Impaired Local Translation of β-actin mRNA in Ighmbp2-Deficient Motoneurons: Implications for Spinal Muscular Atrophy with respiratory Distress (SMARD1).

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal motoneuron disorder in children with unknown etiology. The disease is caused by mutations in the IGHMBP2 gene, encoding a Super Family 1 (SF1)-type RNA/DNA helicase. IGHMBP2 is a cytosolic protein that binds to ribosomes and polysomes, suggesting a role in mRNA metabolism. Here we performed morphological and functional analyses of isolated immunoglobulin μ-binding protein 2 (Ighmbp2)-deficient motoneurons to address the question whether the SMARD1 phenotype results from de-regulation of protein biosynthesis. Ighmbp2-deficient motoneurons exhibited only moderate morphological aberrations such as a slight increase of axonal branches. Consistent with the rather mild phenotypic aberrations, RNA sequencing of Ighmbp2-deficient motoneurons revealed only minor transcriptome alterations compared to controls. Likewise, we did not detect any global changes in protein synthesis using pulsed SILAC (Stable Isotope Labeling by Amino acids in Cell culture), FUNCAT (FlUorescent Non-Canonical Amino acid Tagging) and SUnSET (SUrface SEnsing of Translation) approaches. However, we observed reduced β-actin protein levels at the growth cone of Ighmbp2-deficient motoneurons which was accompanied by reduced level of IMP1/ZBP1, a known interactor of β-actin mRNA. Fluorescence Recovery after Photobleaching (FRAP) studies revealed translational down-regulation of an eGFP-myr-β-actin 3'UTR mRNA in growth cones. Local translational regulation of β-actin mRNA was dependent on the 3' UTR but independent of direct Ighmbp2-binding to β-actin mRNA. Taken together, our data indicate that Ighmbp2 deficiency results in local but modest disruption of protein biosynthesis which might partially contribute to the motoneuron defects seen in SMARD1.

Surrey V ，Zöller C ，Lork AA ，Moradi M ，Balk S ，Dombert B ，Saal-Bauernschubert L ，Briese M ，Appenzeller S ，Fischer U ，Jablonka S ... -  《-》

Characterization of Ighmbp2 in motor neurons and implications for the pathomechanism in a mouse model of human spinal muscular atrophy with respiratory distress type 1 (SMARD1).

Grohmann K ，Rossoll W ，Kobsar I ，Holtmann B ，Jablonka S ，Wessig C ，Stoltenburg-Didinger G ，Fischer U ，Hübner C ，Martini R ，Sendtner M ... -  《HUMAN MOLECULAR GENETICS》

Clinically relevant mouse models of severe spinal muscular atrophy with respiratory distress type 1.

Spinal Muscular Atrophy with Respiratory Distress (SMARD1) is a lethal infantile disease, characterized by the loss of motor neurons leading to muscular atrophy, diaphragmatic paralysis, and weakness in the trunk and limbs. Mutations in IGHMBP2, a ubiquitously expressed DNA/RNA helicase, have been shown to cause a wide spectrum of motor neuron disease. Though mutations in IGHMBP2 are mostly associated with SMARD1, milder alleles cause the axonal neuropathy, Charcot-Marie-Tooth disease type 2S (CMT2S), and some null alleles are potentially a risk factor for sudden infant death syndrome (SIDS). Variant heterogeneity studied using an allelic series can be informative in order to create a broad spectrum of models that better exhibit the human variation. We previously identified the nmd2J mouse model of SMARD1, as well as two milder CMT2S mouse models. Here, we used CRISPR-Cas9 genome editing to create three new, more severe Ighmbp2 mouse models of SMARD1, including a null allele, a deletion of C495 (C495del) and a deletion of L362 (L362del). Phenotypic characterization of the IGHMBP2L362del homozygous mutants and IGHMBP2C495del homozygous mutants respectively show a more severe disease presentation than the previous nmd2J model. The IGHMBP2L362del mutants lack a clear denervation in the diaphragm while the IGHMBP2C495del mutants display a neurogenic diaphragmatic phenotype as observed in SMARD1 patients. Characterization of the Ighmbp2-null model indicated neo-natal lethality (median lifespan = 0.5 days). These novel strains expand the spectrum of SMARD1 models to better reflect the clinical continuum observed in the human patients with various IGHMBP2 recessive mutations.

