Chromogranin B: The ApoE of ALS

Swapping a single cytosine for a thymine in a gene involved in protein secretion more than doubles a person’s risk of amyotrophic lateral sclerosis (ALS), according to a paper published this week in PNAS. Researchers at Laval University in Qubec, Canada, report on a variant in chromogranin B that not only increases one’s likelihood of getting this neurodegenerative disease, but also brings its onset down to younger ages. In essence, first author Franois Gros-Louis and principal investigator Jean-Pierre Julien compare chromogranin B’s effect on ALS risk to that of ApoE4 on Alzheimer disease.

This is the first risk factor for sporadic ALS, thus providing a long-sought entry point into mechanisms and genetics of ALS, wrote Pico Caroni of the Friedrich Miescher Institute in Basel, Switzerland, who was not involved in the study, in an e-mail to ARF.

Approximately one-tenth of ALS cases are inherited, and scientists have identified a few genes to blame (see ARF related news story on Sreedharan et al., 2008; Gitcho et al., 2008; ARF related news story on Kwiatkowski et al., 2009; Vance et al., 2009). However, the roots of the vast majority of ALS cases remain unknown. Julien presented some of these findings at the Andr-Delambre Foundation Symposium on ALS last September (see ARF related news story).

Chromogranins A and B are components of secretory vesicles, but their exact function is unclear. They may be involved in calcium storage, vesicle formation, or peptide secretion, Julien wrote in an e-mail to ARF. Julien’s lab originally pulled chromogranins out of a yeast two-hybrid screen, using as bait mutant superoxide dismutase 1, which causes some 20 percent of familial ALS cases (see ARF related news story on Urushitani et al., 2006). In a mouse ALS model, excess chromogranin A exacerbates disease (ARF related news story).

Now, the researchers have linked chromogranin B to the human disease. In their initial screen, they sequenced the gene in 452 people of French or French Canadian ethnicity, 194 with familial or sporadic ALS, and 258 controls of similar age and geography. They found several different variants in people with ALS, but one in particular stood out: Several people with the disease had a P413L mutation in the gene. When the scientists added more cases, they found that eight out of 80 people with familial ALS, and 40 out of 482 with sporadic ALS carried the variant, compared to only 20 of 760 control cases. That corresponds to more than triple the risk of disease for P413L carriers.

To confirm the association, the researchers repeated their study in 832 people with ALS and 606 controls of Swedish origin. There they found that six of 202 people with familial ALS and 16 of 630 with sporadic ALS had the variant, compared to 11 of 606 controls. Combined, the populations suggest that the P413L variant increases risk for ALS by a more modest factor of 2.2. This number is likely to change with further study. By comparison, the summary odds ratio from some 35 genetics studies of ApoE is 3.68.

In another echo of ApoE’s role in AD, Gros-Louis and colleagues found that people carrying the P413L variant showed symptoms of ALS earlier than did those without it. Among sporadic cases, the median age of onset for carriers was 55, compared to 62 for non-carriers. Similarly, in familial cases the median onset for carriers was 43, versus 55 for non-carriers. The findings provide important support for the notion that sporadic ALS and familial ALS are mechanistically related, Caroni wrote.

Computer models predict that the P413L mutation should change the protein’s structure and function. To evaluate the mutation’s effect in actual experiments, the scientists linked the wild-type and P413L chromogranin B genes to GFP and expressed them in human neuroblastoma cell cultures. They used a marker for the Golgi apparatus to examine how the chromogranins traveled through the protein secretory pathway. Two days after transfection, approximately 40 percent of wild-type protein colocalized with the Golgi, whereas nearly all of the P413L variant remained stuck at the endoplasmic reticulum-Golgi interface.

These results suggest that the mutation might render chromogranin B sorting and secretion defective. They fit well with other studies that have implicated the secretory pathway in ALS. Mutant SOD1 aggregates are found in the endoplasmic reticulum and Golgi (Urushitani et al., 2008), and the motor neurons affected in ALS are particularly vulnerable to endoplasmic reticulum stress (see ARF related news story on Saxena et al., 2009). The authors suggested that in future studies, it would be interesting to examine the impact of the P413L variant on endoplasmic reticulum stress and SOD1 secretion. They are also working on transgenic mice with the variant.

Julien and colleagues have screened for ALS-linked changes in the chromogranin A gene, as well, but have not come up with any significant variation yet.

Reference:
Gros-Louis F, Andersen PM, Dupre N, Urushitani M, Dion P, Souchon F, D’Amour M, Camu W, Meininger V, Bouchard J-P, Rouleau GA, Julien J-P. Chromogranin B P413L variant as risk factor and modifier of disease onset for amyotrophic lateral sclerosis. PNAS 2009 Dec.


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