X-linked Adrenal Hypoplasia Congenita
X-linked adrenal hypoplasia congenita (X-linked AHC) is characterized by primary adrenal insufficiency and/or hypogonadotropic hypogonadism (HH).
Adrenal insufficiency has acute infantile onset (average age 3 weeks) in approximately 60% of affected males and childhood onset (ages 1-9 years) in approximately 40% [Peter et al 1998, Reutens et al 1999, Guran et al 2016].
HH typically manifests in a male with adrenal insufficiency as delayed puberty (i.e., onset age >14 years) and less commonly as arrested puberty at about Tanner Stage 3. Rarely, the initial manifestation of X-linked AHC occurs in early adulthood with a primarily reproductive phenotype (e.g., infertility) [Tabarin et al 2000, Mantovani et al 2002].
Intrafamilial variability in age of onset occurs [Wiltshire et al 2001]; however, in families in which two brothers are affected, the younger one is usually diagnosed earlier as clinical suspicion is greater [Achermann et al 2001].
Adrenal insufficiency. The initial clinical presentation is typically acute, especially in infants, with vomiting, feeding difficulty, dehydration, and shock caused by a salt-wasting episode. In some instances hypoglycemia, frequently presenting with seizures or mineralocorticoid deficiency, may be the presenting manifestation of adrenal insufficiency [Wiltshire et al 2001, Verrijn Stuart et al 2007].
In older children, adrenal insufficiency may be precipitated by intercurrent illness or stress.
If untreated with glucocorticoids and mineralocorticoids, adrenal insufficiency is rapidly lethal as a result of hyperkalemia, acidosis, hypoglycemia, and shock. If not recognized and treated in children, acute adrenal insufficiency and its complications of hypoglycemia and shock may result in neurologic abnormalities and developmental delay.
In rare instances delayed-onset adrenal insufficiency becomes evident in early adulthood [Mantovani et al 2002, Ozisik et al 2003, Guclu et al 2010, Kyriakakis et al 2017]. In these individuals residual glucocorticoid and mineralocorticoid activity in the hypoplastic adrenal cortex may explain the late onset. These individuals may not have overt adrenal insufficiency, but rather biochemical evidence of compensated adrenal insufficiency (e.g., high serum ACTH concentration). In some instances progressive adrenal insufficiency occurs, resulting in clinically significant adrenal insufficiency in early adulthood.
Adrenal insufficiency is usually accompanied by varying degrees of hyperpigmentation caused by increased pituitary production of POMC (proopiomelanocortin); the original report of X-linked AHC described an affected newborn with "coal-black hyperpigmentation" of the skin sparing the palms and soles [Sikl 1948]. Hyperpigmentation present at the time of diagnosis typically regresses over time with appropriate steroid therapy.
Hypogonadotropic hypogonadism (HH) is of mixed hypothalamic and pituitary origin. The "mini puberty" of infancy is normal in boys with X-linked AHC, suggesting that the loss of function of the hypothalamic-pituitary-gonadal axis occurs after early infancy. Although reported in several instances, cryptorchidism has not emerged as a common feature of X-linked AHC. Indeed, macrophallia (large penis) at birth and signs of early puberty in childhood are now increasingly reported in some boys with X-linked AHC [Domenice et al 2001, Landau et al 2010, Durmaz et al 2013]. Of note, one boy with central precocious puberty and normal adrenal function has recently been shown to have a pathogenic variant in NR0B1 [Shima et al 2016].
Typically, HH is manifest in affected males as delayed puberty (onset age >14 years). In addition, a proportion of males may experience pubertal arrest, i.e., they enter puberty normally and progress to about Tanner Stage 3 (or testicular volume of 6-8 cc) after which pubertal development ceases. Without testosterone treatment, full attainment of secondary sexual characteristics is unlikely.
Males with classic X-linked AHC typically have azoospermia and are infertile despite treatment with exogenous gonadotropin therapy or pulsatile gonadotropin-releasing hormone (GnRH) [Seminara et al 1999, Mantovani et al 2006]. Although some men with X-linked AHC have oligospermia, progressive decline in spermatogenesis may occur with time [Tabarin et al 2000, Raffin-Sanson et al 2013].
Other. In one male with a pathogenic variant in NR0B1, tall stature and renal ectopy were associated with adrenal insufficiency [Franzese et al 2005].
Progressive high-frequency sensorineural hearing loss starting at about age 14 years was described in two individuals with X-linked AHC whose NR0B1 status is unknown [Zachmann et al 1992, Liotta et al 1995]. To the authors' knowledge no other individuals with hearing loss and classic X-linked AHC have been reported; thus, it is increasingly unlikely that hearing loss is an associated feature.
Heterozygous females may very occasionally have manifestations of adrenal insufficiency or hypogonadotropic hypogonadism, potentially caused by skewed X-chromosome inactivation.
A heterozygous female with extreme pubertal delay has been described [Seminara et al 1999].
In one family with affected males, a female homozygous for an NR0B1 pathogenic variant had isolated hypogonadotropic hypogonadism [Merke et al 1999].
Xp21 Deletion
The phenotypes resulting from deletion of some of the genes in the Xp21 region are X-linked AHC (deletion of NR0B1), glycerol kinase deficiency (deletion of GK), and Duchenne muscular dystrophy (deletion of DMD).
The clinical findings in glycerol kinase deficiency vary and can include metabolic crisis during starvation, hypoglycemia, seizures, growth restriction, and developmental delay.
Developmental delay has been reported in males with Xp21 deletion when the deletion extends proximally to include DMD or when larger deletions extend distally to include IL1RAPL1 and DMD [Zhang et al 2004].
IL1RAPL1 deletions are associated with global developmental delay / intellectual disability and, sometimes, autistic spectrum disorder.
Heterozygous females. A female with mild adrenal failure and Duchenne muscular dystrophy was reported [Shaikh et al 2008]. Molecular studies confirmed extremely skewed X-chromosome inactivation in the region of Xp21 that resulted in preferential expression of the abnormal allele.
Two girls reported with developmental delay and myopathy due to an Xp21 deletion involving DMD, GK, NR0B1, and IL1RAPL1 did not have adrenal dysfunction [Heide et al 2015].