The ACTH receptor is a seven transmembrane domain receptor encoded by the MC2R gene, which maps onto the short arm of chromosome 18 and consists of 2 exons, the first of which is non-coding/untranslated whilst the latter encodes the full sequence of the receptor. Following isolation and sub-cloning of the human ACTH receptor in 1992 (11), the first reported kindreds with primary adrenal insufficiency secondary to defects in MC2R were characterised by Clark et al. and Tsigos et al. in 1993 (12, 13). Since these initial reports, around 48 mutations have been described in association with FGD (Human Gene Mutation Database http://www.hgmd.cf.ac.uk) (14), the majority of which are missense/nonsense variants ( Figure 2 ) in addition to small genomic deletions and insertions. Variants in MC2R account for about 25% of FGD cases, the majority of which are non-conservative, single amino acid substitutions that likely abrogate cyclic adenosine monophosphate (cAMP) generation and impair receptor trafficking (17–19).

Classically in FGD Type 1, patients are mineralocorticoid replete however, in clinical practice, the picture may be less unambiguous. Several reports have alluded to mild disruptions of the renin-angiotensin-aldosterone axis in patients with MC2R defects, in whom early glucocorticoid replacement may mask aldosterone insufficiency (20, 21). Furthermore, several patients with homozygous frameshift truncating variants have presented in the neonatal period with transient salt wasting (20). Interestingly Mc2r^-/- mice demonstrate reduced serum aldosterone levels (22). This disparity may reflect the fact that the majority of human subjects harbour missense mutations that retain some degree of enzyme activity. Patients also invariably present with early and significant hyperpigmentation due to the action of markedly increased ACTH on the Melanocortin 1 receptor (MC1R). This association was clearly demonstrable in a patient with FGD harbouring homozygous missense variants in both MC2R and MC1R and lacking the ‘classic’ hyperpigmented phenotype (23).

An often overlooked but increasingly recognised feature of FGD Type 1 is hypothyroidism (18, 24–29). Patient phenotypes range from transient neonatal hypothyroidism that normalise with short term thyroxine replacement to subclinical hypothyroidism and persistent thyroid hypo-function (28, 29). Despite assertions that elevated ACTH levels may inhibit thyroid stimulating hormone release (30), the exact mechanism underlying the incidence of hypothyroidism in patients with FGD remain to be elucidated. Interestingly, another feature recognised in patients with ACTH receptor defects is reduced adrenal androgen production or lack of adrenarche. Weber et al. demonstrated consistently sub-optimal serum DHEAS levels in 6 patients with MC2R variants (31). This model of ‘functional’ ACTH deficiency suggests that ACTH at least partially regulates adrenarche given that patients with central ACTH deficiency (hypopituitarism) also exhibit low levels of adrenal androgens (31–34).

Tall stature is an inconsistent but historically common feature of FGD type 1 despite an unaffected Growth hormone-IGF-1 axis (35). Although the exact mechanism underlying this phenotype is unclear, it is likely theorised to be due to the sustained effect of markedly elevated Adrenocorticotropin levels on the growth plate (35, 36). This also correlates to a later median age at diagnosis (2.0 years) when compared to FGD Type 2 (37). ACTH has been shown in vitro to increase rat chondrocyte progenitor cell proliferation and matrix production (38). Interestingly, growth trajectories return to normal once glucocorticoid treatment is instituted. Mc2r knockout mice do not recapitulate this trait and demonstrate similar body lengths to wild-type littermates (22). There is however little genotype phenotype correlation and disease severity and onset is highly variable (39).

After initial characterisation of loss of function defects in MC2R as a primary cause of FGD, it became apparent that other genetic factors were involved in pathogenesis of FGD given the existence of several families with an isolated adrenal insufficiency phenotype and negative MC2R genomic screening. In vitro expression studies by Noon et al. revealed that a fluorescent tagged MC2R cDNA clone was able to traffic to the cell membranes of adrenocorticotropin resistant murine adrenocortical tumour cells but remained restricted to the endoplasmic reticulum in non-adrenal cells. This suggested that an adrenal derived co-factor was necessary for MC2R expression (40, 41) ( Figure 3 ).

In 2005, Metherell et al. conducted autozygosity mapping in a single family in which runs of homozygosity identified a 2.2 Mbp region. Within this region, the adrenally expressed chromosome 21 open reading frame 61 (C21orf61) gene previously encoding a fat-associated low-molecular-weight protein, was identified. Variants in this gene, retitled the melanocortin 2 receptor accessory protein (MRAP), were subsequently identified in several families with FGD (42). Variants in MRAP (FGD Type 2) account for 20-25% of FGD cases, with around 15 mutations being described since initial characterisation of this gene (37, 42–45). The majority of variants are splice site/nonsense that ultimately lead to a truncated and non-functional receptor. Patients generally present at an earlier age (median age of 0.08 years at diagnosis) with normal stature when compared to subjects with MC2R variants (37, 45, 46). This suggests reduced exposure to the unfettered actions of ACTH on the growth plate seen in FGD Type 1. Mrap^-/- mice phenocopy the isolated glucocorticoid deficiency and normal mineralocorticoid function of human subjects. Interestingly, the adrenals from knockout mice are small with indistinct cortical zonation and dysregulated accumulation of WNT4/β-catenin (47).

