Original Article Journal of Child Neurology 1-7 ª The Author(s) 2014 Congenital Visual Impairment and Progressive Reprints and permission: sagepub.com/journalsPermissions.nav Microcephaly Due to Lysyl–Transfer DOI: 10.1177/0883073814553272 Ribonucleic Acid (RNA) Synthetase (KARS) jcn.sagepub.com Mutations: The Expanding Phenotype of Aminoacyl–Transfer RNA Synthetase Mutations in Human Disease Hugh J. McMillan, MD, MSc1, Peter Humphreys, MDCM1, Amanda Smith, PhD1, Jeremy Schwartzentruber, MSc2, Pranesh Chakraborty, MD1, Dennis E. Bulman, PhD1, Chandree L. Beaulieu, MSc1, FORGE Canada Consortium1, Jacek Majewski, PhD3, Kym M. Boycott, MD, PhD1, and Michael T. Geraghty, MBBS, MSc1 Abstract Aminoacyl–transfer ribonucleic acid (RNA) synthetases (ARSs) are a group of enzymes required for the first step of protein translation. Each aminoacyl–transfer RNA synthetase links a specific amino acid to its corresponding transfer RNA component within the cytoplasm, mitochondria, or both. Mutations in ARSs have been linked to a growing number of diseases. Lysyl–transfer RNA synthetase (KARS) links the amino acid lysine to its cognate transfer RNA. We report 2 siblings with severe infantile visual loss, progressive microcephaly, developmental delay, seizures, and abnormal subcortical white matter. Exome sequencing identified mutations within the KARS gene (NM_005548.2):c.1312C>T; p.Arg438Trp and c.1573G>A; p.Glu525Lys occurring within a highly conserved region of the catalytic domain. Our patients’ phenotype is remarkably similar to a phenotype recently reported in glutaminyl–transfer RNA synthetase (QARS), another bifunctional ARS gene. This finding expands the phenotypic spectrum associated with mutations in KARS and draws attention to aminoacyl–transfer RNA synthetase as a group of enzymes that are increasingly being implicated in human disease. Keywords lysyl-tRNA synthetase, aminoacyl–tRNA, microcephaly, epilepsy, vision disorders Received July 06, 2014. Received revised July 06, 2014. Accepted for publication September 07, 2014. Background Aminoacyl–transfer ribonucleic acid (RNA) synthetases (ARSs) are a group of enzymes that are responsible for the first step of protein translation. Each of the 37 aminoacyl– transfer RNA synthetase enzymes links a specific amino acid 1 to its corresponding transfer RNA, a process referred to as Children’s Hospital of Eastern Ontario Research Institute, University of Ottawa, Ottawa, Ontario, Canada aminoacylation or charging. Aminoacylation must occur 2 McGill University and Genome Quebec Innovation Centre, Montre´al, before protein translation can begin. Aminoacyl–transfer Quebec, Canada RNA synthetases are divided into 1 of 3 groups depending 3 Department of Human Genetics, McGill University, Montre´al, Quebec, on the site where transfer RNA aminoacylation occurs: cyto- Canada plasm, mitochondria, or both (ie, bifunctional).1 Each ami- Corresponding Author: noacyl–transfer RNA synthetase has a unique 4-letter Hugh J. McMillan, MD, MSc, Neurology, Children’s Hospital of Eastern Ontario, designation where the first letter corresponds to the amino 401 Smyth Rd, Ottawa, Ontario, Canada K1H 8L1. acid it associates with, followed by ‘‘ARS’’; for example, the Email: [email protected] Downloaded from jcn.sagepub.com at UNIV OF WESTERN ONTARIO on February 17, 2015 2 Journal of Child Neurology convention for alanyl–transfer RNA synthetase is AARS. In normal. Although an electroretinogram was normal at age thecaseofaminoacyl–transfer RNA synthetase functioning 4 months, visual evoked potentials were absent. Magnetic reso- within the mitochondria, the suffix 2 is applied (ie, AARS2). nance imaging (MRI) brain at age 9 months revealed moderate, The 3 bifunctional aminoacyl–transfer RNA synthetases symmetrical thinning of the central cerebral white matter and include lysyl–transfer RNA synthetase (KARS), glycyl–trans- the corpus callosum. On neurologic assessment at age 9 months, fer RNA synthetase (GARS), and glutaminyl–transfer RNA his weight was 10.4 kg (90th percentile) and his head circum- synthetase (QARS). ference was 43.0 cm (2nd percentile). No abnormalities beyond Aminoacyl–transfer RNA synthetases have been linked to a his visual loss were apparent. A hearing test at age 12 months growing number of neurologic disorders with an expanding range was normal. Pregnancy had been significant only for noniden- of phenotypes. Several ARSs have been linked to Charcot-Marie- tical twin gestation. Antenatal ultrasounds were normal. He and Tooth disease, including AARS,2 YARS,3 GARS,4 and KARS.5 The his twin sister were delivered via planned Caesarean section at initial ARS mutations were predominantly autosomal dominant2-4 41 weeks. His birth weight was 2.87 kg (3rd to 10th percentile). although