DiagnosisClinical Diagnosis The diagnosis of leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation (LBSL) can be made with confidence in persons with characteristic abnormalities observed on brain and spinal cord MRI [Van der Knaap et al 2003] and identifiable mutations in the causative gene DARS2, encoding mitochondrial aspartyl tRNA synthetase [Scheper et al 2007].MRI criteria for LBSL [Scheper et al 2007]Major criteria 1. Signal abnormalities ² in the: Cerebral white matter, which is either nonhomogeneous and spotty or homogeneous and confluent, with relative sparing of the U-fibersDorsal columns and lateral corticospinal tracts of the spinal cord (Visualization of such abnormalities in the cervical spinal cord suffices.)Pyramids in the medulla oblongataSupportive criteriaSignal abnormalities ² in theSplenium of the corpus callosumPosterior limb of the internal capsuleMedial lemniscus in the brain stemSuperior cerebellar pedunclesInferior cerebellar pedunclesIntraparenchymal part of the trigeminal nerveMesencephalic trigeminal tractsAnterior spinocerebellar tracts in the medullaCerebellar white matter with subcortical preponderanceElevated lactate in the abnormal cerebral white matter, as measured by proton magnetic resonance spectroscopy (MRS) ^{31. For an MRI-based diagnosis, all major criteria and at least one supportive criterion should be fulfilled.2. ‘Signal abnormalities’ refer to abnormally low signal on T1-weighted images and abnormally high signal on T2-weighted images.3. Lactate is elevated within the abnormal cerebral white matter in most but not all affected individuals [van der Knaap et al 2003, Petzold et al 2006, Labauge et al 2007, Tavora et al 2007]. White matter lactate [Van der Knaap et al 2003]: — Affected individuals: range = 0.5-4.1 mmol/L (SD = 1.1 mmol/L; mean = 2.4 mmol/L) — Controls: mean = 0.2 mmol/L; SD = 0.3 mmol/L TestingRoutine laboratory tests, including CSF analysis, are usually normal. In a few individuals mild and inconsistent elevation of lactate concentration has been noted in blood or CSF or both. No published information is available. Normal values for CSF lactate vary per laboratory, but are typically below 1.5 mmol/L. In LBSL CSF lactate is usually normal, but values between 2 and 3 mmol/L may be seen occasionally [Author, personal observation].Neuropathologic findings have not been reported. Molecular Genetic Testing Gene. DARS2 is the only gene known to be associated with LBSL.Clinical testingSequence analysis of the coding exons and surrounding intronic regions is performed to detect DARS2 genomic DNA (gDNA) mutations in individuals fulfilling the MRI criteria for LBSL. Affected individuals are invariably compound heterozygous for two mutations in DARS2. In a few individuals it has not been possible to determine both pathogenic mutations at the genomic level. No individuals with homozygous mutations have been identified.Sequence analysis of complementary DNA (cDNA), if available, can be performed to:Detect aberrant splice products that occur as a result of mutations outside the regions that are included in the standard analysis of exons and surrounding intronic regions;Determine the effect of mutations close to but not within the acceptor or donor sites, including the c.228-20_-21delTTinsC (p.Arg76Serfs*5) mutation that occurs in the majority of affected individuals. Theoretically, this change would not be predicted to affect splicing; however, cDNA analysis reveals partial skipping of exon 3, leading to a frame shift and a premature stop codon.Table 1. Summary of Molecular Genetic Testing Used in Leukoencephalopathy with Brain Stem and Spinal Cord Involvement and Lactate ElevationView in own windowGene SymbolTest MethodMutation Detection Frequency by This Method 1Test AvailabilityDARS2Sequence analysis~90% 2Clinical1. The ability of the test method used to detect a mutation that is present in the indicated gene2. In four of 43 families the second pathogenic mutation could not be found in gDNA; in two of three the second mutation was detected using cDNA; cells of the fourth individual were not available for isolation of mRNA for cDNA synthesis. In one person fulfilling the MRI criteria for LBSL, no mutations in DARS2 were detected in either gDNA or cDNA [Scheper et al 2007; Scheper & van der Knaap, personal communication].Interpretation of test resultsFor issues to consider in interpretation of sequence analysis results, click here.Finding two-disease causing mutations in DARS2 confirms the diagnosis of LBSL, but absence of DARS2 mutations does not exclude the diagnosis