Mutational spectrum of the ADAR1 gene in dyschromatosis symmetrica hereditaria

Multi-dimensional Insight into the Coexistence of Pathogenic Genes for ADAR1 and TSC2: Careful Consideration is Essential for Interpretation of ADAR1 Variants

Aicardi-Goutières syndrome 6 (AGS6) is a serious auto-immunization-associated acute neurologic decompensation. AGS6 manifests as acute onset of severe generalized dystonia of limbs and developmental regression secondary to febrile illness mostly. Dyschromatosis symmetrica hereditaria (DSH), as pigmentary genodermatosis, is a characterized mixture of hyperpigmented and hypopigmented macules. Both AGS6 and DSH are associated with ADAR1 pathogenic variants. To explore the etiology of a proband with developmental regression with mixture of hyperpigmentation and hypopigmentation macules, we used the trio-WES. Later, to clarify the association between variants and diseases, we used guidelines of ACMG for variants interpretation and quantitative Real-time PCR for verifying elevated expression levels of interferon-stimulated genes, separately. By WES, we detected 2 variants in ADAR1 and a variant in TSC2, respectively, were NM_001111.5:c.1096_1097del, NM_001111.5:c.518A>G, and NM_000548.5:c.1864C>T. Variants interpretation suggested that these 3 variants were both pathogenic. Expression levels of interferon-stimulated genes also elevated as expected. We verified the co-occurrence of pathogenic variants of ADAR1 and TSC2 in AGS6 patients with DSH. Our works contributed to the elucidation of ADAR1 pathogenic mechanism, given the specific pathogenic mechanism of ADAR1, and it is necessary to consider with caution when variants were found in ADAR1.

The role of dsRNA A-to-I editing catalyzed by ADAR family enzymes in the pathogeneses

The process of adenosine deaminase (ADAR)-catalyzed double-stranded RNA (dsRNA) Adenosine-to-Inosine (A-to-I) editing is essential for the correction of pathogenic mutagenesis, as well as the regulation of gene expression and protein function in mammals. The significance of dsRNA A-to-I editing in disease development and occurrence is explored using inferential statistics and cluster analyses to investigate the enzymes involved in dsRNA editing that can catalyze editing sites across multiple biomarkers. This editing process, which occurs in coding or non-coding regions, has the potential to activate abnormal signalling pathways that contributes to disease pathogenesis. Notably, the ADAR family enzymes play a crucial role in initiating the editing process. ADAR1 is upregulated in most diseases as an oncogene during tumorigenesis, whereas ADAR2 typically acts as a tumour suppressor. Furthermore, this review also provides an overview of small molecular inhibitors that disrupt the expression of ADAR enzymes. These inhibitors not only counteract tumorigenicity but also alleviate autoimmune disorders, neurological neurodegenerative symptoms, and metabolic diseases associated with aberrant dsRNA A-to-I editing processes. In summary, this comprehensive review offers detailed insights into the involvement of dsRNA A-to-I editing in disease pathogenesis and highlights the potential therapeutic roles for related small molecular inhibitors. These scientific findings will undoubtedly contribute to the advancement of personalized medicine based on dsRNA A-to-I editing.

Inosine and its methyl derivatives: Occurrence, biogenesis, and function in RNA

Inosine is one of the most common post-transcriptional modifications. Since its discovery, it has been noted for its ability to contribute to non-Watson-Crick interactions within RNA. Rapidly accumulating evidence points to the widespread generation of inosine through hydrolytic deamination of adenosine to inosine by different classes of adenosine deaminases. Three naturally occurring methyl derivatives of inosine, i.e., 1-methylinosine, 2'-O-methylinosine and 1,2'-O-dimethylinosine are currently reported in RNA modification databases. These modifications are expected to lead to changes in the structure, folding, dynamics, stability and functions of RNA. The importance of the modifications is indicated by the strong conservation of the modifying enzymes across organisms. The structure, binding and catalytic mechanism of the adenosine deaminases have been well-studied, but the underlying mechanism of the catalytic reaction is not very clear yet. Here we extensively review the existing data on the occurrence, biogenesis and functions of inosine and its methyl derivatives in RNA. We also included the structural and thermodynamic aspects of these modifications in our review to provide a detailed and integrated discussion on the consequences of A-to-I editing in RNA and the contribution of different structural and thermodynamic studies in understanding its role in RNA. We also highlight the importance of further studies for a better understanding of the mechanisms of the different classes of deamination reactions. Further investigation of the structural and thermodynamic consequences and functions of these modifications in RNA should provide more useful information about their role in different diseases.

