The inheritance pattern of the disease caused by PRKAR1B mutations is autosomal dominant. The mutations, which were found by exome sequencing, occurred de novo in all cases where segregation analysis was possible.
The PRKAR1B gene encodes the regulatory subunit RI-beta of the cyclic AMP (cAMP)-dependent protein kinase A (PKA) complex. PKA is an essential enzyme in the signaling pathway of the second messenger cAMP. Through phosphorylation of target proteins, PKA controls many biochemical events in the cell including regulation of metabolism, ion transport, and gene transcription. The PKA heterotetramer is composed of 2 regulatory and 2 catalytic subunits (summary by Solberg et al., 1992). While PKA complexes are present in all human tissues, the respective composition of PKA varies across cell types, depending on the expression of different subunits. In humans there are 4 different regulatory subunits: R1α, R1β, R2α and R2β, and 6 principal catalytic subunits. Regulatory subunits mediate the localization of PKA to a cellular compartment by binding A-kinase-anchoring proteins (AKAPs; Pidoux & Tasken, 2010; Taylor, Keshwani et al., 2012b), and PKA kinase activity by releasing catalytic subunits in the presence of cAMP (Soberg & Skalhegg, 2018; Taylor, Ilouz et al., 2012a). The subunit RIβ is primarily expressed in the brain (Cadd & McKnight 1989; Uhlen et al., 2015) with the highest levels of expression in the cerebral cortex and hypothalamus Sjöstedt et al., 2020)
Figure 1. The PKA complex (Taylor et al., 2012b)
While molecular and cellular disease mechanisms of MASNS are still unclear, the Arginine 335 residue is emerging as a “hot-spot” for pathogenic missense variant(s) in the PRKAR1B gene: in a cohort of 13 unrelated patients with MASNS, when Marbach et al. (2021, 2022) identified heterozygous missense mutations in the PRKAR1B gene, 11 patients carried the same aforementioned missense mutation (c.1003C>T, p.Arg335Trp).
On the molecular level, the recurrent missense variant (c.1003C>T, p.Arg335Trp) leads to the substitution of an arginine residue by a tryptophan residue within one of the two cAMP-binding domains of R1β (Figure 1, Ilouz et al., 2012), which might impair the cAMP-sensing property of the mutant protein. If this holds true, PKA complexes containing mutant R1β would be less sensitive to rising cAMP concentrations, affecting cellular signalling downstream of PKA in PRKAR1B-expressing cells of the CNS.
This loss of function theory of missense variants in the PRKAR1B gene would be consistent with the observation of a significantly decreased basal PKA enzymatic activity in HEK293 cells transfected with a PRKAR1B p.Arg335Trp expression construct, compared to cells transfected with wild type PRKAR1B construct (Marbach et al., 2021).
Figure 2: Tertiary protein structure of R1β as predicted by the AlphaFold AI system (https://alphafold.ebi.ac.uk/entry/P81377). Color coding: Dark blue = very high confidence, light blue = confident, yellow = low confidence, orange = very low confidence. The position of Arg 335 has been highlighted in red in the left image. The predicted position of the Arg 335 residue lies within the cAMP-binding-domain B (CNB B’) of the protein, with its side chain forming hydrogen bonds with the double-bonded oxygen of the Asp 291 residue’s carboxylate group (right image).
Possible disturbances of several PKA-dependent functions could contribute to the reduced sensitivity to pain in most patients carrying the p.(Arg335Trp) variant, although it is currently unknown whether these pathways obligatory require the R1β subunit. For example, Gs alpha subunit (Gαs) signalling via adenylyl cyclase/cAMP/PKA in response to pain, itch, and inflammation could be impaired, resulting in reduced sensitization of transient receptor potential channels (TRP) and reduced excitability of primary sensory nerves (Geppetti et al., 2015). PKA is also involved in spinal cord central sensitization in response to repeated nociceptive stimuli through phosphorylation of NMDA (N-methyl-D-aspartic acid) and AMPA (α-amino-3-hydroxy- 5-methyl-4-isoxazolepropionic acid) receptors, recruitment of AMPAR to the cell membrane, and persistent strengthening of synapses by phosphorylation of the transcription factor CREB (cAMP response element-binding protein; Basbaum et al., 2009; Latremoliere & Woolf, 2009).