Two new mouse alleles of Ocm and Slc26a5.

The genes Ocm (encoding oncomodulin) and Slc26a5 (encoding prestin) are expressed strongly in outer hair cells and both are involved in deafness in mice. However, it is not clear if they influence the expression of each other. In this study, we characterise the auditory phenotype resulting from two new mouse alleles, Ocmtm1e and Slc26a5tm1Cre. Each mutation leads to absence of detectable mRNA transcribed from the mutant allele, but there was no evidence that oncomodulin regulates expression of prestin or vice versa. The two mutants show distinctive patterns of auditory dysfunction. Ocmtm1e homozygotes have normal auditory brainstem response thresholds at 4 weeks old followed by progressive hearing loss starting at high frequencies, while heterozygotes show largely normal thresholds until 6 months of age, when signs of worse thresholds are detected. In contrast, Slc26a5tm1Cre homozygotes have stable but raised thresholds across all frequencies tested, 3 to 42 kHz, at least from 4 to 8 weeks old, while heterozygotes have raised thresholds at high frequencies. Distortion product otoacoustic emissions and cochlear microphonics show deficits similar to auditory brainstem responses in both mutants, suggesting that the origin of hearing impairment is in the outer hair cells. Endocochlear potentials are normal in the two mutants. Scanning electron microscopy revealed normal development of hair cells in Ocmtm1e homozygotes but scattered outer hair cell loss even at 4 weeks old when thresholds appeared normal, indicating that there is not a direct relationship between numbers of outer hair cells present and auditory thresholds.

The d3GHR carrier epigenome in Druze clan longevity.

The Druze are a distinct group known for their close community, traditions, and consanguineous marriages, dating back to the eleventh century. This practice has led to unique genetic variations, impacting both pathology and gene-associated phenotypes. Some Druze clans, particularly those with exceptional long-lived family heads (ELLI), attracted attention. Given that the bulk of these ELLI were men, the d3GHR polymorphism was the first obvious possibility. Among the 73 clan members, 8.2% carried the d3GHR isoform, with nearly 11% being males. There was a significant age-related increase (p = 0.04) in this isoform among males, leading to examination of potential environmental mediators affecting gene regulation among these carriers during life (namely epigenetic). We focused on DNA methylation due to its crucial role in gene regulation, development, and disease progression. We analyzed DNA samples from 14 clan members with different GHR genotypes, finding a significant (p < 0.05) negative correlation between DNA methylation levels and age. Employing a biological age clock, we observed a significant + 4.229 years favoring the d3GHR group over the WT and heterozygous groups. In conclusion, this study highlights the advantage of d3GHR carriers among this unique Druze clan and underscores the importance of genotype-environment interaction in epigenetic regulation and its impact on health.

Causal interpretations of family GWAS in the presence of heterogeneous effects.

Family-based genome-wide association studies (GWASs) are often claimed to provide an unbiased estimate of the average causal effects (or average treatment effects; ATEs) of alleles, on the basis of an analogy between the random transmission of alleles from parents to children and a randomized controlled trial. We show that this claim does not hold in general. Because Mendelian segregation only randomizes alleles among children of heterozygotes, the effects of alleles in the children of homozygotes are not observable. This feature will matter if an allele has different average effects in the children of homozygotes and heterozygotes, as can arise in the presence of gene-by-environment interactions, gene-by-gene interactions, or differences in linkage disequilibrium patterns. At a single locus, family-based GWAS can be thought of as providing an unbiased estimate of the average effect in the children of heterozygotes (i.e., a local average treatment effect; LATE). This interpretation does not extend to polygenic scores (PGSs), however, because different sets of SNPs are heterozygous in each family. Therefore, other than under specific conditions, the within-family regression slope of a PGS cannot be assumed to provide an unbiased estimate of the LATE for any subset or weighted average of families. In practice, the potential biases of a family-based GWAS are likely smaller than those that can arise from confounding in a standard, population-based GWAS, and so family studies remain important for the dissection of genetic contributions to phenotypic variation. Nonetheless, their causal interpretation is less straightforward than has been widely appreciated.

The genetic analysis of eight families with hemophilia B in Mongolia: Identification of two novel mutation.