The role of RNA editing enzyme ADAR1 in human disease

Adenosine deaminase acting on RNA (ADAR) catalyzes the posttranscriptional conversion of adenosine to inosine in double-stranded RNA (dsRNA), which can lead to the creation of missense mutations in coding sequences. Recent studies show that editing-dependent functions of ADAR1 protect dsRNA from dsRNA-sensing molecules and inhibit innate immunity and the interferon-mediated response. Deficiency in these ADAR1 functions underlie the pathogenesis of autoinflammatory diseases such as the type I interferonopathies Aicardi-Goutieres syndrome and dyschromatosis symmetrica hereditaria. ADAR1-mediated editing of endogenous coding and noncoding RNA as well as ADAR1 editing-independent interactions with DICER can also have oncogenic or tumor suppressive effects that affect tumor proliferation, invasion, and response to immunotherapy. The combination of proviral and antiviral roles played by ADAR1 in repressing the interferon response and editing viral RNAs alters viral morphogenesis and cell susceptibility to infection. This review analyzes the structure and function of ADAR1 with a focus on its position in human disease pathways and the mechanisms of its disease-associated effects. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > RNA Editing and Modification RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Mendelian disease caused by variants affecting recognition of Z-DNA and Z-RNA by the Zα domain of the double-stranded RNA editing enzyme ADAR

Variants in the human double-stranded RNA editing enzyme ADAR produce three well-characterized rare Mendelian Diseases: Dyschromatosis Symmetrica Hereditaria (OMIM: 127400), Aicardi-Goutières syndrome (OMIM: 615010) and Bilateral Striatal Necrosis/Dystonia. ADAR encodes p150 and p110 protein isoforms. p150 incorporates the Zα domain that binds left-handed Z-DNA and Z-RNA with high affinity through contact of highly conserved residues with the DNA and RNA double helix. In certain individuals, frameshift variants on one parental chromosome in the second exon of ADAR produce haploinsufficiency of p150 while maintaining normal expression of p110. In other individuals, loss of p150 expression from one chromosome allows mapping of Zα p150 variants from the other parental chromosome directly to phenotype. The analysis reveals that loss of function Zα variants cause dysregulation of innate interferon responses to double-stranded RNA. This approach confirms a biological role for the left-handed conformation in human disease, further validating the power of Mendelian genetics to deliver unambiguous answers to difficult questions. The findings reveal that the human genome encodes genetic information using both shape and sequence.

Analysis of genotype/phenotype correlations in Japanese patients with dyschromatosis symmetrica hereditaria

Dyschromatosis symmetrica hereditaria (DSH) is one of the genetic pigmentation disorders and shows characteristic mixture of hyper- and hypo-pigmented small macules on the extremities. Heterozygous mutations in the adenosine deaminase acting on RNA1 gene (ADAR1) cause DSH. In the present study, we report five cases of DSH and identify a distinct known mutation in each patient. Furthermore, we review previously described cases with the five ADAR1 mutations found in the present study. We reviewed clinical and molecular findings in the present and previously reported cases and found an identical mutation can result in various phenotypic severities, even in one family. We found novel phenotype-genotype correlations between the presence/absence of facial lesions and the ADAR1 mutation c.3286C>T. The absence of freckle-like macules in the face was found to be more commonly associated with the mutation c.3286C>T than with the other 4 ADAR1 mutations (odds ratio = 0.056 [95% CI: 0.007–0.47, p < 0.005]). We objectively evaluated the severity of skin manifestations in the extremities using our definition of severity levels for such manifestations. This is the first semi-quantitative evaluation of skin manifestations in DSH. Using our definition, we found that patients with facial lesions with or without hypopigmented macules tend to show more severe symptoms on the extremities than patients without facials lesions show. Furthermore, no significant difference in the severity of the skin lesions was observed between the upper and the lower extremities, suggesting that sun exposure does not affect significantly the pathogenesis of DSH skin lesions.

