Exploring the role of neuromuscular excitability in sudden infant death

Sudden infant death syndrome (SIDS) is the unexpected death of a seemingly healthy infant that remains unexplained despite a thorough investigation of the scene and circumstances of death, a comprehensive post-mortem examination, and a multi-professional review of the available information. It is the leading cause of post-neonatal infant death in high income countries. Five children in the UK die each week from SIDS. Current evidence as to the cause of death proposes convergence of "a triple risk": a vulnerable infant, a critical developmental period and an external stressor e.g. fever.
Electrical signals control a muscle's excitability i.e. it's ability to contract and relax. Multiple proteins are responsible for generating and controlling these signals. Mutations in these genes can cause clinical diseases e.g. congenital myasthenia. All of these diseases tend to produce episodic muscle symptoms but in between individuals appear healthy. As they affect a muscle's electrical circuit rather than structure they also leave no post-mortem hallmark. These diseases may impair breathing and be fatal in a manner that is indistinguishable from SIDS. The SCN4A gene codes for one of the most essential proteins for muscle excitability, an ion channel called Nav1.4. Clinical disorders due to mutations in the SCN4A gene cause breathing muscle impairment including life threatening apnoeas (cessation of breathing) in infants. Based on observations I have made in infants I collaborated with my research partner Prof Behr and obtained new genetic and functional evidence that qualitatively similar SCN4A gene variants occur in some cases of (SIDS). I now hypothesise that ion channels like Nav1.4, and other proteins critical for skeletal muscle excitability may collectively account for a larger proportion of SIDS cases (contributing to the vulnerable infant risk) and that normal muscle development may change the clinical severity of these variants in early life. A severity and risk that may get less with age i.e. a critical developmental period.

This proposal aims to:
1. Determine the frequency of rare gene variants encoding proteins that influence skeletal muscle excitability in c.700 SIDS cases and UK biobank controls
2. Determine the frequency of SIDS amongst families with neuromuscular excitability diseases caused by variants in these same genes
3. Characterise the normal developmental changes of muscle fibre types in human skeletal muscle
4. Use a computer model to determine the physiological consequences of combined genetic, developmental and exogenous variables on muscle excitability - the "triple risk"

I will use whole exome sequencing data from c.700 DNA samples of infants who have died from SIDS and screen for very rare or new variants in a candidate gene list (genes critical for muscle excitability) that are predicted to affect protein function. The frequency that these gene variants are found in the SIDS cases will be compared with the UK biobank control sequencing data to see if they are more common in SIDS cases. I will also use clinical data from families with mutations in these genes causing clinical disease to ascertain the incidence of sudden infant deaths and whether this is more common than among the general population. Muscles are made up of subtypes of fast and slow fibres. I think the more fast fibres a muscle has the more severe the effect of the gene variants will be and that infants' muscle will have more fast fibres than older children. A fully automated protocol applying different coloured antibodies to the different fibre types in a single skeletal muscle section will be used on human control biopsies from age 0 to 16years to determine the proportion of each fibre type at different ages. A computer model of a muscle fibre will be used to ascertain the individual and combined effects of developmental variables on normal and mutant (with a gene variant) muscle fibre function and performance.

Five children in the UK die each week from Sudden Infant Death Syndrome (SIDS). The SCN4A gene codes for an ion channel Nav1.4 that is essential for muscle membrane excitability. Clinical disorders due to mutations in the SCN4A gene cause respiratory muscle impairment including life threatening apnoeas in infants. Based on these initial observations in infants I obtained new genetic and functional evidence that qualitatively similar SCN4A gene variants occur in some cases of SIDS. I hypothesise that ion channels and other proteins critical for skeletal muscle excitability may collectively account for a larger proportion of SIDS cases and that normal muscle development may contribute to the severity of these variants in early life.
This proposal aims to:
1. Determine the frequency of rare gene variants encoding proteins that influence skeletal muscle excitability using WES data from c.700 SIDS cases and UKbiobank controls
2. Determine the frequency of SIDS amongst families with neuromuscular excitability diseases caused by variants in these genes
3. Characterise the developmental changes of muscle fibre types in human skeletal muscle
4. Develop an in silico programme to model the physiological consequences of the genetic variation identified and the effect of developmental and exogenous variables on muscle excitability
A fully automated multi-coloured chromogenic IHC protocol capable of detecting myosin heavy chain isoforms I, IIA and IIX in a single skeletal muscle section will be employed on human control biopsies (age 0 to 16years) to determine the relative proportions of fast and slow fibre types at different ages. I hypothesise that excitability gene variants have a greater detriment to fast fibres and that infant muscle has more fast fibres than adults. A linear circuit model of mammalian muscle fibre will be used to ascertain the individual and combined effects of developmental variables on wild type and mutant muscle fibre function and performance.