Manumycin A br Methods br Author contribution statement br C

Methods
Author contribution statement
Competing financial interest
Introduction Retinitis pigmentosa (RP) is just one of a group of heterogeneous inherited retinopathies and monogenic disorders that affect thousands of Americans and millions more worldwide (Daiger et al., 2013; Hamel, 2006, 2014; Hartong et al., 2006). Clinical features of RP include night blindness, and progressive peripheral vision loss that eventually results in complete blindness in some individuals (Daiger et al., 2013; Jin et al., 2014). In most forms of RP, the disease-causing mutations cause a selective death of photoreceptor Manumycin A with only limited injury to the inner layers of the retina. Although thousands of mutations distributed across more than one hundred genes have been reported to cause RP (Hartong et al., 2006; Jin et al., 2014), identifying the causative mutations remains elusive in as many as one fourth of all individuals with the clinical features of this disease. Next-generation sequencing has been extremely valuable in identifying the genes causing some of the rarer forms of RP which had eluded detection by the positional cloning and candidate gene methods of the preceding decades (DeLuca et al., 2016; Jin et al., 2014; Small et al., 2016; Tucker et al., 2013b). Through whole exome sequencing of a single individual with early-onset RP, we previously identified novel compound heterozygous hypomorphic mutations in the tRNA Nucleotidyl Transferase, CCA-Adding, 1 gene, (TRNT1) (DeLuca et al., 2016). Additional screening of this gene in a large population of RP patients revealed a very similar genotype in two members of a second family who both exhibited clinical findings that were almost identical to those of the original patient (DeLuca et al., 2016). TRNT1 is a template independent RNA polymerase that post-transcriptionally adds the CCA sequence required for tRNA aminoacylation at the 3′ end of all tRNAs (Chakraborty et al., 2014; Shi et al., 1998). The enzyme participates in tRNA quality control and stress response (Chakraborty et al., 2014), and is the only human CCA-inserting enzyme involved in the maturation of cytoplasmic and mitochondrial tRNAs (Chakraborty et al., 2014; Czech et al., 2013; Igarashi et al., 2011). Previously, biallelic partial loss-of-function mutations within the TRNT1 gene were shown to cause congenital sideroblastic anemias (CSA) characterized by B-cell immunodeficiency, periodic fevers and development delay (SIFD) (Chakraborty et al., 2014; Liwak-Muir et al., 2016). SIFD pedigrees display an autosomal recessive Mendelian mode of inheritance with some patients also exhibiting severe variable sensorineural hearing loss, cardiomyopathy, central nervous system abnormalities, ataxia and even RP (Chakraborty et al., 2014; Wiseman et al., 2013). This severe multi-organ disease often results in death within the first decade of life (Chakraborty et al., 2014; DeLuca et al., 2016),(Liwak-Muir et al., 2016). The three individuals we identified with hypomorphic TRNT1 mutations had some erythrocytic microcytosis and anisocytosis but only mild anemia – very different from the severe SIFD phenotype of previously reported cases of TRNT1 associated disease (DeLuca et al., 2016). It is interesting that mild dysfunction of a ubiquitous enzyme that is absolutely required for protein synthesis would cause a disease almost entirely limited to photoreceptor cells. Rod photoreceptor cells may be especially sensitive to hypomorphic mutations in genes with roles in RNA and protein synthesis due to the fact that they have one of the highest levels of protein synthesis of any post-mitotic cell type, regenerating approximately 10% of their outer segments daily (Kevany and Palczewski, 2010). Consistent with this notion, mutations in a number of ubiquitously expressed RNA splicing genes (e.g. RP9, PRPF3, PRPF8, PRPF31, and SNRNP200) cause selective photoreceptor cell death without having adverse phenotypic affects in other cell types of the body (Zhao et al., 2009).