CYP11B1 and CYCP11B2 play pivotal roles in the synthesis of adrenal corticosteroids, the former responsible for the synthesis of cortisol and corticosterone, and the latter for the synthesis of aldosterone
[65][59]. The advent of CYP11B2 novel monoclonal antibodies has clarified the zonation of these synthesis enzymes in the normal adrenal gland
[45,46][39][40]. Classically, the adrenal cortex is composed of three main zones, i.e., from the outermost layer, zona glomerulosa (ZG), to the zona fasciculata (ZF) and zona reticularis. Aldosterone and cortisol are produced by ZG and ZF cells, which have been shown by HE staining to be morphologically distinct: ZG cells are smaller, more compact, and have a smaller cytoplasm/nucleus ratio; ZF cells have a larger cytoplasm/nucleus ratio, with more fatty material and transparent appearance. Evaluation of the distribution of each synthase in the normal adrenal gland using specific CYP11B2 and B1 antibodies has shown that the distribution is different in young and elderly individuals
[66,67,68][60][61][62]. In younger individuals, CYP11B2-positive cells are present throughout the ZG
[67][61]; CYP11B1 was detected in both the ZF and zona reticularis. An unstained region is identified between the CYP11B2- and CYP11B1-positive cells that is not detected by either antibody. It is not clear whether this region comprises undifferentiated progenitor-like cells
[69][63]. On the other hand, in the elderly, CYP11B2-positive cells are adjacent to the membrane and form a pattern of CYP11B2-immunopositive cell clusters with no CYP11B2 staining around them. Approximately half of the adrenals of normotensive individuals contain these regions, which have been designated as Aldosterone Producing Cell Clusters (APCCs)
[66,67,68,70,71][60][61][62][64][65]. The nomenclature of APCC was changed to Aldosterone Producing Micronodules (APM) in the HISTALDO study
[42][36]. The question of whether APMs have an autonomous aldosterone secretory capacity has been arisen. This is linked to the fact that the incidence of APMs correlates with the frequency of aldosterone-related hypertension
[68,72][62][66]. Subsequent studies have shown that these APMs when found in adult human adrenal tissue with normal adrenal function also contain somatic mutations of the aldosterne driver mutation; mutations were identified in over 30% of APMs, most frequently in the
CACNA1D mutation
[64,66,71][58][60][65]. This was followed by
ATP2B3,
ATP1A1, and few
KCNJ5 mutations. Interestingly, in case reports suggesting APM to APA transition, APM and micro APA were adjacent, with
ATP1A1 and
ATP2B3 mutations from the former and
KCNJ5 mutations from the latter
[73,74][67][68]. The research efforts have also extended to IHA cases. The study showed somatic mutations in
CACNA1D are the cause of most IHA
[75][69]. This is consistent with previous reports of image-negative PAs
[76][70]. Interpretation is limited due to the small sample of surgically resected IHAs. However, their findings were much appreciated as supporting the concept of a continuum of pathophysiology from normotensive to hypertensive patients
[77][71]. The different distribution of somatic mutations, with
KCNJ5 mutations found specifically in APA and
CACNA1D mutations in IHA, may contribute to the difference in clinical features between APA and IHA
[41][35].
What is the pathological characteristics of APA harboring
KCNJ5, and are they defined by the expression profile of steroidogenic enzymes? (See
Figure 2a.)
KCNJ5 mutated APAs are larger than other APAs; 60% are composed of lipid-laden clear ZF like cells and 40% are composed of compact ZG like cells
[78][72]. Immunohistological studies on steroid synthase have shown that
KCNJ5 mutated APAs are positive for CYP11B2 alone, co-expressing CYP11B2 and CYP17, and in a few cases, cells co-expressing CYP11B2, CYP11B1 and CYP17
[46,78,79][40][72][73]. On the other hand, APA with ATPA2B3 mutations tended to be dominated by compact ZG like cells, while APA with
ATP1A1 and
CACNA1D mutations showed heterogeneity from tumor to tumor with no clear advantage
[55,78][49][72]. Very interestingly, the expression of CYP11B2 and CYP17A1, which are involved in aldosterone and cortisol synthesis, correlated positively in
KCNJ5 mutant APAs, but negatively in
ATP2B3 mutants
[78][72]. The correlation between the two is not clear for
ATP1A1 or
CACNA1D. Inconsistent with this profile of steroidogenic enzymes, the normally negligible hybrid steroid, 18 oxocortisol (see the section “
Development of prediction model for somatic KCNJ5 mutation in PA patinets”), is markedly increased in patients with APAs harboring
KCNJ5 mutations. This increase has not been observed in patients with APAs harboring
ATPase and
CACNA1D mutations
[64,80][58][74] (
Figure 2a). This series of results has suggested that
KCNJ5 mutations are associated with the formation of APA and that the pathogenetic process differs from that of
ATPase and
CACNA1D mutations. APAs harboring
KCNJ5 mutations may have a more disorganized process of tumor cell differentiation and formation than APAs with
ATPase or
CACNA1D mutations. It has not been clear how the
KCNJ5 mutation occurred in APAs.
Figure 2. Distinctive characteristics of APA patients harboring
KCNJ5 mutations, and
CACNA1D or
ATPase mutations. Distinctive characteristics of aldosterone producing adenoma (APA) patients harboring
KCNJ5 mutation and
CACNA1D or
ATPase mutations. (
a)
KCNJ5 mutated APAs showed larger tumor and heterogenous composition of CYP11B2, CYP11B1, and/or CYP17 positive cells, while
CACNA1D and
ATPase mutated APAs showed smaller tumor and homogeneous composition of CYP11B2 positive cells. 18 oxocortisol is elevated in
KCNJ5 mutated APAs, while not in
CACNA1D and
ATPase mutated APAs.
KCNJ5 mutations have been detected mostly in APAs, while
CACNA1D and
ATPase mutations have been dominantly identified in aldosterone producing micronodules (APMs) and idiopathic hyperaldosteronism (IHA) as well as APAs.
WResearche
rs assume distinctive activated pathways of aldosterone synthesis between
KCNJ5 mutated and
CACNA1D or
ATPase mutated APAs, such as the ACTH-cAMP and Ca
2+ signaling pathways. (
b) Conceptual scheme of responsiveness of aldosterone secretion to ACTH between APAs harboring
KCNJ5 and
CACNA1D or
ATPase mutations is shown. Basal aldosterone secretion is higher and its responsiveness to ACTH stimulation is lower in
KCNJ5 mutated APAs than
CACNA1D and
ATPase mutated APAs. ACTH depletion via dexamethasone (Dex) suppression decreased plasma aldosterone levels from
KCNJ5 mutated APAs to that from
KCNJ5 wild APAs
[55,81,82][49][75][76]. Thus
wresearche
rs assume ACTH-cAMP signaling in KCNJ5 mutated APAs is activated to increase basal aldosterone secretion and to lessen response to extra ACTH stimulation. Figure was created with BioRender.com.