21 Hydroxylase Deficiency

Adrenal glands are organs that produce the hormones cortisol, aldosterone, and androgens (delta4-androstenedione and dehydroepiandrosterone). Under certain conditions, the adrenal enzymes –substances needed for the production of the aforementioned hormones– do not work properly. To compensate for this, adrenal glands may increase in volume (adrenal hyperplasia). This mechanism successfully produces some hormones, but can lead to an excess production of others. In the adrenal glands, the enzyme 21 hydroxylase (21OH) is essential for the production of cortisol and aldosterone. The malfunctioning of this enzyme creates a deficit in the production of cortisol and aldosterone, and an increase in the levels of 17hydroxy-progesterone (17OHP) and androgens. 21-hydroxylase deficiency (21OHD) is the most common form of Congenital Adrenal Hyperplasia (CAH), accounting for 95% of cases of Variations of Sex Characteristic (VSCs)/Differences of Sex Development (DSDs). Depending on the extent of the enzyme defect, 21OHD is classified into the classic “salt-wasting” or “simple virilizing” forms or the non-classic form. In the classic forms where the enzyme is practically inactive, the lack of cortisol and aldosterone causes a severe adrenal crisis in the newborn, characterized by dehydration and loss of sodium (hence the name “salt-wasting”). If untreated, shock occurs and leads to death in the first two weeks of life. In the “simple virilizing” form, the activity of the enzyme is greatly reduced, but it produces sufficient levels of cortisol and aldosterone. An adrenal crisis is avoided, but there is an excessive production of androgens which leads to the virilization of the external genitalia, i.e., a typically male appearance of the genitals in individuals with a typically female karyotype (46, XX). This mechanism also occurs in the salt-wasting form. Specifically, newborns with classic forms and karyotype 46,XX can present typically male genitalia but a scrotum without testicles, or atypical external genitalia (not ascribable to those typically considered male or female); the internal reproductive organs are typically female with functioning ovaries. The newborn with a 46,XY karyotype has typically male external genitalia and internal reproductive organs.
In the non-classic forms of 21OHD, the residual activity of the enzyme is such that the production of aldosterone and cortisol is preserved or only minimally reduced, so much so as to be completely asymptomatic. Androgen excess is very slight, so that at birth the external genitalia are typically female in people with karyotype 46,XX. For this reason, this condition is rarely identified in the newborn, more so in individuals with a 46,XY karyotype, who show no sign of hormonal deficiency or excess.
During childhood, the excess of male hormones produced by the adrenal glands can determine the premature appearance of pubic and axillary hair in individuals of both chromosomal sexes, and in some cases also the first signs of puberty. Adult height may turn out to be lower than expected. Individuals with karyotype 46,XX usually have a spontaneous first menstrual period, but they often show acne, abundant body hair, androgenetic alopecia (hair loss), and their menstrual cycle tends to be more irregular than in people without this enzyme deficiency. In people with a 46,XY karyotype, testicular volume may be small, and a specific form of benign testicular tumour may appear.
The classic form of 21OHD has a prevalence of approximately 1:14000-1:18000; the non-classic form has a prevalence of about 1:200 in the Caucasian population, but being a mostly asymptomatic condition, many cases are likely to pass unidentified.

Genetic traits
21OHD results from the mutation of the CYP21A2 gene, located on chromosome 6. It is considered one of the most common autosomal recessive genetic conditions: this means that for the variation of the gene to occur, it must have been inherited from both parents. Individuals who have inherited the gene variation from only one parent will not exhibit it in any way, although they can pass it on to their offspring.

Identification
Since the non-classic form of 21OHD is very common and often asymptomatic, newborns are screened for 17OHP in blood in the days immediately after birth. If 17OHP levels are high, another test (the ACTH test) is done; if positive, genetic testing is performed to confirm the presence of 21OHD, and establish the extent of the deficiency based on the specific gene variation. At this stage, it is also advisable to conduct other hormonal assays and genetic tests to distinguish 21OHD from other, much rarer, forms of congenital adrenal hyperplasia. The same classification can be made if the condition is suspected later in life, for example following the identification of atypical genitalia. Individuals with non-classic forms often come to medical attention after infancy, which is why this form was once termed “late-onset”. In these cases, the first evaluation follows the early appearance of pubic and axillary hair or signs of puberty; in adults, irregular menstrual cycles, infertility, or signs of excess male hormones (acne, abundant body hair) in people with karyotype 46,XX. The classification requires hormonal testing –in particular, a markedly high value of 17OHP is indicative of the condition– as well as genetic analysis.

