Heterozygous mutations in the PRPF31 gene cause autosomal dominant retinitis pigmentosa (adRP), a hereditary disorder leading to progressive blindness. In some cases, such mutations display incomplete penetrance, implying that certain carriers develop retinal degeneration while others have no symptoms at all. Asymptomatic carriers are protected from the disease by a higher than average expression of the PRPF31 allele that is not mutated, mainly through the action of an unknown modifier gene mapping to chromosome 19q13.4. We investigated a large family with adRP segregating an 11-bp deletion in PRPF31. The analysis of cell lines derived from asymptomatic and affected individuals revealed that the expression of only one gene among a number of candidates within the 19q13.4 interval significantly correlated with that of PRPF31, both at the mRNA and protein levels, and according to an inverse relationship. This gene was CNOT3, encoding a subunit of the Ccr4-not transcription complex. In cultured cells, siRNA–mediated silencing of CNOT3 provoked an increase in PRPF31 expression, confirming a repressive nature of CNOT3 on PRPF31. Furthermore, chromatin immunoprecipitation revealed that CNOT3 directly binds to a specific PRPF31 promoter sequence, while next-generation sequencing of the CNOT3 genomic region indicated that its variable expression is associated with a common intronic SNP. In conclusion, we identify CNOT3 as the main modifier gene determining penetrance of PRPF31 mutations, via a mechanism of transcriptional repression. In asymptomatic carriers CNOT3 is expressed at low levels, allowing higher amounts of wild-type PRPF31 transcripts to be produced and preventing manifestation of retinal degeneration.
Worldwide, more than 3 million children are infected with HIV and, without treatment, mortality among these children is extremely high. Both acute and chronic malnutrition are major problems for HIV-positive children living in resource-limited settings. Malnutrition on a background of HIV represents a separate clinical entity, with unique medical and social aetiological factors. Children with HIV have a higher daily calorie requirement than HIV-negative peers and also a higher requirement for micronutrients; furthermore, coinfection and chronic diarrhoea due to HIV enteropathy play a major role in HIV-associated malnutrition. Contributory factors include late presentation to medical services, unavailability of antiretroviral therapy, other issues surrounding healthcare provision and food insecurity in HIV-positive households. Treatment protocols for malnutrition have been greatly improved, yet there remains a discrepancy in mortality between HIV-positive and HIV-negative children. In this review, the aetiology, prevention and treatment of malnutrition in HIV-positive children are examined, with particular focus on resource-limited settings where this problem is most prevalent.
Retinitis pigmentosa (RP) is a genetically heterogenous group of inherited disorders, characterized by death of the retinal photoreceptor cells, leading to progressive visual impairment. One form of RP is caused by mutations in the ubiquitously expressed splicing factor, PRPF31, this form being known as RP11. An intriguing feature of RP11 is the presence of non-penetrance, which has been observed in the majority of PRPF31 mutation-carrying families. In contrast to variable expressivity, which is highly pervasive, true non-penetrance is a very rare phenomenon in Mendelian disorders. In this article, the molecular mechanisms underlying phenotypic non-penetrance in RP11 are explored. It is an elegant example of how our understanding of monogenic disorders has evolved from studying only the disease gene, to considering a mutation on the genetic background of the individual - the logical evolution in this genomic era.
PRPF31-associated retinitis pigmentosa presents a fascinating enigma: some mutation carriers are blind, while others are asymptomatic. We identify the major molecular cause of this incomplete penetrance through three cardinal features: (1) there is population variation in the number (3 or 4) of a minisatellite repeat element (MSR1) adjacent to the PRPF31 core promoter; (2) in vitro, 3-copies of the MSR1 element can repress gene transcription by 50 to 115-fold; (3) the higher-expressing 4-copy allele is not observed among symptomatic PRPF31 mutation carriers and correlates with the rate of asymptomatic carriers in different populations. Thus, a linked transcriptional modifier decreases PRPF31 gene expression that leads to haploinsufficiency. This result, taken with other identified risk alleles, allows precise genetic counseling for the first time. We also demonstrate that across the human genome, the presence of MSR1 repeats in the promoters or first introns of genes is associated with greater population variability in gene expression indicating that copy number variation of MSR1s is a generic controller of gene expression and promises to provide new insights into our understanding of gene expression regulation.
PRPF31, a gene located at chromosome 19q13.4, encodes the ubiquitous splicing factor PRPF31. The gene lies in a head-to-head arrangement with TFPT, a poorly characterized gene with a role in cellular apoptosis. Mutations in PRPF31 have been implicated in autosomal dominant retinitis pigmentosa (adRP), a frequent and important cause of blindness worldwide. Disease associated with PRPF31 mutations is unusual, in that there is often non-penetrance of the disease phenotype in affected families, caused by differential expression of PRPF31. This study aimed to characterize the basic promoter elements of PRPF31 and TFPT. Luciferase reporter constructs were made, using genomic DNA from an asymptomatic individual with a heterozygous deletion of the entire putative promoter region. Fragments were tested by the dual-luciferase reporter assay in HeLa and RPE-1 cell lines. A comparison was made between the promoter regions of symptomatic and asymptomatic mutation-carrying individuals. A patient (CAN493) with adRP was identified, harbouring a regulatory region mutation; both alleles were assayed by the dual-luciferase reporter assay. Luciferase assays led to the identification of core promoters for both PRPF31 and TFPT; despite their shared gene architecture, the two genes appear to be controlled by slightly different regulatory regions. One functional polymorphism was identified in the PRPF31 promoter that increased transcriptional activation. The change was not, however, consistent with the observed symptomatic-asymptomatic phenotypes in a family affected by PRPF31-adRP. Analysis of the mutant promoter fragment from CAN493 showed a >50% reduction in promoter activity, suggesting a disease mechanism of functional haploinsufficiency-the first report of this disease mechanism in adRP.
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