3.3 Structural distribution of
cancer-associated GPCR mutations versus natural
variants
It is notable that the ”hotspots”— well-defined mutation
clusters—are not as common in GPCRs as in those oncogenes such as KRAS
and tumor suppressor genes such us TP53, indicating a diverse landscape
of genetic alterations (Baeissa, Benstead-Hume, Richardson, & Pearl,
2017). Part of the diversity originates from the non-synonymous natural
variants, which represent genetic alterations in GPCRs that result in
amino acid changes in healthy people. These variants play a significant
role in the functional diversity observed among GPCRs across different
individuals and populations. Across all GPCR families, there is a higher
prevalence of non-synonymous natural variants in the N-terminus,
C-terminus, and transmembrane (TM) domains compared to the extracellular
or intracellular loops (A. Lee et al. , 2003). In addition, the
highly conserved DRY and NPxxY motifs have been identified in the
non-synonymous polymorphism analysis, which indicates that mutations in
these structural motifs are inherent features in the diversity of GPCR
function across different individuals and populations (Kim, Duc, &
Chung, 2018). However, even with correction for natural variants, recent
pan-cancer analysis has demonstrated that GPCRs still feature
significant accumulation of mutations in some highly conserved
structural motifs such as E/DRY, CWxP, NPxxY of class A GPCR, and HETx,
GWGxP, PxxG of class B GPCR (Bongers et al. , 2022; Do, Haldane,
Levy, & Miao, 2022). Bongers et al. found that conserved
residues undergo a higher mutational pressure in cancer patients, which
was not observed in natural variants, indicating their importance in
cancer progression.
Most of the conserved motifs in GPCRs mediate their inactive
conformation, and mutations at these motifs would therefore alter
receptor function and stability. For example, the ‘ ’E/DRY’ ’ motif
plays a pivotal role in receptor activation and signaling of class A
GPCRs (Rovati, Capra, & Neubig, 2007). The ionic lock formed by the
aspartic acid and the glutamic acid residue stabilizes the inactive
state of the receptor. Upon ligand binding, conformational changes
disrupt this ionic lock, allowing the transition to the active state and
initiate downstream signaling. Conformational changes caused by
mutations in the E/DRY motif could lead to alternations of receptor
function, including gain of constitutive activity or loss of function
(Huang & Tao, 2014; Römpler, Yu, Arnold, Orth, & Schöneberg, 2006).
For example, cancer-associated CCR2 mutations in the DRY motif lead to a
reduction or complete absence in G protein activation (den Hollanderet al. , 2023). Similar phenotype has been observed for the
muscarinic acid (M1 and M5) receptors (Lu, Curtis, Jones, Pavia, &
Hulme, 1997), gonadotropin-releasing hormone (GnRH) receptor (K. K.
Arora, Cheng, & Catt, 1997), cannabinoid 2 receptor (CB2R) (Feng &
Song, 2003), and the adrenergic receptors (Chung et al. , 2002;
Samama, Cotecchia, Costa, & Lefkowitz, 1993). In addition, mutations
outside the conserved motifs may also affect receptor function. One
example is the N-terminal TSHR mutation found in toxic thyroid adenomas,
which resulted in basal activation of the protein kinase A pathway
(Nanba et al. , 2012).