autosomal recessive inheritance is now increasingly Family history noted his nonconsanguineous parents and twin recognized, particularly with severe and earlier-onset pheno- sister to be well. There was a progressive decline in head types.6 Next-generation sequencing is an effective tool that has growth velocity after age 9 months: 44.0 cm at 21 months and advanced our understanding of clinical phenotypes associated 45.2 cm at 3.5 years (both well below the 2nd percentile). He with ARS mutations. Next-generation sequencing permits in- developed seizures at age 2 years characterized by 30 to 60 sec- parallel sequencing of all protein-encoding regions within a onds of behavioral arrest, blank stare, and chewing movements human genome. Although exons constitute only about 1% to of lips. Electroencephalogram (EEG) showed generalized spike 2% of the human genome, they contain 85% of Mendelian and wave discharges. His seizures were well controlled with disease-causing mutations,7 making this technique highly advan- phenobarbital and later valproic acid. Baseline and surveillance tageous when investigating patients with rare clinical presenta- transaminase levels were normal. Developmentally, he demon- tions,8 phenotypes associated with significant genetic strated global developmental delay. He sat unsupported at age 8 heterogeneity (eg, spinocerebellar ataxia, Charcot-Marie-Tooth months and rolled front-to-back at age 10 months. He could disease, type 2), or a broad differential diagnosis.8 As this technol- transition from crawl to sit at age 14 months and started to ogy becomes increasing affordable and available, there is a con- cruise at age 22 months. At the age of 5 years, he had less than comitant increase in testing opportunities for patients with such 10 words and could understand some simple instructions. Bio- clinical presentations.8 As a result, our understanding of aminoa- chemical testing revealed normal serum creatine kinase, lac- cyl–transfer RNA synthetase related disease has thus expanded tate, plasma amino acids, 7 dehydrocholesterol, very-long- considerably, with mutations now identified in 19 of the 37 ARS chain fatty acids, transferrin isoelectric focusing, vitamin B12, genes, particularly ARSs that are active within mitochondria. The karyotype, and chromosome microarray (Baylor College, range of clinical phenotypes associated with ARS mutations has v5.0, in 2006). Urine organic acids were unremarkable. Repeat similarly expanded and now includes microcephaly, seizures, leu- brain MRI at age 20 months revealed some progress of myeli- koencephalopathy, peripheral neuropathy, vision and hearing nation; however, symmetric abnormalities were still noted in impairment, and/or hepatic failure. the deep white matter. MR spectroscopy of the basal ganglia We report 2 siblings who presented with visual impair- was normal. At his most recent neurologic follow-up at the age ment from birth, progressive microcephaly, epilepsy, and of 10 years, his EEG continued to show persistent focal and severe cognitive impairment due to 2 mutations in the generalized epileptiform discharges although his seizures bifunctional ARS, lysyl–transfer RNA synthetase (KARS). remained clinically under good control. KARS mutations have been linked to autosomal recessive A younger sister was born 3 years after the twins. Pregnancy syndromic peripheral neuropathy phenotype5 and nonsyn- was uncomplicated, and she was delivered at 40 weeks. Birth dromic hearing impairment.9 Our patients’ symptoms were weight was 3.46 kg (just below the 50th percentile) and birth more severe than those noted in previous patients with head circumference was 33.5 cm (just above the 10th percentile). mutations in KARS, yet were strikingly similar to those At age 5 weeks, her parents noted her to have the same impair- recently reported in another bifunctional ARS, glutaminyl– ment of visual fixation and nystagmus that had been noted in her transfer RNA synthetase (QARS).6 older brother. Neurologic examination noted her head circumfer- ence to be 37.4 cm (below the 2nd percentile). At age 4 months, her visual impairment was less severe than her brother’s; she Cases could visually fixate and follow a nearby face. She would also A boy was referred to Pediatric Ophthalmology at 6 weeks of reach for objects in front of her. She had an intact smile and was age because of concern regarding visual impairment. He was cooing and laughing. Neurologic exam was otherwise unremark- not visually fixating and lacked a social smile. Severe visual able. Routine EEG at age 10 months was normal. Hearing test at loss and gross searching pendular nystagmus were noted. He 2 years of age was normal. Video EEG was performed at 2 years was unable to visually fixate or follow with either