Eight Novel Mutations of the ADAR1 Gene in Chinese Patients with Dyschromatosis Symmetrica Hereditaria

AIMS

To identify potential novel gene mutations in Chinese patients with dyschromatosis symmetrica hereditaria (DSH).

METHODS

We enrolled 8 Chinese patients with familial DSH, 5 Chinese patients with sporadic DSH, and 100 randomly selected healthy individuals in this study. The genome of each participant was extracted from peripheral blood samples. Sanger sequencing of the ADAR1 gene was performed after polymerase chain reaction amplifications. Comparisons between the DNA sequences of the affected individuals and the NCBI database were performed.

RESULTS

We detected eight novel heterozygous mutations and five previously reported mutations in the ADAR1 gene in our patients. The novel mutations include c.1934 + 3A>G, c.2749A>G, c.2311insA, c.3233G>A, c.3019 + 1G>T, c.2894C>A, c.1202_1205del, and c.2280C>A. These detected novel mutations are predicted to induce two frame-shift mutations, one nonsense mutation, three missense mutations, and two splice-site mutations.

CONCLUSIONS

The findings of this study expand our knowledge of the range of ADAR1 gene mutations in DSH and will contribute to identifying correlations between the various DSH phenotypes and genotypes. Furthermore, they may provide insight into the underlying pathogenic mechanism.

Updated review of genetic reticulate pigmentary disorders

Reticulate pigmentary disorders are a group of disorders characterized by hyper- and/or hypopigmented macules with varying sizes and amounts of pigment. Some of the disorders are heritable, such as Dowling-Degos disease, dyschromatosis universalis hereditaria, dyschromatosis symmetrica hereditaria, reticulate acropigmentation of Kitamura and X-linked reticulate pigmentary disorder. Although each condition possesses unique phenotypic characteristics and the prognosis for each is somewhat different, there is a large degree of overlap between the disorders and therefore they are difficult to differentiate in the clinical setting. This updated review provides a clinical and molecular delineation of these genetic reticulate pigmentary disorders and aims to establish a concise diagnostic strategy to allow clinical dermatologists to make an accurate diagnosis, as well as to provide useful information for clinical and genetic counselling.

Adenosine Deaminases That Act on RNA (ADARs)

Inosine is one of the most common modifications found in human RNAs and the Adenosine Deaminases that act on RNA (ADARs) are the main enzymes responsible for its production. ADARs were first discovered in the 1980s and since then our understanding of ADARs has advanced tremendously. For instance, it is now known that defective ADAR function can cause human diseases. Furthermore, recently solved crystal structures of the human ADAR2 deaminase bound to RNA have provided insights regarding the catalytic and substrate recognition mechanisms. In this chapter, we describe the occurrence of inosine in human RNAs and the newest perspective on the ADAR family of enzymes, including their substrate recognition, catalytic mechanism, regulation as well as the consequences of A-to-I editing, and their relation to human diseases.

Two novel ADAR1 gene mutations in two patients with dyschromatosis symmetrical hereditaria from birth

Dyschromatosis symmetrica hereditaria (DSH) is a rare type of pigmentary genodermatosis, which is autosomal dominantly inherited with high penetrance. The onset of DSH is typically during infancy or childhood. Cases of patients born with skin lesions have rarely been reported. Additionally, there have been few significant non‑cutaneous complications reported with DSH. The present study reported two sporadic cases of patients born with DSH, confirmed by the identification of ADAR1 mutations. Additionally, comorbidity of DSH, congenital heart disease (CHD) and hemangioma disease were first reported. In the patient with isolated DSH from birth, a nonsense mutation (p.Y1192X) was identified, whereas in the second patient with DSH, CHD and hemangioma from birth, a frameshift mutation (p.Glu673ValfsX652) in ADAR1 was identified. To the best of the authors' knowledge, >120 mutations in ADAR1 have been reported to cause DSH; however, no previous studies have reported mutations in ADAR1 in DSH at birth, with CHD and hemangioma. The novel variants described in the current study add to the current knowledge of ADAR1 mutations in DSH.

