Deficient Geranylgeranylation of Ram/Rab27 in Choroideremia

Abstract

Choroideremia, an X-linked form of retinal degeneration, results from defects in the Rab escort protein-1 (REP-1) gene. REP-1 and REP-2 assist in the attachment of geranylgeranyl groups to Rab GTPases, a modification essential for their action as molecular switches regulating intracellular vesicular transport. If Rabs that depend preferentially on REP-1 for prenylation exist, they will accumulate unprenylated in choroideremia cells. Using recombinant Rab geranylgeranyl transferase and REPs to label unprenylated cytosolic proteins, we identified one unprenylated protein in choroideremia lymphoblasts that was prenylated in vitro more efficiently by REP-1 than by REP-2. This protein was purified and identified as Ram (renamed Rab27), a previously cloned Rab of unknown function. Immunohistochemistry of rat retina showed that Ram/Rab27 is expressed in the pigment epithelium and choriocapillaris, the two retinal cell layers that degenerate earliest in choroideremia. These results raise the possibility that the retinal degeneration in choroideremia results from the deficient geranylgeranylation of Ram/Rab27 or a closely related protein.

Rab27b regulates exocytosis of secretory vesicles in acinar epithelial cells from the lacrimal gland

“…Secondary antibodies and fluorescent affinity probes for microscopy, including Texas Red 10,000 MW dextran and FM 1-43, were purchased from Molecular Probes/Invitrogen (Carlsbad, CA). Purified recombinant His-tagged Rab27a and Rab27b protein were prepared as described (41). All other chemicals were reagent grade and obtained from standard suppliers.…”

Apoptosis of the mammalian retina and lens

“…(Papermaster, 1995). Indeed, some mutations affect genes whose expression is widespread on many other tissues, yet the primary clinical effect is in the retina or elsewhere in the visual system (Johns et al, 1993;Jun et al, 1994;Neuman, 1993;Seabra et al, 1992Seabra et al, , 1993Seabra et al, , 1995. This clinical dilemma in disorders of the visual system is shared by other degenerative disorders elsewhere in the nervous system including the spinal cord and central nervous system such as amyotrophic lateral sclerosis and Alzheimer's disease.…”

Gene Therapy for Choroideremia Using an Adeno-Associated Viral (AAV) Vector

“…REP1 also helps escort prenylated Rabs to their specific destination membrane by binding the hydrophobic, insoluble prenyl groups and carrying them through the cytoplasm (Pylypenko et al 2003). Rabs themselves have an important function in intracellular vesicle trafficking, for which the addition of a prenyl moiety "lipid anchor" is essential (Seabra et al 1995). Thus, in choroideremia, the affected gene plays a role in fundamental cellular processes.…”

“…Instead, the gene product, REP1, is involved in lipid modification of Rab GTPases, members of the Ras superfamily of monomeric G proteins ( Fig. 2) (Seabra et al 1992(Seabra et al , 1995Pylypenko et al 2003). This modification, known as prenylation, involves the covalent attachment of one or more prenyl groups (geranylgeranyl or farnesyl) to carboxy-terminal cysteines of a Rab protein.…”

“…Through its action on Rab GTPases, REP1 plays a role in the fundamental cellular processes of intracellular vesicle trafficking and recycling. Despite about two decades of work to understand the role of REP1 deficiency in the pathogenesis of the disease (e.g., Seabra et al 1992Seabra et al , 1993Seabra et al , 1995Larijani et al 2003;Pylypenko et al 2003;Tolmachova et al 2006Tolmachova et al , 2010, no established treatments currently exist to stop or even slow the progression of retinal degeneration in choroideremia.…”