Discussion
To the best of our knowledge, the present study is the first population-scale study to investigate relationships between HGS and C-section. We performed observational analysis and GWEIS using over 160,000 participants from the UK Biobank. We showed significant associations between HGS and C-section and proved several Gene×C-section interactions effecting on HGS.
Our observational analysis proved a significant association between C-section and HGS in offspring. Based on previous studies, we hypothesized that this association might be related to the changes caused by C-section in intestinal flora 24 and immune responses in offspring 25. These changes might act as prenatal environmental factors that have long-term or permanent effects on skeletal muscle growth and decline in offspring 26. Previous studies have observed the cross talk between gut microbiota and skeletal muscle in pathological conditions27 and proposed that intestinal flora regulates skeletal muscle function through skeletal muscle-gut axis 27, 28. Lahiri et al.29 observed that the mice lacked gut microbiota had skeletal muscle atrophy, decreased expression of insulin-like growth factor 1, and reduced transcription of genes associated with skeletal muscle growth and mitochondrial function. These phenomena could be improved by transplanting intestinal flora from pathogen-free mice into germ-free mice 29. A population-based study found that C-section lead to a decrease in the abundance of Bifidobacteriumin the intestines of infants 30.Bifidobacterium may influence gut-muscle communication and regulate muscle size 31. Data showed that treatment which could increase the abundance of Bifidobacterium in older individuals may prophylactically moderate aging muscle loss31. In addition, immune responses also might be implicated in the mechanism of this association between C-section and HGS. Lipopolysaccharide (LPS), sitting at the neonatal immune system priming, was significantly under-represented in C-section neonates32, which may result in persistent effects on human physiology in later life 33. Immune responses and related factors play a role in skeletal muscle formation. For example, the TGF-β family members are implicated in the regulation of myogenic differentiation and have been shown to be potent inhibitors of myoblasts differentiation 34. These results suggested a role for the gut microbiota and immune system in regulating skeletal muscle mass and function.
ADGRV1 gene showed a significant G×E interaction effect on left grip strength. ADGRV1 encodes a member of the G-protein coupled receptor superfamily and act as an important regulator of bone homeostasis and muscle function directly or indirectly. Mutations in this gene contribute to epilepsy with myoclonic seizures 35. Mouse models and qRT-PCR analysis demonstrated that ADGRV1-deficient osteoblasts have increased RANKL expression, which result in low BMD36. RANKL as a member of the TNF receptor superfamily is essential for osteoclastogenesis, and its activation induces high-turnover osteoporosis37. In addition, RANKL is also expressed in skeletal muscle and activates the NF-κB pathway, which inhibits myogenic differentiation and leads to skeletal muscle dysfunction and loss38. Inhibition of RANKL expression could improve muscle strength in osteoporotic mice and humans39. Previous studies have reported muscle-bone interactions and possible bone-to-muscle communication40. Our result supported previous findings and suggested that the effects of C-section on hand grip might be related to muscle-bone interactions.
POLR3G gene and WASL gene showed significant G×E interaction effects on left grip strength while CRPPA gene and APBB2 gene showed significant G×E interaction effects on right grip strength. POLR3G participants in the transcription of RNA polymerase III (Pol III) and express in the skeletal muscle cell lineage, which results in a significant reduction in the activation of muscle genes 41. In addition, the POLR3G overexpression had some ability to reverse differentiation41. WASL gene encodes a member of the Wiskott-Aldrich syndrome (WAS) protein family named N-WASP, which is expressed ubiquitously42. WASL could promote actin polymerization and the formation of branched actin polymers which plays an important role in the differentiation of myoblast to myotubes43. Cai et al. suggested that N-WASP is a key intermediate in the regulation of myofibroblast differentiation and maturation 44. CRPPA gene, locating at chromosome 7p21, is a frequent cause of Walker–Warburg syndrome45. Although the functional role of human CRPPA remains unknown, Riemersma et al. 46 suggested that CRPPA deficiency contribute to tertiary dystroglycanopathies, which is strongly associated with congenita muscular dystrophy (CMD)45. APBB2 gene encodes a kind of cytoplasmic adaptor protein which mediates the assembly of multimolecular complexes47. APBB2 knockout (KO) mice showed several phenotypes in muscle that are comparable to the clinical features of CMD in humans, including motor impairments and neuromuscular junction (NMJ) abnormalities 47 48.
Interestingly, gene set enrichment analyses results suggested a role of immune system involved in the association between C-section and HGS. The relationship between C-section and immunity has been discussed above. In addition, the relationship between immunity and HGS has been observed in a monozygotic-twins epigenome-wide study 49. CLEC16A gene was reported as a candidate gene associated with both left and right HGS in present study. CLEC16A gene encodes a member of the C-type lectin domain containing family and has been showed to be associated with susceptibility for various autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematous (SLE) 50. One of the possible potential functional links between CLEC16A and autoimmunity is autophagy 51, which has been showed to have crosstalks with signaling cascades in both innate and adaptive immune responses 52. According to a quantitative proteomics analysis, Tam et al. 51 identified that CLEC16A may enhance mTORC1 activity and act as a negative regulator through mTOR pathway in autophagy. The mTOR is a serine/threonine kinase capable of integrating several stimuli from the medium and growing evidence suggests that mTOR signaling pathway plays an important role in maintaining muscle fiber size in healthy muscle 53. For example, mTORC1 could mediate the signaling of substrates and control the initiation of translation by which consequently promote the synthesis of muscle protein 54.
What’s more, the results of gene set enrichment analysis suggested the role of central nervous system (CNS) in the relationship between C-section and HGS. Besides the muscle structure, the ability of muscle to produce force is also associated with neuromotor control. Evidence showed that the primary motor cortex (M1) contributes to the control of a power grip of finger muscles during a variety of grasping behaviors55. Kara et al. demonstrated that neuromotor control dysfunction contributes to the deterioration of the force generating capacity of skeletal muscles 56. More research is needed to verify this hypothesis.
One of the innovations of this study is to consider the G×E interaction effects on hand grip strength, which has been demonstrated to contribute to the etiology of complex traits. The GWEIS analyses complement the genetic relationships between C-section and HGS and provide several novel loci contributing to the variation of HGS. In addition, the large samples of HGS in UK biobank increased the accuracy and effectiveness of our analyses. Previous studies focused on the effect of C-section on child health, less study have assessed its effect on the overall health status of offspring. Our study demonstrated the interaction association between C-section and the HGS for offspring. As far as we known, this is the first systemic study exploring the effect of C-section as environmental factor on HGS for offspring. Our study holds great potential for clarifying the functional relevance of C-section with HGS.
Admittedly, there are still limitations in present study. First, because of the sample population, our results cannot be generalized across ethnic groups. Secondly, although several genes have been reported to have interaction effects with C-section, the specific biological functions and mechanisms of action need further experimental studies.
In summary, we performed an observational analysis and a genome-wide interaction analysis of C-section and HGS. We reported significant interactions whether from phenotypic or genetic perspective. Our study reported several novel candidate loci and indicated the role of C-section in HGS.