Molecular pathophysiology of human MICU1 deficiency

Abstract

Aims MICU1 encodes the gatekeeper of the mitochondrial Ca2+ uniporter, MICU1 and biallelic loss‐of‐function mutations cause a complex, neuromuscular disorder in children. Although the role of the protein is well understood, the precise molecular pathophysiology leading to this neuropaediatric phenotype has not been fully elucidated. Here we aimed to obtain novel insights into MICU1 pathophysiology. Methods Molecular genetic studies along with proteomic profiling, electron‐, light‐ and Coherent anti‐Stokes Raman scattering microscopy and immuno‐based studies of protein abundances and Ca2+ transport studies were employed to examine the pathophysiology of MICU1 deficiency in humans. Results We describe two patients carrying MICU1 mutations, two nonsense (c.52C>T; p.(Arg18*) and c.553C>T; p.(Arg185*)) and an intragenic exon 2‐deletion presenting with ataxia, developmental delay and early onset myopathy, clinodactyly, attention deficits, insomnia and impaired cognitive pain perception. Muscle biopsies revealed signs of dystrophy and neurogenic atrophy, severe mitochondrial perturbations, altered Golgi structure, vacuoles and altered lipid homeostasis. Comparative mitochondrial Ca2+ transport and proteomic studies on lymphoblastoid cells revealed that the [Ca2+] threshold and the cooperative activation of mitochondrial Ca2+ uptake were lost in MICU1‐deficient cells and that 39 proteins were altered in abundance. Several of those proteins are linked to mitochondrial dysfunction and/or perturbed Ca2+ homeostasis, also impacting on regular cytoskeleton (affecting Spectrin) and Golgi architecture, as well as cellular survival mechanisms. Conclusions Our findings (i) link dysregulation of mitochondrial Ca2+ uptake with muscle pathology (including perturbed lipid homeostasis and ER–Golgi morphology), (ii) support the concept of a functional interplay of ER–Golgi and mitochondria in lipid homeostasis and (iii) reveal the vulnerability of the cellular proteome as part of the MICU1‐related pathophysiology.

Uncontrolled mitochondrial calcium uptake underlies the pathogenesis of neurodegeneration in MICU1-deficient mice and patients

“…Heterozygous micu1 neurons and nKO mice have no significant impairments, which is consistent with literature on most individuals with heterozygous MICU1 mutation ( 14 , 15 , 62 ). This is likely because lymphoblasts derived from humans with heterozygous MICU1 mutation ( 15 ), and neurons derived from HT mice (fig. S4) showed no decrease in the mtCU protein abundance, suggesting that one normal allele is sufficient to provide a normal amount of MICU1.…”

“…In this study, we showed that neuron-specific homozygous MICU1 deletion in mouse leads to altered neuronal Ca 2+ homeostasis and progressive motor and cognitive dysfunction likely in both males and females. These phenotypes recapitulate those previously reported in patient fibroblasts/lymphoblasts and the symptoms displayed by many MICU1-deficient male and female patients ( 12 – 15 , 62 , 63 ). Heterozygous micu1 neurons and nKO mice have no significant impairments, which is consistent with literature on most individuals with heterozygous MICU1 mutation ( 14 , 15 , 62 ).…”

“…6F ). The studies of the patient fibroblasts were also performed with the lymphoblasts of another MICU1-deficient patient ( 15 ), and the results showed the same outcome observed in the MICU1-deficient patient fibroblasts (fig. S5, E to G).…”

Identification of a novel homozygous synthesis of cytochrome c oxidase 2 variant in siblings with early‐onset axonal Charcot‐Marie‐Tooth disease

“…MICU1 was identified as being dysregulated by our immunoblot studies. Recessive MICU1 mutations were linked to myopathy with extrapyramidal signs and learning disability in pediatric cases (Kohlschmidt et al, 2021; Logan et al, 2014).…”

“…Mutations in MFN2 (most commonly autosomal dominant) cause Charcot‐Marie‐Tooth disease type 2A (CMT2A), the commonest axonal form of CMT (Pipis et al, 2020). Recessive mutations in the MICU1 gene cause a very rare neuromuscular disease characterized by myopathy with extrapyramidal signs, due to primary alterations in mitochondrial calcium signaling (Bitarafan et al, 2021; Kohlschmidt et al, 2021). However, further functional studies on patient‐derived cells such as fibroblasts or neuronal cells would be needed to study the precise effect of the p.(Gly121Arg)‐ SCO 2 variant on mitochondrial fusion and calcium signaling.…”