**2.** *PRKAR1A* **Structure and Function**

Cyclic adenosine monophosphate (cAMP)-dependent protein kinase type 1-alpha regulatory subunit is encoded by the *PRKAR1A* gene. *PRKAR1A* consists of 11 exons; ten of them (2–11) are coding. Protein kinase A (PKA) (Figure 1), a serine/threonine kinase, is a second messenger-dependent enzyme and it is involved in G-protein coupled intracellular pathways. It is the main mediator of cAMP actions for various cellular processes in mammals, including cell differentiation, proliferation, and apoptosis [30–32].

**Figure 1.** Schematic representation of cyclic adenosine monophosphate (cAMP) signaling pathway. *C* catalytic subunit of PKA, *GDP* guanosine diphosphate, *GPCR* G-protein coupled receptor, *GTP* guanosine triphosphate, *PKA* protein kinase, *R* regulatory subunit of PKA, *α*, *β*, *γ* subunits. After the GPCR is activated, adenylate cyclase is activated and produces cAMP, which binds to the R subunit and activates PKA. Then, conformational changes ensue and the C subunits are released and phosphorylate cytoplasmic targets.

> The PKA holoenzyme is a hetero-tetramer composed of two regulatory (R) subunits and each is bound to one catalytic (C) subunit [33]. Four subtypes of R (RIα, RIβ, RIIα, RIIβ) and four subtypes of C (Cα, Cβ, Cγ and Prkx) subunits have been identified so far. A gene is coding each R (*PRKR1A*, *PRKR1B*, *PRKR2A*, *PRKR2B*) and each C (*PRKACA*, *PRKACB*, *PRKACG*, *PRKX*) subunit, respectively [33,34]. Two major isozymes have been identified, type I and type II PKA, based on their chromatographic elution patterns [32]; they are comprised of homodimers of either RIα and RIβ or RIIα and RIIβ, respectively [31,35]. In the basal state, the catalytic subunits bind mostly to type II subunits [31,35–37]. When cAMP binds to the R subunits, it alters their conformation; this causes the dissociation of each active C subunit from the dimer with the corresponding R subunit. Following that, the free C subunits phosphorylate threonine and serine residues of proteins that are critical to the activation of downstream processes [38–40].

> RIα haploinsufficiency, as shown by mice and human studies, predisposes to the development of tumors [29,41]. The majority of *PRKAR1A* mutations result in premature stop codons with unstable mRNAs undergoing nonsense-mediated decay [25,42]. In the thyroid, PKA through the production of cAMP, signals downstream of thyrotropin (TSH) on cell proliferation and differentiation; increased levels of TSH in humans have been associated with thyroid tumors [43]. In addition, in a series of thyroid tumors, the Cα subunit was investigated but no mutations were detected [44].
