Molecular pathogenesis of sialic acid storage diseases: insight gained from four missense mutations and a putative polymorphism of human sialin.
BACKGROUND
Free sialic acid storage diseases are caused by mutations of a lysosomal sialic acid transporter called sialin. We showed recently that the milder clinical form, Salla disease, and a related non-Finish case, are characterized by residual transport, whereas sialin mutants found in lethal infantile cases are inactive. In the present study, we have characterized the molecular effects of a putative polymorphism (M316I) and of four pathogenic mutations associated with either infantile (G127E and R57C) or Salla-like (G409E) phenotypes, or both (G328E). The transport activity of human sialin was analysed using a novel assay that was based on a construct without the functional lysosomal sorting motif, which is expressed at the plasma membrane.


RESULTS
The lysosomal localization of human sialin was not (M316I and G328E) or only partially (R57C, G127E and G409E) affected by the missense mutations. In contrast, all pathogenic mutations abolished transport, whereas the putative M316I polymorphism induced an approx. 5-fold decrease of sialic acid transport.


CONCLUSIONS
The molecular effects of the R57C and G127E mutations strengthen the conclusion that the infantile phenotype is caused by loss-of-function mutations. On the other hand, the milder severity of the heterozygous G409E patient may reflect an incomplete expression of the splicing mutation present on the second allele. In the case of the G328E mutation, found in the homozygous state in a clinically heterogeneous family, the apparent severity of the transport phenotype suggests that the genetic or environmental factors underlying this clinical heterogeneity might be protective.
Ruivo
Raquel
R
Institut de Biologie Physico-Chimique, Centre National de la Recherche Scientifique, UPR 1929, Université Paris 7 Denis Diderot, 13 rue Pierre et Marie Curie, 75005 Paris, France.
Sharifi
Azita
A
Boubekeur
Samira
S
Morin
Pierre
P
Anne
Christine
C
Debacker
Cécile
C
Graziano
Jean-Charles
JC
Sagné
Corinne
C
Gasnier
Bruno
B
D002460:Cell Line; D002648:Child; D006801:Humans; D008247:Lysosomes / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D020125:Mutation, Missense; D027361:Organic Anion Transporters / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D011110:Polymorphism, Genetic; D029461:Sialic Acid Storage Disease / Q000235:genetics* / Q000378:metabolism; D027981:Symporters / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
D027361:Organic Anion Transporters; D027981:Symporters; C421224:sialic acid transport proteins