Rigid spine syndrome is a rare childhood-onset myopathy characterised by slowly progressive or non-progressive scoliosis, neck and spine contractures, hypotonia, and respiratory insufficiency. Biallelic variants in SELENON account for most cases of rigid spine syndrome, however, the underlying genetic cause in some patients remains unexplained. We used exome and genome sequencing to investigate the genetic basis of rigid spine syndrome in patients without a genetic diagnosis. In five patients from four unrelated families, we identified biallelic variants in HMGCS1 (3-hydroxy-3-methylglutaryl-coenzyme A synthase). These included six missense variants and one frameshift variant distributed throughout HMGCS1. All patients presented with spinal rigidity primarily affecting the cervical and dorsolumbar regions, scoliosis, and respiratory insufficiency. Creatine kinase levels were variably elevated. The clinical course worsened with intercurrent disease or certain drugs in some patients; one patient died from respiratory failure following infection. Muscle biopsies revealed irregularities in oxidative enzyme staining with occasional internal nuclei and rimmed vacuoles. HMGCS1 encodes a critical enzyme of the mevalonate pathway and has not yet been associated with disease. Notably, biallelic hypomorphic variants in downstream enzymes including HMGCR and GGPS1 are associated with muscular dystrophy resembling our cohort’s presentation. Analyses of recombinant human HMGCS1 protein and four variants (p.S447P, p.Q29L, p.M70T, p.C268S) showed that all mutants maintained their dimerization state. Three of the four mutants exhibited reduced thermal stability, and two mutants showed subtle changes in enzymatic activity compared to the wildtype. Hmgcs1 mutant zebrafish displayed severe early defects, including immobility at 2 days and death by day 3 post-fertilisation and were rescued by HMGCS1 mRNA. We demonstrate that the four variants tested (S447P, Q29L M70T, and C268S) have reduced function compared to wildtype HMGCS1 in zebrafish rescue assays. Additionally, we demonstrate the potential for mevalonic acid supplementation to reduce phenotypic severity in mutant zebrafish. Overall, our analyses suggest that these missense variants in HMGCS1 act through a hypomorphic mechanism. Here, we report an additional component of the mevalonate pathway associated with disease and suggest biallelic variants in HMGCS1 should be considered in patients presenting with an unresolved rigid spine myopathy phenotype. Additionally, we highlight mevalonoic acid supplementation as a potential treatment for patients with HMGCS1-related disease.

中文翻译：

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HMGCS1 变体导致脊柱僵硬综合征，适合在动物模型中进行甲羟戊酸治疗


脊柱僵硬综合征是一种罕见的儿童期发病肌病，其特征是缓慢进展或非进行性脊柱侧凸、颈部和脊柱挛缩、肌张力减退和呼吸功能不全。SELENON 中的双等位基因变异是大多数脊柱僵硬综合征病例的原因，然而，一些患者的潜在遗传原因仍未解释。我们使用外显子组和基因组测序来研究无基因诊断的患者脊柱僵硬综合征的遗传基础。在来自 4 个不相关家庭的 5 例患者中，我们发现了 HMGCS1 （3-羟基-3-甲基戊二酰辅酶 A 合酶） 的双等位基因变异。其中包括 6 个错义变体和 1 个移码变体，分布在整个 HMGCS1 中。所有患者均表现为主要影响颈部和背腰椎区域的脊柱强直、脊柱侧凸和呼吸功能不全。肌酸激酶水平不同程度地升高。一些患者的临床病程因并发疾病或某些药物而恶化;1 例患者死于感染后呼吸衰竭。肌肉活检显示氧化酶染色不规则，偶尔有内核和有边缘的空泡。HMGCS1 编码甲羟戊酸途径的一种关键酶，尚未与疾病相关。值得注意的是，下游酶（包括 HMGCR 和 GGPS1）中的双等位基因亚型变异与肌肉萎缩症有关，类似于我们队列的表现。对重组人 HMGCS1 蛋白和四种变体 （p.S447P、p.Q29L、p.M70T、p.C268S） 的分析表明，所有突变体都保持了其二聚化状态。与野生型相比，四个突变体中有三个表现出降低的热稳定性，两个突变体的酶活性表现出细微的变化。 Hmgcs1 突变斑马鱼表现出严重的早期缺陷，包括受精后 2 天不动和受精后第 3 天死亡，并被 HMGCS1 mRNA 挽救。我们证明，在斑马鱼救援试验中，与野生型 HMGCS1 相比，测试的四种变体 （S447P、Q29L、M70T 和 C268S） 的功能降低。此外，我们证明了补充甲羟戊酸以降低突变斑马鱼表型严重程度的潜力。总体而言，我们的分析表明 HMGCS1 中的这些错义变体通过亚态机制起作用。在这里，我们报告了与疾病相关的甲羟戊酸通路的附加成分，并建议在表现未解决的僵硬脊柱肌病表型的患者中考虑 HMGCS1 中的双等位基因变异。此外，我们强调甲羟戊酸补充剂是 HMGCS1 相关疾病患者的潜在治疗方法。