Leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) is regarded a relatively mild leukodystrophy, diagnosed by characteristic long tract abnormalities on MRI and biallelic variants in DARS2, encoding mitochondrial aspartyl-tRNA synthetase (mtAspRS). DARS2 variants in LBSL are almost invariably compound heterozygous; in 95% of cases, 1 is a leaky splice site variant in intron 2. A few severely affected patients, still fulfilling the MRI criteria, have been described. We noticed highly unusual MRI presentations in 15 cases diagnosed by WES. We examined these cases to determine whether they represent consistent novel LBSL phenotypes.

We reviewed clinical features, MRI abnormalities, and gene variants and investigated the variants' impact on mtAspRS structure and mitochondrial function.

We found 2 MRI phenotypes: early severe cerebral hypoplasia/atrophy (9 patients, group 1) and white matter abnormalities without long tract involvement (6 patients, group 2). With antenatal onset, microcephaly, and arrested development, group 1 patients were most severely affected. DARS2 variants were severer than for classic LBSL and severer for group 1 than group 2. All missense variants hit mtAspRS regions involved in tRNA^Asp binding, aspartyl-adenosine-5'-monophosphate binding, and/or homodimerization. Missense variants expressed in the yeast DARS2 ortholog showed severely affected mitochondrial function.

DARS2 variants are associated with highly heterogeneous phenotypes. New MRI presentations are profound cerebral hypoplasia/atrophy and white matter abnormalities without long tract involvement. Our findings have implications for diagnosis and understanding disease mechanisms, pointing at dominant neuronal/axonal involvement in severe cases. In line with this conclusion, activation of biallelic DARS2 null alleles in conditional transgenic mice leads to massive neuronal apoptosis.

脑干和脊髓受累和乳酸升高 (LBSL) 的白质脑病被认为是一种相对轻微的脑白质营养不良，通过 MRI 上的特征性长道异常和DARS2中的双等位基因变异体诊断，编码线粒体天冬氨酰-tRNA 合成酶 (mtAspRS)。LBSL中的 DARS2 变体几乎总是复合杂合的；在 95% 的病例中，1 是内含子 2 中的泄漏剪接位点变异。已经描述了一些仍符合 MRI 标准的严重受影响的患者。我们注意到 WES 诊断出的 15 例患者的 MRI 表现非常不寻常。我们检查了这些病例以确定它们是否代表一致的新型 LBSL 表型。

我们回顾了临床特征、MRI 异常和基因变异，并研究了这些变异对 mtAspRS 结构和线粒体功能的影响。

我们发现了 2 种 MRI 表型：早期严重脑发育不全/萎缩（9 例患者，第 1 组）和白质异常而无长道受累（6 例患者，第 2 组）。由于产前发病、小头畸形和发育停滞，第 1 组患者受到的影响最为严重。DARS2变体比经典 LBSL 更严重，第 1 组比第 2 组更严重。所有错义变体均命中 mtAspRS 区域，这些区域涉及 tRNA ^Asp结合、天冬氨酰-腺苷-5'-单磷酸结合和/或同源二聚化。在酵母DARS2直系同源物中表达的错义变体显示严重影响线粒体功能。

DARS2变体与高度异质的表型相关。新的 MRI 表现是严重的脑发育不全/萎缩和白质异常，没有长道受累。我们的研究结果对诊断和理解疾病机制具有重要意义，指出严重病例中占主导地位的神经元/轴突受累。与该结论一致，条件性转基因小鼠中双等位基因DARS2无效等位基因的激活导致大量神经元凋亡。