BACKGROUND: This study aimed to conduct molecular diagnostics among individuals with hemophilia B (HB) and carriers of hemophilia in Mongolia. METHODS: Eight patients (six severe, two mild) with HB and their 12 female relatives were enrolled from eight families. Sanger sequence was performed for mutation identification. The questionnaire survey was conducted to evaluate carrier symptoms in female relatives. RESULTS: Two families had a history of HB. A total of five different variants (c.223C > T; c.344A > G; c.464G > C; c.187_188del; and c.1314_1314delA) were identified in six patients with severe HB. Of these, two (c.187_188del and c.1314_1314delA) were novel. No variant in the entire F9 was found in two patients with mild HB. Nonsense c.223C > T (p.Arg75*) mutation was detected in two unrelated patients. Carrier testing identified five mothers as carriers, while one younger sister was a non-carrier. The carrier status of six female relatives of the two mild patients remained undetermined. By questionnaire survey, only one of the five genetically identified carriers displayed noticeable symptoms of being a carrier. CONCLUSION: The novel variants c.187_188del and c.1314_1314delA can cause severe hemophilia B. This study did not observe a significant association between symptoms and carrier status in the five carriers.

Frameshift Mutation in PAX2 Related to Focal Segmental Glomerular Sclerosis: A Case Report and Literature Review.

BACKGROUND: Paired box gene 2 (PAX2) heterozygous mutations can cause renal coloboma syndrome, but its role in patients with focal segmental glomerular sclerosis (FSGS) has been rarely reported. METHODS: Based on the clinical manifestations and renal pathological characteristics of the patient, as well as familial whole exome sequencing, the diagnosis of FSGS related to PAX2 mutation was confirmed. Treatment such as lowering urinary protein and blood pressure was given, and the patient was followed up and observed. RESULTS: There is a familial heterozygous case presented with chronic kidney disease secondary to FSGS, which was related to PAX2 frameshift mutation due to the deletion of G at the position 76 (c.76delG). To our knowledge, this is the first report of PAX2 c.76delG variant related to adult-onset FSGS. CONCLUSION: Here, we further expand the phenotypic spectrum of FSGS. Genetic screening especially PAX2 mutation is recommended in patients with adult-onset FSGS of unknown etiology.

Molecular investigation of MEFV gene polymorphisms among patients with familial mediterranean fever-like symptoms.

The diagnosis of familial Mediterranean fever (FMF) is primarily based on clinical standards. The purpose of this study was to investigate the relevance of Mediterranean fever (MEFV) genetic testing in the diagnosis of FMF as well as to identify the most frequent variant alleles and their relationship to clinical symptoms in Egyptian patients. Egyptian patients with a clinical suspicion of having FMF were studied in order to determine MEFV genotypes. Each patient was meticulously evaluated through an extensive collection of their medical history, a thorough clinical examination, and a series of laboratory tests, encompassing CBC, ESR, and CRP measurements. The MEFV variant screening procedure included the use of reverse dot blot hybridization. The average age of our patients when they were given a diagnosis was 22.8 ± 1.404 years old. The predominant clinical manifestations identified were abdominal pain, fever, and arthralgia.  Molecular interrogation of the MEFV gene unveiled that a significant proportion of the cohort, constituting 72 individuals (60%), displayed heterozygosity, whereas a smaller fraction, comprising 12 subjects (10%), demonstrated homozygosity and an equivalent number (10%) exhibited compound heterozygosity. Pertaining to the distribution of allele variants, E148Q emerged as the most prevalent, succeeded by M694I, accounting for 12.5% of the cases, and M680I (G/A), representing 10.41%. This notable prevalence of heterozygous genotypes among the Egyptian demographic, preliminarily identified as potential FMF cases, underscores the imperative for molecular diagnostics to enhance the precision of FMF identification.

The risk of endocrine interventions in carriers of a genetic predisposition for breast and gynecologic cancers: recommendations of the German Consortium for Hereditary Breast and Ovarian Cancer.

PURPOSE: To support doctors in counselling women with genetic predisposition for breast or gynecologic cancers on endocrine interventions. METHODS: Evidence on the safety of endocrine interventions for fertility treatment, contraception, hormone replacement therapy after risk-reducing salpingo-oophorectomy (RRSO) or treatment of symptoms during peri- and postmenopause was analysed for carriers of probably pathogenic and pathogenic variants in BRCA1 or BRCA2 (BRCA1/2-pV), in other breast and ovarian cancer genes and the Lynch Syndrome. Cancer risks were compared with data on risks for the general population. RESULTS: Data on risk modulation of endocrine interventions in women with genetic predisposition is limited. Ovarian hyperstimulation for fertility treatment may be performed. Oral contraceptives should not be used to reduce ovarian cancer risk in BRCA1/2-pV carriers. Premenopausal BRCA1/2-pV carriers and carriers of pV in Lynch Syndrome genes should be offered hormone replacement therapy (HRT) after RRSO, to prevent diseases caused by estrogen deficiency. CONCLUSION: Effect direction and strength of risk modulation by endocrine interventions is similar to the general population. Participation of individuals at risk in prospective registries is recommended.

