MOLECULAR-BASIS OF SPECTRIN DEFICIENCY IN BETA-SPECTRIN DURHAM - A DELETION WITHIN BETA-SPECTRIN ADJACENT TO THE ANKYRIN-BINDING SITE PRECLUDES SPECTRIN ATTACHMENT TO THE MEMBRANE IN HEREDITARY SPHEROCYTOSIS
H. Hassoun et al., MOLECULAR-BASIS OF SPECTRIN DEFICIENCY IN BETA-SPECTRIN DURHAM - A DELETION WITHIN BETA-SPECTRIN ADJACENT TO THE ANKYRIN-BINDING SITE PRECLUDES SPECTRIN ATTACHMENT TO THE MEMBRANE IN HEREDITARY SPHEROCYTOSIS, The Journal of clinical investigation, 96(6), 1995, pp. 2623-2629
We describe a spectrin variant characterized by a truncated beta chain
and associated with hereditary spherocytosis. The clinical phenotype
consists of a moderate hemolytic anemia with striking spherocytosis an
d mild spiculation of the red cells. We describe the biochemical chara
cteristics of this truncated protein which constitutes only 10% of the
total beta spectrin present on the membrane, resulting in spectrin de
ficiency, Analysis of reticulocyte cDNA revealed the deletion of exons
22 and 23, We show, using Southern blot analysis, that this truncatio
n results from a 4,6-kb genomic deletion. To elucidate the basis for t
he decreased amount of the truncated protein on the membrane and the o
verall spectrin deficiency, we show that (a) the mutated gene is effic
iently transcribed and its mRNA abundant in reticulocytes, (b) the mut
ant protein is normally synthesized in erythroid progenitor cells, (c)
the stability of the mutant protein in the cytoplasm of erythroblasts
parallels that of the normal beta spectrin, and (d) the abnormal prot
ein is inefficiently incorporated into the membrane of erythroblasts,
We conclude that the truncation within the beta spectrin leads to inef
ficient incorporation of the mutant protein into the skeleton despite
its normal synthesis and stability, We postulate that this misincorpor
ation results from conformational changes of the beta spectrin subunit
affecting the binding of the abnormal heterodimer to ankyrin, and we
provide evidence based on binding assays of recombinant synthetic pept
ides to inside-out-vesicles to support this model.