs in someone raised as female. Long-term health considerations may include bone health issues if hormone levels are not balanced [PMID:24647359].
Genetic causes
5-alpha-reductase deficiency is caused by pathogenic variants in the SRD5A2 gene. This gene provides instructions for making an enzyme called steroid 5-alpha-reductase type 2. This enzyme is crucial for converting testosterone, a male sex hormone, into its more potent form, dihydrotestosterone (DHT). DHT is essential for the development of external male genitalia during early foetal development and for many male secondary sexual characteristics that typically emerge during puberty. When variants in SRD5A2 reduce or eliminate the function of this enzyme, the body cannot produce enough DHT, leading to the characteristic features of this condition [PMID:33990425].
- SRD5A2 steroid 5 alpha-reductase 2
Inheritance pattern
This condition has an autosomal recessive inheritance pattern. This means that a person must inherit two copies of the altered gene - one from each parent - to develop the condition. Individuals who inherit only one copy of the altered SRD5A2 gene are called carriers. Carriers typically do not show symptoms of the condition themselves but can pass the altered gene on to their children. If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit two altered copies and develop 5-alpha-reductase deficiency.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of 5-alpha-reductase deficiency often begins with the clinical observation of ambiguous genitalia at birth or atypical pubertal development. Hormonal studies, such as measuring testosterone and DHT levels and their ratio, can provide strong indications of the condition. Confirmatory diagnosis is achieved through genetic testing of the SRD5A2 gene. In the UK, genetic testing for disorders of sex development falls under the NHS Genomic Medicine Service and is typically initiated following a referral to a clinical genetics service or specialist paediatric endocrinologist. Relevant R-codes for genetic testing may include R148 for disorders of sex development.
Management & lifestyle
Management of 5-alpha-reductase deficiency involves a multidisciplinary team approach, often including endocrinologists, geneticists, surgeons, and psychological support. Decisions about sex assignment and future interventions must be made carefully, considering the potential for pubertal virilisation and individual preferences. Hormone replacement therapy may be offered to promote masculine development at puberty or to support bone health. Surgical interventions to normalise genital appearance may also be considered, though timings and types of surgery are individualised decisions taken in discussion with clinical teams. Regular monitoring of hormones and physical development is an ongoing part of care within the NHS framework.
UK care pathway
In the UK, individuals and families affected by rare genetic conditions like 5-alpha-reductase deficiency are typically supported through the NHS Genomic Medicine Service. A referral to a clinical genetics specialist or a paediatric endocrinology team is the usual starting point. These specialists can offer accurate diagnosis, genetic counselling, and coordination of care. Genetic counsellors play a vital role in explaining the condition, its inheritance, and discussing implications for other family members. Diagnostic genetic testing often falls under specific NHS Genomic R-codes, such as those for disorders of sex development (e.g., R148).
Frequently asked questions
Can individuals with 5-alpha-reductase deficiency have children?
Individuals with 5-alpha-reductase deficiency are typically infertile. While they usually have functioning testes, the deficiency of DHT means that sperm production is severely affected or absent, making natural conception unlikely.
Is this condition more common in certain ethnic groups?
While 5-alpha-reductase deficiency is rare globally, some studies suggest a higher prevalence in specific populations, such as communities in certain regions of the Dominican Republic, Papua New Guinea, and Turkey, due to founder effects [PMID:33678070].
What is the typical age for diagnosis?
Diagnosis often occurs at birth due to the appearance of ambiguous external genitalia. However, in cases where the external genitalia are more typically female, diagnosis might be delayed until puberty when spontaneous masculinisation occurs, or if infertility is investigated later in life.
Is there a cure for 5-alpha-reductase deficiency?
There is no cure for the underlying genetic cause of 5-alpha-reductase deficiency. However, various medical and surgical interventions can help manage the symptoms and support individuals, such as hormone therapy to promote masculine development or address bone health concerns.
Can genetic testing identify carriers of this condition?
Yes, genetic testing can identify carriers of 5-alpha-reductase deficiency. If you have a family history of the condition, or if you and your partner are planning a family and are concerned, you can discuss carrier testing with a genetic counsellor or specialist.
References
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- Avendaño A, Paradisi I, Cammarata-Scalisi F. 5-α-Reductase type 2 deficiency: is there a genotype-phenotype correlation? A review. Hormones (Athens, Greece). 2018. PMID: 29858846
- Okeigwe I, Kuohung W. 5-Alpha reductase deficiency: a 40-year retrospective review. Current opinion in endocrinology, diabetes, and obesity. 2014. PMID: 25321150
- Imperato-McGinley J. 5 alpha-reductase deficiency: human and animal models. European urology. 1994. PMID: 8287893
- Biglieri EG, Kater CE. 17 alpha-hydroxylation deficiency. Endocrinology and metabolism clinics of North America. 1991. PMID: 1879398
- Randall VA. Role of 5 alpha-reductase in health and disease. Bailliere's clinical endocrinology and metabolism. 1994. PMID: 8092979
- Alswailem MM, Alzahrani OS, Alghofaili L. Molecular genetics and phenotype/genotype correlation of 5-α reductase deficiency in a highly consanguineous population. Endocrine. 2019. PMID: 30269266