[Genetic analysis of a child with Primary hypertrophic osteoarthropathy].

OBJECTIVE: To explore the genetic etiology of a child with Primary hypertrophic osteoarthropathy. METHODS: A child who was admitted to Zhongnan Hospital of Wuhan University on July 27, 2021 was selected as the study subject. Genomic DNA was extracted from peripheral blood samples of the child and his parents and subjected to whole exome sequencing. Suspected splicing variant was verified by Sanger sequencing of family members. In vitro function was validated through a minigene assay, whilst the suspected exonic deletion was validated by long-fragment PCR. RESULTS: Whole exome sequencing revealed that the child has harbored compound heterozygous variants of HPGD gene, including a heterozygous deletion (exon 3 del) derived from his father and a splicing variant (c.421+1G>T) derived from his mother. Long-fragment PCR verified that the child and his father had both harbored a 7 565 bp heterozygous deletion (c.218-1304_324+6156del), whilst the minigene assay proved that the splicing variant has resulted in skipping of exon 4. CONCLUSION: The heterozygous c.218-1304_324+6156del deletion and the c.421+1G>T splicing variant of the HPGD gene probably underlay the pathogenesis in this child. Above finding has enriched the mutational spectrum of the HPGD gene and provided a basis for genetic counseling and prenatal diagnosis for this family.

[Genetic analysis and prenatal diagnosis for a Chinese pedigree affected with Autosomal dominant polycystic kidney disease].

OBJECTIVE: To explore the clinical phenotype and genetic etiology for a Chinese pedigree affected with Autosomal dominant polycystic kidney disease (ADPKD). METHODS: A pedigree with ADPKD diagnosed at the Department of Gynaecology of the First Affiliated Hospital of Zhengzhou University in December 2020 was selected as the study subject. Clinical data of the pedigree was collected, and whole exome sequencing (WES) was carried out for the proband. Candidate variants were verified by Sanger sequencing of the proband and her relatives. RESULTS: Fetal ultrasonography showed increased volume and parenchymal echogenicity in both kidneys. The fetus was found to harbor c.11098C>T (p.R3700C) and c.11039T>C (p.F3680S) compound heterozygous variants of the PKD1 gene, which were respectively inherited from its mother and father. Based on the guidelines from the American College of Medical Genetics and Genomics (ACMG), both variants were predicted to be likely pathogenic (PM1+PM2_supporting+PP3). CONCLUSION: The c.11098C>T (p.R3700C) and c.11039T>C (p.F3680S) compound heterozygous variants of the PKD1 gene probably underlay the ADPKD in the fetus. Above finding has provided guidance for the genetic counseling and prenatal diagnosis for this pedigree.

[Genetic analysis of a child with pachygyria due to variant of ADGRG1 gen].

OBJECTIVE: To explore the genetic basis for a child with pachygyria. METHODS: A proband who had visited Qinzhou Maternal and Child Health Care Hospital for pachygyria and mental retardation in June 2020 was selected as the study subject. Clinical data was collected. The child was subjected to whole exome sequencing (WES), and candidate variant was verified by Sanger sequencing. RESULTS: The proband, a 4-year-and-6-month-old female, was clinically diagnosed with megagyrus deformity. WES revealed that she has harbored compound heterozygous variants of the ADGRG1 gene, namely c.781G>T (p.E261*) in exon 6 and c.1369A>C (p.S457R) in exon 11, which were verified by Sanger sequencing to be derived from her mother and father, respectively. Her younger sister was also heterozygous for the c.1369A>C (p.S457R) variant. Based on the guidelines from the American College of Medical Genetics and Genomics (ACMG), both variants were rated as likely pathogenic (PVS1+PM2_ Supporting; PM1+PM2_Supporting+PM3+PP3). CONCLUSION: The c.781G>T (p.E261*) and c.1369A>C (p.S457R) compound heterozygous variants of the ADGRG1 gene probably underlay the pachygyria malformation in this child.

Characterization of cryptic complex chromosome rearrangements in balanced chromosomal rearrangement carriers and their PGT-SR clinical outcome assessments.

