The function of ARMC5 and the types of disease-causing variants in PBMAH
ARMC5 is a cytosolic protein with no known enzymatic functions, but critical roles in embryonic development, immune responses, adrenal gland biology and is a putative tumor suppressor. The gene is located on chromosome 16 (16p11.2) and is highly conserved across eukaryotic organisms.
Disease-causing variants in ARMC5 are inherited in an autosomal dominant manner, with incomplete penetrance. Additionally, adrenal nodules in PBMAH also possess a second hit somatic disease-causing variant, consistent with the Knudson two-hit hypothesis for tumor suppressor genes. Interestingly, the mutability of ARMC5 gene is high, with multiple somatic disease-causing variants reported from the same patient. This suggests that ARMC5 deficiency causes a genomic instability even if PBMAH is a benign disorder without known malignant progression to date.
Two thirds of pathogenic variants are frameshift and non-sense, leading to loss of function. Very few are splice site variants and the remaining ones are missense, where their real pathogenicity is revealed just by the presence of the second hit. There is no apparent hot-spot region for this gene and so far, no gross deletion/duplication was identified at germline level, but the presence of those structural variations is possible.
The main challenge for the molecular characterization in PBMAH patients is to identify the pathogenicity of a germline missense variant, particularly if tumor DNA is unavailable for analysis, or if a variant in the tumor is not found. There are many in silico predictor tools that can be used to access the putative damage of a missense variant on the proteins function or its overall conservation throughout different species. Some characteristics of the variant could also help its classification, such as the overall frequency in control databases, or if the variant is de novo. Since the function of ARMC5 protein is still not fully understood, functional in vitro tests are very limited for missense variants.
Suspected Pathophysiological Mechanisms
In vitro studies indicate ARMC5 may be involved in regulating the steroidogenic pathway. Interestingly, several studies have shown a reduction in cortisol production after inactivation of ARMC5 in adrenocortical cell lines. Therefore, hypercortisolemia is believed to result from the large size of the nodules rather than an increase in cortisol production per individual cell. Additionally, it has been hypothesized that ARMC5 may regulate glucocorticoid production later in life. Initially, ARMC5-knockout mice presented with hypocorticosteronemia and suppressed expression of steroidogenic enzymes, followed by excess corticosterone production with increasing age. These findings may explain the prevalence of PBMAH in older adults.
Pathogenic variants are detected using Sanger sequencing analysis of the ARMC5 gene. Targeted genetic testing is recommended in conjunction with clinical evaluation to determine if the pathogenic variant was inherited or occurred de novo.