Exons B and C were only detected at very low levels, indicating that they are not major players in retinal expression. encode distinct proteins. Further, the proportions of transcripts and proteins in human retina are different from those in mouse retina. Conclusions Identification of unique retinal isoforms supports the existence of a novel IMPDH1 function in the retina, one that is probably altered by disease-causing mutations. This alone, or coupled with the high levels of IMPDH1 in the retina, may explain the retina-specific phenotype associated with IMPDH1 mutations. Elucidating the functional properties of these unique, human retinal isoforms is crucial to understanding the pathophysiology of IMPDH1 mutations. Retinitis pigmentosa (RP) is a heterogeneous form of inherited retinal degeneration that affects 100,000 individuals in the United States and approximately 1.5 million individuals worldwide.1 Initial symptoms of RP include night blindness followed by loss of peripheral vision. Vision loss progresses as a result of photoreceptor death from the midperipheral retina toward the macula, culminating in tunnel vision, legal blindness, and all too often, total loss of sight.2 RP is caused by mutations in many distinct genes. To date, 16 autosomal dominant, 16 autosomal recessive, and 6 X-linked forms have been identified in addition to many other syndromic, systemic, and complex forms (RetNet; http//:www.sph.uth.tmc.edu/RetNet/ provided in the public domain by the University of Texas Houston Health Science Center, Houston, TX). Several forms of RP are caused by mutations in photoreceptor-specific or abundant proteins involved in phototransduction or the visual cycle. Understanding BRD7552 the pathophysiology of these is well advanced due to the large amount already known about the pathways involved.3 Other forms of RP are caused by mutations in more widely expressed genes whose mechanisms of action are largely unknown. One such gene is inosine monophosphate dehydrogenase 1 (cause the RP10 form of autosomal dominant RP (adRP) and are also a rare cause of isolated Leber congenital amaurosis (LCA).4-6 is located on chromosome 7, region q32.1, and codes for the enzyme IMPDH type 1. Genes coding for IMPDH are found in all eukaryotes and most prokaryotes, and are highly conserved across species at both the gene and protein levels.7,8 Like most mammals, humans have an and an gene. These genes encode enzymes that are 84% identical at the amino acid level.9 All IMPDH proteins form active homotetramers that catalyze the rate-limiting step of de novo guanine synthesis by converting inosine monophosphate (IMP) to xanthosine monophosphate (XMP) with the reduction of NAD. Each IMPDH monomer is composed of an eight-stranded barrel structure, which performs the enzymatic function, BRD7552 and a flanking subdomain, which is composed of two CBS regions similar to the cystathionine transcripts were identified and described in human cells and tissues.17 These transcripts differ in size (4.0, 2.7, and 2.5 kb) but contain identical coding sequences, derived from 14 exons, and identical 3-untranslated regions (UTRs; for example, see Fig. 4A). The three transcripts differ only in splicing of three alternate untranslated 5 exons, historically designated A, B, and C. Each of these transcripts encodes an identical protein that we refer to as canonical IMPDH1, a 55.6-kDa protein 514 amino acids in length. In contrast, the human gene, though identical in exonic structure within the coding region, does not contain sequences homologous to the 5 exons A, B, or C of Rabbit polyclonal to Complement C3 beta chain and are from the exon 1F to 3UTR R product subcloning. Transcripts are from the exon A F to 3UTR R product subcloning. Transcript percentages reflect results of separate subcloning experiments. Only transcripts seen in greater than 2% of clones from each subcloning experiment are reported. mutations cause retinal degeneration BRD7552 only, despite a much wider expression pattern.4,5,18 The disease mechanism of these mutations and the reason that they manifest as a retina-specific phenotype are currently unknown. One possibility is that mutations are null and cause a loss of enzyme activity that only affects photoreceptors or that is compensated for by in other tissues. However, several studies have shown that mutations do not affect enzyme activity and hence make this an unlikely mechanism for disease.6,19,20 Another possibility is that the mutations.