Several reports have presented that balanced chromosomal rearrangements (BCRs) carriers with normal phenotypes may be carriers of complex rearrangements. However, the incidence and PGT clinical outcomes of cryptic complex chromosome rearrangements (CCCRs) in individuals with BCRs is remain unknown. We recruited a cohort of 1,264 individuals with BCR carriers from 2016 to 2021 at the Reproductive and Genetic Hospital of CITIC Xiangya. Peripheral blood was collected for karyotyping and genomic DNA extraction and the PGT-SR clinical outcomes of CCCRs carriers were analyzed and compared with those of BCR carriers. Our findings revealed that 3.6% (45/1,264) of BCR carriers had CCCRs, involving 3-25 breakpoints on 1-3 chromosomes. Furthermore, when mate-pair sequencing was employed, 63.3% (19/30) of CCCR carriers were found to have chromosome rearrangements that were different from those identified by the MicroSeq technique. And the transferable embryo rate of CCCR carriers with 3 chromosomes was significantly lower than that of CCCR carriers with only 1-2 chromosomes. In this research, we revealed that some of the BCR carriers were actually CCCR carriers, and the prognosis of PGT in CCCR carriers with one or two chromosomes is better than that of CCCR carriers with three chromosomes.

A novel heterozygous missense variant of PANX1 causes human oocyte death and female infertility.

Pannexin1 (PANX1) is a highly glycosylated membrane channel-forming protein, which has been found to implicate in multiple physiological and pathophysiological functions. Variants in the PANX1 gene have been reported to be associated with oocyte death and recurrent in vitro fertilization failure. In this study, we identified a novel heterozygous PANX1 variant (NM_015368.4 c.410 C > T (p.Ser137Leu)) associated with the phenotype of oocyte death in a non-consanguineous family, followed by an autosomal dominant (AD) mode. We explored the molecular mechanism of the novel variant and the variant c.976_978del (p.Asn326del) that we reported previously. Both of the variants altered the PANX1 glycosylation pattern in cultured cells, led to aberrant PANX1 channel activation, affected ATP release and membrane electrophysiological properties, which resulted in mouse and human oocyte death in vitro. For the first time, we presented the direct evidence of the effect of the PANX1 variants on human oocyte development. Our findings expand the variant spectrum of PANX1 genes associated with oocyte death and provide new support for the genetic diagnosis of female infertility.

Utility of Clinical-Pathological Parameters for Exclusion of BRCA1/2 Mutation Carriers as Candidates for Partial Breast Irradiation.

PURPOSE: Several international groups have published guidelines to identify low-risk breast cancer (BC) patients who are eligible for partial breast irradiation (PBI). These include the American Society for Radiation Oncology (ASTRO), the European Society for Radiotherapy and Oncology (ESTRO), and ESTRO subgroups such as the Intraoperative radiation (IORT) Task Force and Groupe Européen de Curiethérapie (GEC) -ESTRO. Only ASTRO guidelines recommend against the use of PBI in known carriers of germline pathogenic variants (PVs) in BRCA1/2. The aim of this study was to evaluate the proportion of BC patients, subsequently found to be BRCA1/2 PV carriers who would be eligible for PBI based on clinical-pathologic criteria of the above-mentioned international guidelines. METHODS AND MATERIALS: Data were extracted from the medical records of consecutive BC BRCA1/2 PV carriers treated at a single institution between 2006 and 2023. Data included patient demographics, tumor characteristics, treatment, and disease outcomes. RESULTS: Overall, 498 patients with 518 primary tumors were analyzed. Of these, 282 (12 of them with synchronous bilateral disease) presented with unknown genetic status at diagnosis and formed the study cohort. The median age at diagnosis was 42.7 years (range, 23.8-77.9). Based on the recent ASTRO guidelines (not including conditionally recommended criteria), 17 of 294 (5.8%) of the carriers had tumors that would be eligible for PBI, including 3 Her2-positive tumors and 5 patients diagnosed between ages 40 and 49 years. Using the ESTRO IORT and the ACROP-ESTRO PBI criteria, 9 of 294 (3%) would be eligible, whereas with the GEC-ESTRO low-risk criteria, 31 of 294 (10.5%) of the carriers would be eligible, and their intermediate risk criteria would increase eligibility for PBI by an additional 8.2% (overall 18.7%). CONCLUSIONS: Using clinical-pathologic criteria published in international guidelines, 3% to 18% of BRCA1/2 PV carriers will have tumors eligible for PBI. Therefore, especially in populations who are at high risk for being BRCA1/2 PV carriers, we recommend adhering to stricter guidelines. In our cohort, ASTRO, ESTRO-IORT, and ESTRO PBI had the lowest probability of identifying BRCA1/2 PV carriers as eligible for PBI.