Identification a nonsense mutation of APC gene in Chinese patients with familial adenomatous polyposis

Familial adenomatous polyposis (FAP; Mendelian of Inherintance in Man ID, 175100) is a rare autosomal dominant disorder characterized by the development of numerous adenomatous polyps throughout the colon and rectum associated with an increased risk of colorectal cancer. FAP is at time accompanied with certain extraintestinal manifestations such as congenital hypertrophy of the retinal pigment epithelium, dental disorders and desmoid tumors. It is caused by mutations in the adenomatous polyposis coli (APC) gene. The present study reported on a Chinese family with FAP. Polymerase chain reaction and direct sequencing of the full coding sequence of the APC gene were performed to identify the mutation in this family. A nonsense mutation of the APC gene was identified in this pedigree. It is a heterozygous G>T substitution at position 2,971 in exon 15 of the APC gene, which formed a premature stop codon at amino acid residue 991 (p.Glu991*). The resulting truncated protein lacked 1,853 amino acids. The present study expanded the database on APC gene mutations in FAP and enriched the spectrum of known germline mutations of the APC gene. Prophylactic proctocolectomy may be considered as a possible treatment for carriers of the mutation.

Functions of the RNA Editing Enzyme ADAR1 and Their Relevance to Human Diseases

Adenosine deaminases acting on RNA (ADARs) convert adenosine to inosine in double-stranded RNA (dsRNA). Among the three types of mammalian ADARs, ADAR1 has long been recognized as an essential enzyme for normal development. The interferon-inducible ADAR1p150 is involved in immune responses to both exogenous and endogenous triggers, whereas the functions of the constitutively expressed ADAR1p110 are variable. Recent findings that ADAR1 is involved in the recognition of self versus non-self dsRNA provide potential explanations for its links to hematopoiesis, type I interferonopathies, and viral infections. Editing in both coding and noncoding sequences results in diseases ranging from cancers to neurological abnormalities. Furthermore, editing of noncoding sequences, like microRNAs, can regulate protein expression, while editing of Alu sequences can affect translational efficiency and editing of proximal sequences. Novel identifications of long noncoding RNA and retrotransposons as editing targets further expand the effects of A-to-I editing. Besides editing, ADAR1 also interacts with other dsRNA-binding proteins in editing-independent manners. Elucidating the disease-specific patterns of editing and/or ADAR1 expression may be useful in making diagnoses and prognoses. In this review, we relate the mechanisms of ADAR1′s actions to its pathological implications, and suggest possible mechanisms for the unexplained associations between ADAR1 and human diseases.

A novel P53/POMC/Gαs/SASH1 autoregulatory feedback loop activates mutated SASH1 to cause pathologic hyperpigmentation

p53-Transcriptional-regulated proteins interact with a large number of other signal transduction pathways in the cell, and a number of positive and negative autoregulatory feedback loops act upon the p53 response. P53 directly controls the POMC/α-MSH productions induced by ultraviolet (UV) and is associated with UV-independent pathological pigmentation. When identifying the causative gene of dyschromatosis universalis hereditaria (DUH), we found three mutations encoding amino acid substitutions in the gene SAM and SH3 domain containing 1 (SASH1), and SASH1 was associated with guanine nucleotide-binding protein subunit-alpha isoforms short (Gαs). However, the pathological gene and pathological mechanism of DUH remain unknown for about 90 years. We demonstrate that SASH1 is physiologically induced by p53 upon UV stimulation and SASH and p53 is reciprocally induced at physiological and pathophysiological conditions. SASH1 is regulated by a novel p53/POMC/α-MSH/Gαs/SASH1 cascade to mediate melanogenesis. A novel p53/POMC/Gαs/SASH1 autoregulatory positive feedback loop is regulated by SASH1 mutations to induce pathological hyperpigmentation phenotype. Our study demonstrates that a novel p53/POMC/Gαs/SASH1 autoregulatory positive feedback loop is regulated by SASH1 mutations to induce pathological hyperpigmentation phenotype.