Holbrook SE ，Hicks AN ，Martin PB ，Hines TJ ，Castro HP ，Cox GA ... -  《-》

RNA helicase IGHMBP2 regulates THO complex to ensure cellular mRNA homeostasis.

RNA helicases constitute a large protein family implicated in cellular RNA homeostasis and disease development. Here, we show that the RNA helicase IGHMBP2, linked to the neuromuscular disorder spinal muscular atrophy with respiratory distress type 1 (SMARD1), associates with polysomes and impacts translation of mRNAs containing short, GC-rich, and structured 5' UTRs. The absence of IGHMBP2 causes ribosome stalling at the start codon of target mRNAs, leading to reduced translation efficiency. The main mRNA targets of IGHMBP2-mediated regulation encode for components of the THO complex (THOC), linking IGHMBP2 to mRNA production and nuclear export. Accordingly, failure of IGHMBP2 regulation of THOC causes perturbations of the transcriptome and its encoded proteome, and ablation of THOC subunits phenocopies these changes. Thus, IGHMBP2 is an upstream regulator of THOC. Of note, IGHMBP2-dependent regulation of THOC is also observed in astrocytes derived from patients with SMARD1 disease, suggesting that deregulated mRNA metabolism contributes to SMARD1 etiology and may enable alternative therapeutic avenues.

Prusty AB ，Hirmer A ，Sierra-Delgado JA ，Huber H ，Guenther UP ，Schlosser A ，Dybkov O ，Yildirim E ，Urlaub H ，Meyer KC ，Jablonka S ，Erhard F ，Fischer U ... -  《-》

Severe phenotypes of SMARD1 associated with novel mutations of the IGHMBP2 gene and nuclear degeneration of muscle and Schwann cells.

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a very rare autosomal recessive form of spinal muscular atrophy manifested in low birth weight, diaphragmatic palsy and distal muscular atrophy. Caused by a mutation in the IGHMBP2 gene, the disease is addressed here by reference to five Polish patients in which SMARD1 has been confirmed genetically. All presented a severe form of the disease and had evident symptoms during the second month of life; with four displaying weak cries, feeding difficulties and hypotonia from birth. Two were afflicted by severe dysfunction of the autonomic nervous system. Ultrastructural analysis of a muscle biopsy revealed progressive degeneration within the nuclei of the muscle cells and Schwann cells. Neuromuscular junctions were also defective. It proved possible to identify in our patients 6 novel IGHMBP2 mutations: three missense (c.595G>C, c.1682T>C and c.1794C>A), two nonsense (c.94C>T and c.1336C>T) and one in-frame deletion (c.1615_1623del). One nonsense mutation (c.429C>T) that had been described previously was also identified. Observation of our patients makes it clear that clinical picture is still the most important factor suggesting diagnosis of SMARD1, though further investigations concerning some of the symptoms are required. As the IGHMBP2 gene is characterized by significant heterogeneity, genetic counseling of affected families is rendered more complex. IGHMBP2 protein deficiency can lead to the degeneration of nuclei, in both muscle and Schwann cells.

Jędrzejowska M ，Madej-Pilarczyk A ，Fidziańska A ，Mierzewska H ，Pronicka E ，Obersztyn E ，Gos M ，Pronicki M ，Kmieć T ，Migdał M ，Mierzewska-Schmidt M ，Walczak-Wojtkowska I ，Konopka E ，Hausmanowa-Petrusewicz I ... -  《-》