A Novel Heterozygous Intronic FBN1 Variant Contributes to Aberrant RNA Splicing in Marfan Syndrome.

BACKGROUND: Marfan syndrome (MFS) is a complex genetic systemic connective tissue disorder. It is well known that genetic factors play a critical role in the progression of MFS, with nearly all cases attributed to variants in the FBN1 gene. METHODS: We investigated a Chinese family with MFS spanning two generations. Whole exome sequencing, in silico analysis, minigene constructs, transfection, RT-PCR, and protein secondary structure analysis were used to analyze the genotype of the proband and his father. RESULTS: The main clinical manifestations of the proband and his father were subluxation of the left lens and high myopia with pectus deformity. Whole exome sequencing identified a novel single nucleotide variant (SNV) in the FBN1 gene at a non-canonical splice site, c.443-3C>G. This variant resulted in two abnormal mRNA transcripts, leading to a frameshift and an in-frame insertion. Further in vitro experiments indicated that the c.443-3C>G variant in FBN1 was pathogenic and functionally harmful. CONCLUSION: This research identified a novel intronic pathogenic FBN1: c.443-3C>G gene variant, which led to two different aberrant splicing effects. Further functional analysis expands the variant spectrum and provides a strong indication and sufficient basis for preimplantation genetic testing for monogenic disease (PGT-M).

Perinatal lethal form Gaucher disease with compound heterozygosity of single nucleotide variants and copy number variations presenting as nonimmune hydrops fetalis and cerebellar hypoplasia: A case report.

OBJECTIVE: To present the ultrasound imaging and genetic diagnosis of a fetus with prenatal lethal form of Gaucher disease. CASE REPORT: A 37-year-old primiparous woman was pregnant at her 23 weeks of gestation and the prenatal fetal ultrasound revealed hydrops fetalis, cerebellum hypoplasia, and fetal immobility. The pregnancy was terminated due to major fetal anomaly, and whole exome sequencing (WES) analysis of fetal tissue and parental blood unveiled a pathogenic variant in exon 10 of the GBA gene (NM_001005741.3: c.1265T > G: p.L422R) originating from the mother. Additionally, a novel CNV (chr1: 155204785-155205635 deletion, 0.85 kb) spanning exon 10-12 in the GBA gene was identified from the father. This compound heterozygosity confirmed the diagnosis of prenatal lethal form of Gaucher disease and was informative for genetic counseling. CONCLUSION: WES is a powerful tool to detect pathogenic variants among fetuses with nonimmune hydrops fetalis and complex abnormality from prenatal ultrasound. Compound heterozygosity consisted of single nucleotide variants (SNV) and copy number variations (CNVs) may lead rare inherited metabolic disorders including prenatal lethal form of Gaucher disease.

Incidence of endometrial cancer in BRCA mutation carriers.

OBJECTIVE: Whether or not women who harbor a germline pathogenic variant ('mutation') in the BRCA1 or BRCA2 genes are at elevated risk of developing endometrial cancer is yet to be determined. METHODS: We conducted a prospective analysis of 4959 BRCA mutation carriers with no prior history of cancer (except for breast or melanoma) and an intact uterus. RESULTS: After a mean of 6.7 years of follow-up there were 38 incident cases of endometrial cancer diagnosed; 30 among BRCA1 carriers and eight among BRCA2 carriers. The mean age at diagnosis was 58.4 years (range 46.8-76.1). The majority were of the endometrioid subtype (n = 16), followed by mixed endometroid and serous (n = 4), serous (n = 3) or clear cell (n = 1) (missing = 13). The cumulative incidence from age 40 to age 70 was 3.4% for BRCA1 carriers and was 1.6% for BRCA2 mutation carriers. Prior tamoxifen use was associated with a significant two-fold increased risk (HR = 2.24; 95% CI 1.10-4.55). There was no significant association between exogenous hormone use, oophorectomy, smoking or BMI at age 40 and risk (P ≥ 0.32). CONCLUSIONS: Compared to the general population, we observed higher rates of endometrial cancer among young BRCA1 mutation carriers; however, lifetime risks were similar. Women with prior tamoxifen exposure were at a significantly increased risk. These findings were based. on a small number of incident cases and require confirmation with additional follow-up of our aging cohort.

