The TRNT1 gene encodes tRNA nucleotidyltransferase 1, which catalyzes the addition of cytosine-cytosine-adenosine (CCA) to the ends of cytoplasmic and mitochondrial tRNAs. The most common clinical phenotype associated with TRNT1 is autosomal recessive sideroblastic anemia with B-cell immunodeficiency, periodic fever, and developmental delay (SIFD). Muscle involvement has rarely been reported in TRNT1-related disorders. Here we report a Chinese patient with incomplete SIFD and hyperCKemia, and explored the skeletal muscle pathological changes. The patient was a 3-year-old boy with sensorineural hearing loss, sideroblastic anemia, and developmental delay since infancy. At the age of 11 months, significantly increased levels of creatine kinase were noted, accompanied by mild muscle weakness. Whole-exome sequencing revealed compound heterozygous variants of the TRNT1 gene, c.443C > T (p.Ala148Val) and c.692C > G (p.Ala231Gly), in the patient. Western blot showed a decreased expression of TRNT1 and cytochrome c oxidase subunit IV (COX IV) in the skeletal muscle of the patient. Electron microscopy observation of skeletal muscle pathology revealed abnormal mitochondria of various sizes and shapes, supporting a diagnosis of mitochondrial myopathy. The present case indicates that in addition to the classic SIFD phenotype, TRNT1 mutations can cause mitochondrial myopathy, a rare clinical phenotype of TRNT1-related disorders.

Congenital sideroblastic anemias (CSAs) are a group of inherited bone-marrow disorders manifesting with erythroid hyperplasia and ineffective erythropoiesis. We describe a detailed clinical and genetic characterization of three siblings with CSA. Two of them had limb-girdle myopathy and global developmental delay. The two elder siblings performed allogenic hematopoietic stem-cell transplantation 5 and 3 years prior with stabilization of the hematological features. Exome sequencing in the non-transplanted sibling revealed a novel homozygous nonsense variant in SLC25A38 gene NM_017875.2:c.559C > T; p.(Arg187*) causing autosomal-recessive sideroblastic anemia type-2, and a second homozygous pathogenic previously reported variant in GMPPB gene NM_013334.3:c.458C > T; p.(Thr153Ile) causing autosomal-recessive muscular dystrophy-dystroglycanopathy type B14. With the established diagnosis, hematopoietic stem cell transplantation is now being scheduled for the youngest sibling, and a trial therapy with acetylcholine esterase inhibitors was started for the two neurologically affected patients with partial clinical improvement. This family emphasizes the importance of whole-exome sequencing for familial cases with complex phenotypes and vague neurological manifestations.

The sideroblastic anemias are a heterogeneous group of inherited and acquired disorders characterized by the presence of ring sideroblasts in the bone marrow. X-linked sideroblastic anemia (XLSA) is caused by germline mutations in ALAS2. Hemizygous males have a hypochromic microcytic anemia, which is generally mild to moderate and is caused by defective heme synthesis and ineffective erythropoiesis. XLSA is a typical iron-loading anemia; although most patients are responsive to pyridoxine, treatment of iron overload is also important in the management of these patients. Autosomal recessive sideroblastic anemia attributable to mutations in SLC25A38, a member of the mitochondrial carrier family, is a severe disease: patients present in infancy with microcytic anemia, which soon becomes transfusion dependent. Conservative therapy includes regular red cell transfusion and iron chelation, whereas allogenic stem cell transplantation represents the only curative treatment. Refractory anemia with ring sideroblasts (RARS) is a myelodysplastic syndrome characterized mainly by anemia attributable to ineffective erythropoiesis. The clinical course of RARS is generally indolent, but there is a tendency to worsening of anemia over time, so that most patients become transfusion dependent in the long run. More than 90% of these patients carry somatic mutations in SF3B1, a gene encoding a core component of the RNA splicing machinery. These mutations cause misrecognition of 3' splice sites in downstream genes, resulting in truncated gene products and/or decreased expression attributable to nonsense-mediated RNA decay; this explains the multifactorial pathogenesis of RARS. Variants of RARS include refractory cytopenia with multilineage dysplasia and ring sideroblasts, and RARS associated with marked thrombocytosis; these variants involve additional genetic lesions. Inhibitors of molecules of the transforming growth factor-β superfamily have been shown recently to target ineffective erythropoiesis and ameliorate anemia both in animal models of myelodysplastic syndrome and in RARS patients.