In 2012, dual reports of partial MCM4 deficiency in 14 probands from the Irish Traveller population highlighted a unique syndrome that encompassed adrenal insufficiency, intrauterine and postnatal growth restriction, microcephaly and natural killer cell (NK) deficiency (85, 86). The variant highlighted (and to date, the only variant associated with this disorder) was a homozygous splice site substitution c.71-2A>G leading to a single base cDNA insertion c.70_71insG and frameshift truncation, p.P24Rfs*4. Unlike other forms of FGD, the adrenal phenotype was relatively mild with onset of adrenal insufficiency often in late childhood. In healthy control derived peripheral lymphocytes, two MCM4 isoforms are detectable at 96KDa and 85KDa. In patient cells, only the minor isoform is present. This smaller protein is touted to lack the N-terminal MCM4 domain and may partially rescue patient phenotype (85, 86).

Complete loss of Mcm4 in mice is lethal. Hypomorphic Mcm4 ^{Chaos3/–Mcm3+/–} mice are viable and demonstrate abnormal adrenal morphology. The adrenal capsule is thinned with significant numbers of non-steroidogenic Cyp11a1/Cyp11b1 negative cells that are on the contrary, Gata-4 and Gli1-positive. This remodelled adrenal cortex demonstrates a lack of steroidogenic output in keeping with human disease (85, 87).

Until recently, inborn errors of sphingolipid metabolism due to single enzyme defects within the sphingolipid pathway have been characterised by their predominantly neurological phenotype. Sphingosine-1 phosphate lyase insufficiency syndrome (SPLIS) due to defects in SGPL1, which coordinates the final degradative step in the sphingolipid pathway, is a newly described multi-systemic disorder, in which adrenal failure features prominently (88–90).

Sphingolipid synthesis involves a series of tightly regulated, enzyme-catalysed steps that initiate in the endoplasmic reticulum from non-sphingolipid precursors to biosynthesis of higher order complex glycosphingolipids within the Golgi apparatus. Despite the diversity within the biosynthetic pathway, sphingolipid metabolism begins with a common entry point and exit via a single degradative pathway. This common initial step involves the coupling of cytosolic serine and palmitoyl CoA to 3-ketodihydrosphingosine through the action of serine palmitoyltransferase (SPT) whilst SGPL1 executes the penultimate step of the metabolic pathway, catalytic cleavage of sphingosine-1 phosphate (S1P), into 2E-hexadecanal and phosphoethanolamine (91, 92). SGPL1 is the major modulator of S1P signalling (93). Under normal physiological conditions, S1P is largely pro-proliferative, suppressing the pro-apoptotic actions of ceramide however loss of function mutations in SGPL1 result in a pathological accumulation of S1P which studies have shown to be associated with induction of apoptosis.

In 2017, Prasad et al. (88) described loss of function human mutations in SGPL1 and a novel syndrome characterised by primary adrenal insufficiency. Extra-adrenal phenotypic features included steroid-resistant nephrotic syndrome, hypothyroidism, ichthyosis, neurodevelopmental delay, hypogonadism and lymphopenia. Mass spectrometric analysis of plasma sphingolipids in one patient and heterozygous parents revealed elevated ceramides and S1P levels when compared to age and sex matched controls suggesting that the underlying multi-system pathology in these patients may be due to organ-specific cytosolic accumulation of sphingolipid intermediates. Correspondingly, Lovric et al. described a cohort of 7 families who were found on next generation sequencing to harbour 9 unique, recessive mutations in SGPL1 (89). Several studies have subsequently corroborated these initial findings and expanded the phenotypic spectrum to include microcephaly, sensorineural deafness, and progressive neurological deterioration (94). A further neurological phenotype has been described in siblings bearing compound heterozygous loss of function mutations in SGPL1, involving axonal mononeuropathy giving rise to Charcot Marie Tooth-like disease (95).

Sgpl1 ^-/- mice exhibit early postnatal mortality but those that survive demonstrate disrupted adrenal morphology. Adrenocortical zonation is disordered and cells of the zona fasciculata have reduced expression of steroidogenic enzymes and contain fewer lipid droplets when compared to wild type mice (88). Electron microscopy of Sgpl1 ^-/- kidneys demonstrated foot process effacement and absent slit diaphragms (89). Tamoxifen-inducible Sgpl1-ablated (SPL^Flox/Flox Cre+) mice with partial lyase deficiency, interestingly, demonstrated glomerulopathy with progressive proteinuria and markedly increased intra-renal S1P levels (96). These mice additionally showed dermal irritation and hyperkeratosis whilst other phenotypic features of Sgpl1 silencing were less evident suggesting that some degree of wild type activity may be protective against developing multi-systemic disease.

From initial descriptions of defects in the ACTH receptor to disorders of sphingolipid metabolism, the genomic landscape of FGD has dramatically transformed over the last three decades. Pending the discovery of novel genes implicated in pathogenesis of glucocorticoid deficient disorders, the likelihood of oligogenic inheritance is augmented in the diagnosis of unsolved FGD cases. One report suggests that digenic, tri-allelic inheritance of variants in both STAR and CYP11A1 account for an isolated case of adrenal failure (97). Oligogenic heterozygosity is increasingly pertinent particularly in cases of haplotype ambiguity. Given the increasing accessibility of next generation sequencing techniques, the heritability of adrenal insufficiency-related phenotypes may be more easily uncovered allowing earlier patient intervention with significantly disease modulating impacts.

AM: Writing – original draft, Writing – review & editing.