A 3-year-old Tanzanian Female with Glucose-6-Phosphate Dehydrogenase A- and a Novel Heterozygous PIEZO1 Mutation (2744A>G, N915S) Presenting with Severe Hemolytic Anemia.

A 3-year-old Tanzanian female presented with severe hemolytic anemia of unknown etiology, necessitating multiple red blood cell transfusions. The patient was found to have glucose-6-phosphate dehydrogenase (G6PD) deficiency A- and a heterozygous Piezo-type mechanosensitive ion channel component 1 (PIEZO1) mutation (2744A>G, N915S). This case identifies a novel PIEZO1 mutation implicated in erythrocyte channelopathies occurring in conjunction with an X-linked enzymopathy in a female patient. This underscores the importance of keeping X-linked disorders in the differential diagnosis of hemolytic anemia in females, as well as presents the possibility for novel coexisting mutations to augment the phenotype.

RésuméUne Tanzanienne de 3 ans a présenté une anémie hémolytique sévère d'étiologie inconnue, nécessitant de multiples transfusions de globules rouges. Le patient présentait un déficit en glucose-6-phosphate déshydrogénase (G6PD) A- et un mécanosensible hétérozygote de type piézo.mutation du composant 1 du canal ionique (PIEZO1) (2744A>G, N915S). Ce cas identifie une nouvelle mutation PIEZO1 impliquée dans les érythrocytes canalopathies survenant en conjonction avec une enzymopathie liée à l'X chez une patiente. Cela souligne l'importance de garder troubles liés à l'X dans le diagnostic différentiel de l'anémie hémolytique chez les femmes, et présente la possibilité de nouvelles mutations coexistantes pour augmenter le phénotype.

Successful intracytoplasmic sperm injection in a macrozoospermia case with novel compound heterozygous aurora kinase C (AURKC) mutations.

PURPOSE: To explore the application possibility of macrocephalic sperm from a patient with 100% macrocephalic sperm and AURKC gene variations. METHODS: We diagnosed a case of macrozoospermia with 100% macrocephalic sperm and 39.5% multi-tailed spermatozoa by morphological analysis. Whole-exome sequencing (WES) was used for the patient and his wife. Sanger sequencing technique was used to verify the AURKC mutations in the patient's parents and his offspring. Sperm's ploidy was tested by flow cytometry. The couple asked for intra-couple ART therapy. RESULTS: The patient presented novel compound heterozygous AURKC mutations (c.434C > T, c.497A > T) by WES. Sanger sequencing validation showed that variant of c.434C > T was observed in his father and c.497A > T was observed in his mother. Flow cytometry revealed that there existed a certain proportion of haploid sperm. Macrocephalic spermatozoa whose heads were smaller than the diameter of injection needle were selected for microinjection. A singleton pregnancy was achieved after embryo transfer. Prenatal diagnosis revealed that the fetus had normal chromosomal karyotype. Sanger sequencing technique showed that the fetus carried a c.434C > T mutation in one AURKC allele. A 3730 g healthy male fetus was delivered at term. CONCLUSION: Our study reported a successful live birth from a patient with definite AURKC gene variants and may provide insights for such patients to choose donor sperm or their own sperm.

Heterozygous de novo dominant negative mutation of REXO2 results in interferonopathy.

Mitochondrial RNA (mtRNA) in the cytosol can trigger the innate immune sensor MDA5, and autoinflammatory disease due to type I IFN. Here, we show that a dominant negative mutation in the gene encoding the mitochondrial exonuclease REXO2 may cause interferonopathy by triggering the MDA5 pathway. A patient characterized by this heterozygous de novo mutation (p.T132A) presented with persistent skin rash featuring hyperkeratosis, parakeratosis and acanthosis, with infiltration of lymphocytes and eosinophils around small blood vessels. In addition, circulating IgE levels and inflammatory cytokines, including IFNα, are found consistently elevated. Transcriptional analysis highlights a type I IFN gene signature in PBMC. Mechanistically, REXO2 (T132A) lacks the ability to cleave RNA and inhibits the activity of wild-type REXO2. This leads to an accumulation of mitochondrial dsRNA in the cytosol, which is recognized by MDA5, leading to the associated type I IFN gene signature. These results demonstrate that in the absence of appropriate regulation by REXO2, aberrant cellular nucleic acids may accumulate and continuously trigger innate sensors, resulting in an inborn error of immunity.