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GPCR can activate diverse signaling pathways critical for cellular function. This receptor signaling is tightly regulated through a process known as desensitization that is mediat- ed by GPCR phosphorylation, allowing its binding to b- arrestin which triggers receptor internalization, conse- quently dampening intracellular signaling.30 A common phosphorylation code (motif) has been identified in the C- terminal cytoplasmic tail of most GPCRs, and its phospho- rylation enables electrostatic interactions with b-arrestin.31
GPR34 contains 7 transmembrane domains, followed by a C-terminal cytoplasmic tail. The majority of GPR34 mutations identified in MALT lymphoma are nonsense changes or frameshift indels that are clustered in its C-ter- minal region, resulting in truncated proteins. A phosphory- lation code (motif) (S351S353T356) is seen in the C-terminal cytoplasmic tail of GPR34 according to Zhou et al.,31 and all the above nonsense and frameshift changes would elimi- nate or impair these phosphorylation sites, thus potentially deregulating the receptor desensitization process.
The remaining GPR34 mutations are missense changes including R84H, D151A and Y327N. R84H affects the tri- basic motif (RKR) in the first intracellular loop, which is the key topogenic signal determining the orientation of the first transmembrane domain.32 D151A affects the highly con- served E/DRY motif and is predicted to cause the receptor constitutive activation.33 Y327N is close to the interface of the seventh TM domain and the cytoplasmic tails, and is predicted to be highly damaging by PolyPhen-2, although its potential functional impact is unclear.
Similarly, CCR6 also contains 7 transmembrane domains, followed by a C-terminal cytoplasmic tail, and the majority of CCR6 mutations are nonsense or frameshift changes that are clustered in its C-terminal region. A phosphorylation code (motif) (S357T360T363) is present in the C-terminal cyto- plasmic tail of CCR6,31 and all the nonsense and frameshift changes identified would eliminate these phosphorylation sites, potentially impairing the receptor desensitization process. The functional impact of the two CCR6 missense
A
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mutations (R159S and Y352C) is unclear.
The nature and distribution of mutations in GPR34 and
CCR6 identified in this study are very similar to those seen in CXCR4 in Waldenström macroglobulinemia and WHIM syndrome, as well as those found in CCR4 in adult T-cell leukemia/lymphoma, which have been shown to be gain- of-function changes.34-36 Despite their similar mutation pat- tern, GPR34 and CCR6 have distinct ligands, and thus respond to different environmental cues, potentially explaining their differential involvement in MALT lym- phoma of different sites. GPR34 has been shown to be trig- gered by lyso-phosphatidylserine, while CCR6 is activated by CCL20.37-39 Overexpression of wild-type GPR34 in vitro resulted in ERK, PI3K/AKT and PKC signaling, inducing AP1 and NF-κB-mediated gene transcription.20,40 It still remains to be investigated whether the above GPR34 and CCR6 mutations impact similar downstream signaling pathways, and if they would enhance these intracellular signaling pathways through impaired receptor desensitiza- tion and internalization or via independent mechanisms.
Distinct genetic profile in MALT lymphoma of various sites
The present study provides further evidence showing dis- tinct genetic profiles among MALT lymphoma of various sites (Figure 1). MALT lymphoma of the salivary gland fea- tures frequent mutation of TBL1XR1 and GPR34, while those of the thyroid are characterized by frequent muta- tions in TET2, TNFRSF14 and PIK3CD. MALT lymphoma of the ocular adnexa is noted for frequent TNFAIP3 muta- tion. In contrast, MALT lymphomas of the stomach and lung are distinguished by a high prevalence of t(11;18)(q21;q21).
The present study also unravels several distinct associa- tions among genetic changes in MALT lymphoma. In MALT lymphoma of the salivary gland, there is a significant positive association between TBL1XR1 mutation and GPR34 mutation/translocation. The effect of TBL1XR1 mutations has not been fully characterized, but a previous
Figure 3. Nature and distribution of TBL1XR1 mutations in mucosa-associated lymphoid tissue (MALT) lymphoma, and their potential functional impact. (A) TBL1XR1 mutations identified in this study. The site of MALT lymphoma in which mutation was identified is indicated by color: red: salivary gland; red: thyroid; green: ocular adnexa; orange: stomach; purple: lung; gray: other sites. (B) Detailed analyses of TBL1XR1 mutations within the WD40 domains. TBL1XR1 mutations identified in the present and a previous study by Jung et al. are included in the analyses.41 The majority of TBL1XR1 mutations are localized in the regions of struc- tural importance, such as HSDW tetrad indicated by arrows and the residues predicted to be top facing (WDSP predictor, indicated in red text). The approximate position in the HMM logo for WD40 repeats (Prosite PS00678) is shown below the sequence.
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