when the clinical and MRI findings are characteristic.Testing Strategy To confirm/establish the diagnosis in a proband. Because the clinical picture of LBSL is consistent with a spinocerebellar ataxia of any type, it is the MRI findings that raise the suspicion of LBSL. In some instances affected individuals have minimal neurologic signs but MRI abnormalities typical of LBSL.If the MRI fulfills the criteria for LBSL, molecular genetic testing of DARS2 should be performed.If the MRI is suggestive of LBSL, but the MRI criteria are not fully met, molecular genetic testing of DARS2 should still be considered. If the MRI fulfills the criteria for LBSL and molecular genetic testing of DARS2 does not identify disease-causing mutations, LBSL is not excluded.If the clinical picture is consistent with a spinocerebellar ataxia and MRI of the brain and spinal cord is normal or shows abnormalities that are not compatible with LBSL, molecular genetic testing of DARS2 is not warranted. Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.Genetically Related (Allelic) Disorders No other phenotypes are known to be associated with mutations in DARS2.}

Natural History Initial development is normal in most affected children. In some children, independent walking is late and unstable from the beginning. Deterioration of motor skills usually starts in childhood or adolescence [Van der Knaap et al 2003, Linnankivi et al 2004, Serkov et al 2004, Tavora et al 2007, Uluc et al 2008] and occasionally in adulthood [Petzold et al 2006, Labauge et al 2007]. The clinical picture of LBSL consists of slowly progressive cerebellar ataxia, spasticity, and dorsal column dysfunction, involving the legs more than the arms. Tendon reflexes are retained. Most affected individuals have decreased position and vibration sense of the legs more than the arms, leading to increased difficulty walking in the dark.Manual dexterity becomes impaired to a variable degree. Dysarthria develops over time.Some affected individuals develop epilepsy. Seizures are infrequent and easily controlled with medication [Van der Knaap et al 2003]. Some have learning problems from early on, but most have normal intellectual capacities. Cognitive decline may occur and is usually mild [Van der Knaap et al 2003, Serkov et al 2004]. Some affected individuals experience lowered consciousness, neurologic deterioration, and fever following minor head trauma [Serkov et al 2004]. Recovery is only partial.Evidence of an axonal neuropathy is found in some but not in all affected individuals [Van der Knaap et al 2003, Tavora et al 2007, Uluc et al 2008, Isohanni et al 2010].The disease is slowly progressive. Most affected individuals become wheelchair-dependent in their teens or twenties; however, disease severity varies. Some affected individuals become wheelchair dependent before age ten and are totally incapacitated in their twenties, whereas others have the first signs of the disease in their twenties and still walk in their forties.

Genotype-Phenotype Correlations No genotype-phenotype correlation has been described, but the subject has not been investigated systematically. So far, no major intrafamilial variation has been observed.

Differential DiagnosisThe clinical picture of LBSL consists of slowly progressive cerebellar ataxia, spasticity, and dorsal column dysfunction, involving the legs more than the arms. The tendon reflexes are retained. Based on these findings alone, many disorders can be considered [Finsterer 2009a]; however, the MRI findings distinguish LBSL from other spinocerebellar ataxias [Van der Knaap et al 2003].The clinical findings of a spinocerebellar ataxia in combination with MRI abnormalities of the dorsal columns, lateral corticospinal tracts, and cerebral white matter would be compatible with vitamin B₁₂ deficiency (combined cord degeneration) [Locatelli et al 1999]. The brain stem abnormalities typically seen in LBSL do not occur in vitamin B₁₂ deficiency. In vitamin B₁₂ deficiency the cervical spinal cord is mainly affected [Locatelli et al 1999], whereas in LBSL the entire spinal cord is affected [Van der Knaap et al 2003].Elevated lactate in MRS or body fluids or both in combination with clinical findings of a spinocerebellar ataxia or white matter abnormalities on MRI or both should lead to the consideration of mitochondrial disorders [Finsterer 2009b]. Although the brain stem and spinal cord are frequently affected in mitochondrial disorders, the selective involvement of specific brain stem and spinal cord tracts is unique for LBSL [Van der Knaap & Valk 2005].