Probing RNA recognition by human ADAR2 using a high-throughput mutagenesis method

Adenosine deamination is one of the most prevalent post-transcriptional modifications in mRNA. In humans, ADAR1 and ADAR2 catalyze this modification and their malfunction correlates with disease. Recently our laboratory reported crystal structures of the human ADAR2 deaminase domain bound to duplex RNA revealing a protein loop that binds the RNA on the 5′ side of the modification site. This 5′ binding loop appears to be one contributor to substrate specificity differences between ADAR family members. In this study, we endeavored to reveal detailed structure–activity relationships in this loop to advance our understanding of RNA recognition by ADAR2. To achieve this goal, we established a high-throughput mutagenesis approach which allows rapid screening of ADAR variants in single yeast cells and provides quantitative evaluation for enzymatic activity. Using this approach, we determined the importance of specific amino acids at 19 different positions in the ADAR2 5′ binding loop and revealed six residues that provide essential structural elements supporting the fold of the loop and key RNA-binding functional groups. This work provided new insight into RNA recognition by ADAR2 and established a new tool for defining structure–function relationships in ADAR reactions.

Mutations in the mevalonate pathway genes in Chinese patients with porokeratosis

BACKGROUND

Porokeratosis (PK, MIM 175800) is a chronic autosomal dominant cutaneous keratinization disorder, which has a wide variety of clinical manifestations.

OBJECTIVES

We analysed the molecular basis of 10 families and 12 sporadic cases with different subtypes of porokeratosis in the Chinese population.

METHODS

Genomic DNA was extracted from peripheral blood samples. Mutation screening was performed by direct sequencing of exons and flanking intron-exon boundaries for the entire coding region of four mevalonate pathway genes and SLC17A9 gene.

RESULTS

We detected three novel mutations and seven previously described mutations by direct sequence analysis of the PCR products. Mutations p.Phe249Ser and p.Asn292Ser in mevalonate decarboxylase (MVD) were the most common mutations in this PK cohort; their presence was 27.3% and 13.6% respectively.

CONCLUSIONS

This study extended the mutation spectrum of PK in the Chinese Han population and provided further evidence for the genetic basis of PK. We first identified MVD simultaneously responsible for porokeratosis palmaris et plantaris disseminate development and confirmed the genotype-phenotype correlations.

Genetic spectrum of dyschromatosis symmetrica hereditaria in Chinese patients including a novel nonstop mutation in ADAR1 gene

Background

Dyschromatosis symmetrica hereditaria (DSH) is a rare autosomal dominant cutaneous disorder caused by the mutations of adenosine deaminase acting on RNA1 (ADAR1) gene. We present a clinical and genetic study of seven unrelated families and two sporadic cases with DSH for mutations in the full coding sequence of ADAR1 gene.

Methods

ADAR1 gene was sequenced in seven unrelated families and two sporadic cases with DSH and 120 controls. Functional significance of the observed ADAR1 mutations was analyzed using PolyPhen 2, SIFT and DDIG-in.

Results

We describe six novel mutations of the ADAR1 gene in Chinese patients with DSH including a nonstop mutation p.Stop1227R, which was firstly reported in ADAR1 gene. In silico analysis proves that all the mutations reported here are pathogenic.

Conclusion

This study is useful for functional studies of the protein and to define a diagnostic strategy for mutation screening of the ADAR1 gene. A three-generation family exhibiting phenotypic variability with a single germline ADAR1 mutation suggests that chilblain might aggravate the clinical phenotypes of DSH.

