The Role of Rab38 in Rat and Human Platelet Function Defects.

The fawn-hooded hypertensive (FHH) rat is a model of a moderate platelet function defect manifest by severely prolonged tail bleeding times, decreased platelet aggregation to ADP and collagen, and decreased ADP release from dense granules. The FHH rat has a mutation at the ATG start site of the gene encoding the small GTP-binding protein Rab38, which we have identified as being strongly associated with the bleeding phenotype. We have further characterized expression of Rab38 in the FHH rat and in humans. As assessed by reverse transcriptase PCR, we found that Rab38 is expressed in a wide variety of tissues from FHH and control Brown-Norway (BN) rats. However, by western blotting there was no protein expression of Rab38 in the FHH tissues, confirming that the mutation results in a severe defect at the protein level. Confocal immunofluorescence microscopy demonstrated Rab38 localized in punctuate cytoplasmic granules in human megakaryocytic cell lines. The Rab38 staining colocalized with von Willebrand factor (VWF) found in alpha-granules. However, we did not demonstrate any significant colocalization of Rab38 with serotonin, a marker of dense granules or Lamp-1, a marker of lysosomes, in the megakaryocytic cell lines. We also demonstrated colocalization of Rab38 with VWF in normal human platelets. We hypothesized that some humans with mild to moderate platelet dysfunction may have abnormalities of Rab38. We sequenced the entire coding region and intron-exon boundaries of the Rab38 gene in six patient samples collected at Emory University for the CDC Women with Bleeding Disorders and Menorrhagia Study. Four of these patients had detectable platelet aggregation abnormalities and two did not. In this limited initial study, no obvious mutations or new polymorphisms were detected in the Rab38 gene. Numerous known polymorphisms were detected, but no clear pattern was associated with the platelet aggregation abnormalities as of yet. These results along with our previous findings in the FHH rat suggest that Rab38 may be involved in a general secretion mechanism in platelets. It remains to be seen whether mutations in the Rab38 gene account for some of the platelet function defects seen in humans, but we are actively pursuing this hypothesis.