Medical options
According to scientific literature, karyotype 46,XX newborns with 21OHD and atypical external genitalia are generally assigned to the female gender at birth, and later tend to affirm a female gender identity. Obviously, only the individual can express their gender identity, which may or may not be congruent with the gender assigned at birth. Therefore, the most recent recommendations support the need to postpone partially reversible or irreversible interventions (including any kind of surgery to feminize genitals) to when the person can express their informed consent. Where the presence of atypical genitalia represents a source of distress for the person or the family, multidisciplinary professional support, and counselling should be offered: it can improve the psychosocial well-being of the person, and help them explore their wishes and needs related to possible gender-affirming treatments. This includes feminization surgery of the external genitalia, to be performed by a team of expert surgeons. If the person assigned to the female gender at birth has a male gender identity, they can undergo, if they so desire, a gender affirmation medical and/or surgical procedure exactly like people without 21OHD.
The classic forms of 21OHD generally require replacement therapy for the deficient hormones. First, treatment with glucocorticoids is necessary, not only to replace the needed hormones, but also to reduce the production of excess androgens. On the other hand, this treatment must be carefully balanced because excessive glucocorticoid therapy during childhood and puberty can impair height and bone health. Furthermore, at least in the first year of life, treatment with fludrocortisone is typically necessary to compensate for the deficiency in aldosterone. In classic salt-wasting forms, the early detection of this condition and subsequent replacement treatment for deficient hormones is a medical emergency because it can lead to infant death if untreated.
The use of glucocorticoids in children and adolescents with non-classic forms is also common. They might not need replacement treatment, but require glucocorticoids to avoid excessive androgens, which could result in early puberty or a final adult height lower than expected. In people with classic forms of 21OHD, replacement treatment is usually maintained for life. In people with non-classic forms, an individual assessment is necessary once they reach adulthood. For example, some people with karyotype 46,XX continue treatment with glucocorticoids to improve fertility or reduce signs of excess androgens, while other people may not need them.
People with 21OHD can exhibit varying degrees of reduced fertility. In people with karyotype 46,XY and 21OHD, fertility may be impaired due to the presence of benign tumours in the testicle, as well as excessive androgens. In subjects with karyotype 46,XX and 21OHD, reduced fertility can be associated with excess androgens, and is generally proportional to the degree of hormone deficiency. However, the scientific literature reports spontaneous conception in most cases.
In any case, adequate glucocorticoid treatment enhances fertility by suppressing excessive androgen production by the adrenal glands. If this approach is unsuccessful, medically assisted procreation procedures can be implemented. During pregnancy, the excess of androgens in the mother does not influence the development of the fetus. Thus, pre-natal genetic counselling is important to assess the risk of transmission of 21OHD to the offspring, so as to promptly identify the signs of an adrenal crisis in the newborn.

Bibliography
Claahsen-van der Grinten HL et al. MANAGEMENT OF ENDOCRINE DISEASE: Gonadal dysfunction in congenital adrenal hyperplasia. [Abstract]. European journal of endocrinology. 2021 Mar;184(3):R85-R97
Claahsen-van der Grinten HL, et al. Congenital Adrenal Hyperplasia-Current Insights in Pathophysiology, Diagnostics, and Management. Endocrine reviews. 2022 Jan 12;43(1):91-159
El-Maouche D, Arlt W, Merke DP. Congenital adrenal hyperplasia. [Abstract]. Lancet. 2017 Nov 11;390(10108):2194-2210. Epub 2017 May 30. Erratum in: Lancet. 2017 Nov 11;390(10108):2142

Further Links
NHS. Great Ormond Street Hospital for Children. Congenital adrenal hyperplasia (CAH) 
Orphanet. Iperplasia surrenalica congenita classica da deficit di 21-idrossilasi