ManagementEvaluations Following Initial Diagnosis To establish the extent of disease in an individual diagnosed with LBSL, the following evaluations are recommended:Neurologic examination Brain and spinal cord MRIIf possible, proton MRS of abnormal cerebral white matter Physical therapy/occupational therapy assessment Treatment of ManifestationsSupportive therapy includes:Physical therapy and rehabilitation to improve motor functionThe following as needed: Antiepileptic drugs (AED) if epileptic seizures are presentSpecial education Speech therapyPrevention of Secondary ComplicationsRehabilitation and physical therapy are helpful in the prevention of secondary complications, such as contractures and scoliosis.SurveillanceLBSL is very slowly progressive. Annual evaluations suffice. In case of rapid worsening more frequent evaluations are appropriate.Evaluation of Relatives at RiskSee Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationSearch Clinical Trials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.OtherStudies of muscle biopsies, fibroblasts, and lymphoblasts show no evidence of mitochondrial dysfunction; therefore, there is no rationale for the “mitochondrial cocktail” of vitamins and cofactors, often given to persons with mitochondrial dysfunction.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Leukoencephalopathy with Brain Stem and Spinal Cord Involvement and Lactate Elevation: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDDARS21q25​.1Aspartyl-tRNA synthetase, mitochondrialDARS2 homepage - Mendelian genesDARS2Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Leukoencephalopathy with Brain Stem and Spinal Cord Involvement and Lactate Elevation (View All in OMIM) View in own window 610956ASPARTYL-tRNA SYNTHETASE 2; DARS2 611105LEUKOENCEPHALOPATHY WITH BRAINSTEM AND SPINAL CORD INVOLVEMENT AND LACTATE ELEVATION; LBSLNormal allelic variants. The genomic copy of the gene comprises 33,725 bases; it contains 17 exons. The cDNA has 3348 base pairs. Two variants in the dbSNP database have frequency data that indicate their role as normal variants. Only one of those, rs35515638 (c.587A>G), has frequency data with an average heterozygosity of 0.07. Another nonsynonymous SNP, c.1013G>A was observed in 12 out of 360 control chromosomes (Scheper and van der Knaap, personal communication).Pathologic allelic variants. In almost all affected individuals one mutation is present upstream of exon 3. The c.228-20_-21delTTinsC is most often observed. In other affected individuals nucleotide changes are seen in the same region, within a stretch of 10 or 11 C-residues that lies 10 nucleotides upstream of exon 3 [Scheper et al 2007].Several individuals share haplotypes involving five or six microsatellite markers on chromosome 1p25. The mutations c.492+2T>C and c.455G>T are correlated with two of these haplotypes and are often seen in affected individuals of North-East European origin [Scheper et al 2007, Isohanni et al 2010].Table 2. Selected DARS2 Allelic Variants View in own windowClass of
Variant AlleleDNA Nucleotide ChangeProtein Amino Acid ChangeReference
SequencesNormalc.587A>Gp.Lys196ArgNM_108122​.4
NP_060592​.2c.1013G>Ap.Gly338GluPathologicc.228-20_-21delTTinsCp.Arg76Serfs*5c.492+2T>Cp.Met134_Lys165delc.455G>Tp.Cys152PheSee Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www​.hgvs.org). Normal gene product. The protein length is 645 amino acids; its predicted molecular weight is 74 kd. It is a mitochondrial aspartyl-tRNA synthetase (mtAspRS), which charges tRNA^Asp in mitochondria. Upon entry into mitochondria, the predicted N-terminal mitochondrial targeting sequence of 47 amino acids is removed. The functional protein contains 598 amino acids with a predicted molecular weight of 68 kd. Multiple alignment with related enzymes reveals 36−43% identity of the full-length sequence with bacterial sequences, 32% with the mitochondrial sequence from the lower eukaryote S. cerevisiae, and below 23% with enzymes from archæa and cytosol of eukaryotes, including human cytosolic aspartyl-tRNA synthetase. Alignment also shows that human mt-AspRS possesses strictly conserved residues found in all known AspRS sequences, including those for ATP binding and tRNA binding. Residues involved in amino acid binding include those typical for class II aaRSs and those specific for aspartic acid recognition [Bonnefond et al 2005]. Based on the homology to its bacterial counterparts, mtAspRS is thought to form homo-dimers [Delarue et al 1994].Abnormal gene product. The majority of affected individuals have a mutation that affects splicing of exon 3. Incorrect splicing of this exon results in a frame shift in the reading frame and nonsense-mediated decay of the wrongly-spliced mRNA. It should be noted that these mutations upstream of exon 3 diminish but not completely abolish correct splicing. As a consequence, in patient cells a low amount of wild type protein is produced. A total lack of mtAspRS activity is thought to be incompatible with life. Another common mutation, c.492+2T>C (p.Met134_Lys165del) leads to a deletion of part of the protein. The effect of this deletion is unclear but is likely to have a severe effect on the function of the protein. Several missense mutations have been shown to severely reduce the amino acylation function in assays with purified bacterially-expressed recombinant proteins [Scheper et al 2007].