Autosomal recessive mutations in IGHMBP2, a ubiquitously expressed DNA/RNA helicase, have been linked to childhood neuromuscular degenerative diseases (NMDs). C57BL/6J-Ighmbp2em3Cx is a SMARD1-like strain, or Spinal Muscular Atrophy with Respiratory Distress, created via CRISPR-Cas9 targeting of the IGHMBP2 gene and hereafter referred to as em3. SMARD1 is characterized by muscle weakness starting in the distal extremities and diaphragmatic paralysis leading to respiratory failure. Most patients are diagnosed in early infancy and die in early childhood. The em3 mouse has more severe muscle atrophy than the historical SMARD1-like model (nmd2J) in the hind limb, diaphragm, and intercostal muscles. The em3 mouse model also has an average lifespan of ~3 weeks compared to the 2J's ~3 month lifespan. C57BL/6J-Ighmbp2em5Cx is a Charcot-Marie-Tooth disease type 2S model that has not impact on lifespan but does impact motor and sensory function beginning around the 4 week timepoint.
Gene therapy has shown promise in another NMD, Spinal Muscular Atrophy (SMA). In collaboration with the Meyer lab at Nationwide Children's Hospital in Columbus, OH, we are testing 2 different AAV9-Ighmbp2 vectors. Each has a different promoter with one having a Chicken ?-Actin (CBA) Promoter [higher expression levels than endogenous levels] and the other having a truncated Methyl-CpG binding protein 2 (MECP2 aka P546) promoter [expression levels close to endogenous levels expressed by muscles and neurons]. We performed postnatal day 1 intracerebroventricular injections on em3 and em5 mutants and unaffected sibling pups to determine the efficacy of each treatment, respectively, and if there are toxic effects associated with overexpression of IGHMBP2 in wild type mice. Using a variety of assays to determine strength and neuromuscular degeneration, we determined that the P546 promoter is more effective in EM3 mice and that either virus causes the EM5 mice to show no significant difference between mutant mice and wildtype mice.