Recognition of duplex RNA by the deaminase domain of the RNA editing enzyme ADAR2

Adenosine deaminases acting on RNA (ADARs) hydrolytically deaminate adenosines (A) in a wide variety of duplex RNAs and misregulation of editing is correlated with human disease. However, our understanding of reaction selectivity is limited. ADARs are modular enzymes with multiple double-stranded RNA binding domains (dsRBDs) and a catalytic domain. While dsRBD binding is understood, little is known about ADAR catalytic domain/RNA interactions. Here we use a recently discovered RNA substrate that is rapidly deaminated by the isolated human ADAR2 deaminase domain (hADAR2-D) to probe these interactions. We introduced the nucleoside analog 8-azanebularine (8-azaN) into this RNA (and derived constructs) to mechanistically trap the protein–RNA complex without catalytic turnover for EMSA and ribonuclease footprinting analyses. EMSA showed that hADAR2-D requires duplex RNA and is sensitive to 2′-deoxy substitution at nucleotides opposite the editing site, the local sequence and 8-azaN nucleotide positioning on the duplex. Ribonuclease V1 footprinting shows that hADAR2-D protects ∼23 nt on the edited strand around the editing site in an asymmetric fashion (∼18 nt on the 5′ side and ∼5 nt on the 3′ side). These studies provide a deeper understanding of the ADAR catalytic domain–RNA interaction and new tools for biophysical analysis of ADAR–RNA complexes.

Mutations in POFUT1, encoding protein O-fucosyltransferase 1, cause generalized Dowling-Degos disease

Dowling-Degos disease (DDD), or reticular pigmented anomaly of the flexures, is a type of rare autosomal-dominant genodermatosis characterized by reticular hyperpigmentation and hypopigmentation of the flexures, such as the neck, axilla, and areas below the breasts and groin, and shows considerable heterogeneity. Loss-of-function mutations of keratin 5 (KRT5) have been identified in DDD individuals. In this study, we collected DNA samples from a large Chinese family affected by generalized DDD and found no mutation of KRT5. We performed a genome-wide linkage analysis of this family and mapped generalized DDD to a region between rs1293713 and rs244123 on chromosome 20 [corrected]. By exome sequencing, we identified nonsense mutation c.430G>T (p.Glu144(∗)) in POFUT1, which encodes protein O-fucosyltransferase 1, in the family. Study of an additional generalized DDD individual revealed the heterozygous deletion mutation c.482delA (p.Lys161Serfs(∗)42) in POFUT1. Knockdown of POFUT1 reduces the expression of NOTCH1, NOTCH2, HES1, and KRT5 in HaCaT cells. Using zebrafish, we showed that pofut1 is expressed in the skin and other organs. Morpholino knockdown of pofut1 in zebrafish produced a phenotype characteristic of hypopigmentation at 48 hr postfertilization (hpf) and abnormal melanin distribution at 72 hpf, replicating the clinical phenotype observed in our DDD individuals. At 48 and 72 hpf, tyrosinase activities decreased by 33% and 45%, respectively, and melanin protein contents decreased by 20% and 25%, respectively. Our findings demonstrate that POFUT1 mutations cause generalized DDD. These results strongly suggest that the protein product of POFUT1 plays a significant and conserved role in melanin synthesis and transport.

Mechanistic insights into editing-site specificity of ADARs

Adenosine deaminases that act on RNA (ADARs) deaminate adenosines in dsRNA to produce inosines. ADARs are essential in mammals and are particularly important in the nervous system. Altered levels of adenosine-to-inosine (A-to-I) editing are observed in several diseases. The extent to which an adenosine is edited depends on sequence context. Human ADAR2 (hADAR2) has 5' and 3' neighbor preferences, but which amino acids mediate these preferences, and by what mechanism, is unknown. We performed a screen in yeast to identify mutations in the hADAR2 catalytic domain that allow editing of an adenosine within a disfavored triplet. Binding affinity, catalytic rate, base flipping, and preferences were monitored to understand the effects of the mutations on ADAR reactivity. Our data provide information on the amino acids that affect preferences and point to a conserved loop as being of key importance. Unexpectedly, our data suggest that hADAR2's preferences derive from differential base flipping rather than from direct recognition of neighboring bases. Our studies set the stage for understanding the basis of altered editing levels in disease and for developing therapeutic reagents.

