PDCD10 mutation carriers have a greater chance of spontaneous mutation, an increased CCM burden, and a younger age of presentation, which is often associated with clinical hemorrhage[13]. There is also a significant association with other manifestations including skin CCMs, scoliosis, spinal cord cavernous malformations, cognitive disability, and benign brain tumor including meningioma, vestibular schwannoma, and astrocytoma[11, 38–40]. Indeed, there is a lack of knowledge of why some patients that harbor PDCD10/CCM3 mutations might develop this syndromic clinical phenotype. Here we provided two important information to the clinical phenotype of the syndromic type of cerebral cavernous malformation. First, our PDCD10 mutation carriers have a greater CCM burden and a younger age of presentation, which is often associated with clinical hemorrhage. Besides that, we performed an extensive in silico structural analysis with a profound docking site study of a frameshift and a splicing site mutation with the major proteins knowing to interact with PDCD10 that might enhances the comprehension of this rare disease.
Similar to the known CCM mutations, the PDCD10/CCM3 c.222delT (p.Asn75ThrfsTer14) mutation identified can result in a PTC[41]. Abnormal mRNAs containing PTCs are normally degraded by the nonsense-mediated mRNA decay (NMD) system, but can escape from the NMD system[42, 43]. Our qPCR results showed a significant reduction of mRNA in the blood samples of the patient that had this mutation. In this line, reduction of mRNA in the blood sample suggested that mutated transcript of PDCD10 be degraded by the quality control NMD-system and thus, truncated protein might not exist or be present at a very low level. Specifically, in relation to P2 patient the mutation c.475-2 A > G (p.A119Gfs*42) in the splicing site also reduced the expression of PDCD10, for the first time demonstrated here. The consequences of splice mutations on the expression of the corresponding transcript variants are diverse, thus, the functional evaluation of this type of variant is increasingly necessary[44].
Protein structure prediction revealed that the frameshift (c.222delT; p.Asn75ThrfsTer14) mutation lead to a substantial loss of αG-helix of the focal adhesion targeting (FAT)-homology domain, which is responsible for direct interaction with MGC4607 and with paxillin. This interaction is essential for cell-cell junctions regulation, cell-extracellular matrix adhesion, cell cycle regulation, proliferation and migration of cells and cell cycle regulation[45].
The surface of this domain contains a hydrophobic patch, which is found between α7 and α8 helix being the site for MGC4607 binding. This interaction is mostly made up of hydrophobic residues from MGC4607 (T225, I226, F228, L229, A232, I233, G236, and A237) binding to hydrophobic residues from PDCD10 (I131, I134, A135, I138, L142, V168, F174, L178, S171, K132, and K139)[17, 46]. Interestingly, the interaction of MGC4607 with PDCD10 reciprocally protects the proteins from degradation[47]. Thus, it is reasonable to hypothesize that this the mutations studied here may influence the formation of the MGC4607-PDCD10 complex.
Once the in silico structural analysis prediction reveals that PDCD10FS only disrupted the FAT domain one could conclude that the formation of the PDCD10-GCKIII complex was intact. Nonetheless, docking analyses revealed that this frameshift mutation implied in a more severe disruption. We demonstrated that both docking scores predicted that PDCD10FS had reduced affinity with the three STK proteins. Since previous studies had demonstrated that the interplay between PDCD10 and GCKIII family is essential to maintain proper levels of the GCKIII proteins and that PDCD10-GCKIII complex seems crucial for the proper position of the Golgi apparatus and cell polarity this disturbance might influence the correct function of several cellular mechanisms in different organs of syndromic cerebral cavernous malformation patients[48, 49]. You et al (2013)[30] previously reported that PDCD10 deficiency also is able to downregulation of DLL4-Notch signalling, an important regulator for hyper-angiogenetic phenotype, in surgical specimens of CCMs[50]. Accumulating evidence indicates a crucial role of PDCD10 also in Erk1/2 activation and consequently VEGF pathway[51, 52]. In the context of pathological relevance between PDCD10 mutations, this data reveals the importance of the interaction of PDCD10 with other signaling pathways[53].
Some reports had associated low-grade astrocytomas to the PDCD10/CCM3 genotype, however there is no pathogenesis model to this interplay[54]. Here we demonstrated that, low-grade astrocytomas do not have statistically difference in the expression of PDCD10 or any of the GSKIII proteins when comparing to normal brain tissue, but had statistically higher expression of TLR4, CD14, ERK2 and PIK3CA. There is some new evidence that the TLR4-MEKK3-KLF2/4 and PIK3CA pathway may be up regulated in PDCD10 individuals duo to a permissive microbiome since the PDCD10 protein have a distinct cellular role on the gut epithelium[53, 55]. These is a unique finding, opening a road to a mechanistic link between PDCD10 genotype and the occurrence of low-grade astrocytomas in the syndromic phenotype of cerebral cavernous malformations.
Differently from patients with other genotypes, PDCD10/CCM3 individuals tend to harbor higher bleeding risk associated with the precocity of symptoms and higher lesion burden. Although we had presented here some evidence that aberrant protein-protein interaction of PDCD10 may influence the proper cellular mechanism, further studies should be done with a larger cohort of patients assessing the genetic profile as a possible prognostic biomarker. This may be a new avenue to expand the spectrum of CCM mutations and assist to guide genetic counseling and early genetic diagnosis.