Protein rescue

While the exact function of CMG2 remains to be unraveled, studies showed that the majority of the HFS mutations lead to loss of function due either to the instability of the CMG2 messenger (Class III mutations) (1)—in the case of frame-shift mutations in exon 13 (2) —or to improper folding of the ectodomain of the protein in the ER followed by ER retention and degradation (Class II mutations) (3,4).

Class II mutations identified in patients such as p.C39F, p.L45P, p.I189T, p.V310F, or p.C315W are localized in the ectodomain of CMG2. These mutations affect the folding/stability of the either the vWA domain or the Ig-like domain (both composing the ectodomain of CMG2) leading to protein degradation through the protein quality surveillance mechanism called Endoplasmic-Reticulum Associated Degradation (ERAD) pathway. Therefore, the protein degradation results in undetectable protein level of the correspondent mutant proteins in the tested patient-derived cells. In view of a possible therapeutic intervention, ERAD pathways inhibitors were considered as a potential therapeutic target for patients harboring these mutations. A biochemical study runned by the laboratory of Prof. Gisou van der Goot showed that some CMG2 proteins harboring these Class II mutations can partially be rescued through treatment with proteasome inhibitors. This rescue, which not only leads to partial recovery in terms of protein levels but also plasma membrane targeting of a functional protein, might to be more efficient for mutations that map to the vWA domain than mutations mapping to the Ig-like domain (4).

Among all the Class III mutations identified in patients, there are three mutations - c. 1074delT, c. 1073-1074insC and c. 1073-1074insCC - that account for about 60% of all different mutations mapping to exons encoding the cytoplasmic tail of CMG2 protein. The occurrences of these mutations lead to premature stop codons in terms of mRNA, which will then be detected and degraded by the mRNA surveillance mechanism nonsense-mediated decay (NMD) pathway. Therefore, the low mRNA level results in undetectable protein level of the correspondent mutant proteins in the tested patient-derived cells. This observation points to the NMD pathway as a potential therapeutic target for patients with these three mutations. However, studies carried out in ectopic expression system showed that the well-synthesized CMG2insCC and CMG2delT proteins are efficiently targeted to ER associated degradation (ERAD) pathway whereas the other mutant protein CMG2insC could be properly transported to cell surface and bind the pathological ligand. Patients with insCC and delT mutations would therefore not benefit from treatment inhibiting NMD pathway. In contrast, the protein level of CMG2insC could be restored by silencing one of the central factors in NMD pathway and the rescued protein is properly targeted to the membrane as it is showed in patient-derived cells, which suggest that NMD pathway in this case would be a promising target (2).

The outcome from these 2 studies highlighted the importance of in-depth analysis of the molecular consequences of specific mutations in order to give the most suitable and efficient treatment for patients with different mutations (2,4).

(1) Deuquet et al, 2011 (review) (2) Yan et al, 2013 (submitted and reviewed publication) (3) Deuquet et al, 2009 (4) Deuquet et al, 2011


May 9, 2013
The Hyalinosis Fibromatosis Syndrome website is finally complete and online!
Jan 22, 2013

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HFS meeting 2013
October 7,8 and 9 2013 in Lausanne, Switzerland