Mutations in ADAR1 cause Aicardi-Goutières syndrome associated with a type I interferon signature

Adenosine deaminases acting on RNA (ADARs) catalyze the hydrolytic deamination of adenosine to inosine in double-stranded RNA (dsRNA) and thereby potentially alter the information content and structure of cellular RNAs. Notably, although the overwhelming majority of such editing events occur in transcripts derived from Alu repeat elements, the biological function of non-coding RNA editing remains uncertain. Here, we show that mutations in ADAR1 (also known as ADAR) cause the autoimmune disorder Aicardi-Goutières syndrome (AGS). As in Adar1-null mice, the human disease state is associated with upregulation of interferon-stimulated genes, indicating a possible role for ADAR1 as a suppressor of type I interferon signaling. Considering recent insights derived from the study of other AGS-related proteins, we speculate that ADAR1 may limit the cytoplasmic accumulation of the dsRNA generated from genomic repetitive elements.

Nucleoside analog studies indicate mechanistic differences between RNA-editing adenosine deaminases

Adenosine deaminases acting on RNA (ADAR1 and ADAR2) are human RNA-editing adenosine deaminases responsible for the conversion of adenosine to inosine at specific locations in cellular RNAs. Since inosine is recognized during translation as guanosine, this often results in the expression of protein sequences different from those encoded in the genome. While our knowledge of the ADAR2 structure and catalytic mechanism has grown over the years, our knowledge of ADAR1 has lagged. This is due, at least in part, to the lack of well defined, small RNA substrates useful for mechanistic studies of ADAR1. Here, we describe an ADAR1 substrate RNA that can be prepared by a combination of chemical synthesis and enzymatic ligation. Incorporation of adenosine analogs into this RNA and analysis of the rate of ADAR1 catalyzed deamination revealed similarities and differences in the way the ADARs recognize the edited nucleotide. Importantly, ADAR1 is more dependent than ADAR2 on the presence of N7 in the edited base. This difference between ADAR1 and ADAR2 appears to be dependent on the identity of a single amino acid residue near the active site. Thus, this work provides an important starting point in defining mechanistic differences between two functionally distinct human RNA editing ADARs.

Acromelanosis albo-punctata: a distinct inherited dermatosis with acral spotty dyspigmentation without systemic involvement

We describe an otherwise healthy 7-year-old boy who developed confetti-like hypopigmented macules on the dorsal aspects of the hands and feet, spreading to the palms and soles a few months after birth. In 1964 Siemens introduced the term acromelanosis albo-punctata to describe the skin features of a patient who has remained the only reported case in the literature so far and who strongly resembles our patient. By genetic testing we excluded mutations in genes known to be involved in diseases with acral hypo- or hyperpigmentation. We review the differential diagnosis of acral localized spotty dyspigmentation and conclude that acromelanosis albo-punctata may represent a distinct entity.

Identification of two novel splice mutations of the ADAR1 gene in two Chinese families with dyschromatosis symmetrica hereditaria

Dyschromatosis symmetrica hereditaria (DSH) is a rare, autosomal dominant dermatosis, characterized by a mixture of hyperpigmented and hypopigmented macules on the dorsa of the hands and feet. The DSH locus has been mapped to chromosome 1q21, and in 2003, pathogenic mutations were identified in the ADAR1 (adenosine deaminase acting on RNA1) gene. In this study, we performed mutation detection of the ADAR1 gene in two Chinese families with DSH. PCR and direct sequencing of the ADAR1 gene were used to identify and confirm the mutations in the two families. Furthermore, we analysed the RNA transcripts by reverse transcriptase (RT)-PCR. Two aberrant splice products were confirmed with RT-PCR and DNA direct sequence analysis. These novel findings further extend our understanding of the role of ADAR1 in DSH.

A-to-I RNA editing: a contribution to diversity of the transcriptome and an organism's development

The complexity of multicellular organisms requires both an increase in genetic information and fine tuning in regulation of gene expression. One of the mechanisms responsible for these functions is RNA editing. RNA editing is a complex process affecting the mechanism of changes in transcriptome sequences. The best studied example of this process is A-to-I RNA editing. On the organism's level, RNA editing plays a key role during ontogenesis and in the defense against pathogens. Disorders in A-to-I RNA editing lead to serious abnormalities. The importance of RNA editing increases with an increase in the organism's complexity. Correct RNA editing is an indispensable factor of an organism's development and probably determines the lifespan of higher eukaryotes.