Intervertebral Disc Degeneration (IVDD) is the primary etiology of chronic lower back pain, which increases in incidence with advancing age. Progressive structural alterations coupled with severe disruptions in metabolic homeostasis facilitate IVDD progression (including abnormal apoptosis, senescence, and pyroptosis of IVD cells; ECM degradation and infiltration of immune cells)^[61-63]. Therefore, comprehensively understanding the regulatory mechanisms of IVDD, particularly the primary genetic and epigenetic mechanisms, is essential.

Recent studies have demonstrated that RBPs can regulate degenerative changes in neural progenitor cells (NPCs), thereby influencing the risk of IVDD. Decreased levels of RBP motif single-stranded interacting protein 3 (RBMS3) and HuR have been noted in the nucleus pulposus tissue of patients with IVDD. RBMS3 enhances nucleus pulposus cell proliferation and inhibits their apoptosis and inflammation by inactivating the Wnt/β-catenin signaling pathway, thereby delaying IVDD progression. HuR mitigates inflammation, ECM degradation, and senescence by enhancing NF-kappaB repressing factor (NKRF) mRNA stability and autophagy^[64-66]. Conversely, increased protein levels of WTAP enhance lnc-NORAD m6A modification in senescent NPCs, whereas YTHDF2-mediated lnc-NORAD degradation facilitates cellular senescence during IVDD^[67].

Ossification of Posterior Longitudinal Ligament (OPLL) is an emerging spinal condition among older adults, leading to intractable myelopathy and radiculopathy due to heterotopic ossification of the posterior longitudinal ligament^[68]. Recent evidence demonstrated increased levels of cell division cycle 5-like (CDC5L) and METTL3 in patients with OPLL, where they respectively enhance chondrogenesis and osteogenic differentiation^[69,70].

Ankylosing spondylitis is a common, highly heritable spondyloarthropathy that primarily affects the spine joints and pelvis, leading to severe chronic pain^[71]. Limited information exists regarding RBP function in spondylitis, with one report indicating that decreased YTHDF2 is a risk factor for new-onset ankylosing spondylitis^[72]. Another study found Rio Kinase 3 enriched in peripheral blood mononuclear cells obtained from ankylosing spondylitis, and its suppression reduces osteogenic differentiation in mouse MSC^[73].

Osteoarthritis (OA) is a leading cause of severe joint pain, creating a significant socioeconomic burden owing to the aging population, with no existing disease-modifying therapies available^[74]. This progressive degenerative disease of the cartilage is marked by cartilage degradation, subchondral bone remodeling, and synovial inflammation^[75]. Given recent comprehensive reviews regarding aberrant RBP expression and its role in OA regulation, we will concentrate on targeted RBP therapy for OA^[76]. Recently, LIN28a was identified in damaged cartilage in humans and mice, where its overexpression enhanced ECM production. In OA mice (with Col2a1-Cre), the loss of Lin28a exacerbates cartilage degradation through high mobility group AT-hook 2 (HMGA2) and Sry HMG box protein 9 (SOX9)^[77]. Cre-inducible Lin28 transgenic mice demonstrate increased articular cartilage thickness, which is diminished by the injection of the extracellular regulated protein kinase (ERK) signaling pathway inhibitor U0126^[78]. In a surgical destabilization of the medial meniscus model of OA, intra-articular injection of synovium-targeted METTL3 siRNA, adenovirus-mediated shZFP36L1, and lentiviral-mediated pumilio1 (PUM1) overexpression mitigates cartilage damage, preserves cartilage integrity, and inhibits OA pathogenesis^[30,79,80]. Notably, ncRBP fascin actin-bundling protein 1 inhibition by NP-G2-044 could mitigate the progression of mechanical overload-induced OA in mice^[81]. These data indicate that RBPs and their associated pathway activators or inhibitors may be potential candidates for targeted therapies in OA.

Rheumatoid Arthritis (RA) is a chronic immune-mediated disease characterized by inflammation and destruction of the bone and cartilage in affected joints^[82]. M6A regulator level, including ALKBH5, FTO, and YTHDF2, is suppressed in patients with RA, indicating a potential risk factor associated with m6A regulators in RA^[83,84]. Recent analyses have proficiently addressed RA-related m6A regulators, including those mentioned above^[85].

Poly(rC)-binding protein 1 (PCBP1) and heterogeneous nuclear ribonucleoprotein U-like 1 (hnRNP UL1) - expressed at reduced levels in RA - modulate AS of immune response-related genes and inhibit NF-κB-mediated inflammation, respectively. The conditional deletion of hnRNP UL1 in macrophages through Lyz2-cre enhances the synthesis of inflammatory cytokines^[86,87]. Additionally, synovial tissues from patients with RA demonstrate increased levels of cold-inducible RBP (CIRP), p54^nrb, and SAM68. They enhance NF-κB-induced pro-inflammatory response, proliferation, migration, and invasion of fibroblast-like synoviocytes in RA^[87-89]. MS-444, a potential small-molecule inhibitor of HuR, impeded its dimerization and translocation, thereby diminishing the type I interferon (IFN) response in RA fibroblast-like synoviocytes^[90]. Notably, HuR activators mitigate osteoporosis-related phenotypes in mice, whereas HuR inhibitors influence the IFN response in RA. Consequently, the future lies in targeting RBP as a potential therapeutic for skeletal disorders and in developing associated small-molecule compounds as novel therapeutic agents.

TARP syndrome (MIM # 311900) is an X chromosome-linked recessive genetic disorder characterized by Robin sequence (micrognathia, glossoptosis, and cleft palate), congenital heart defects, developmental delay, feeding difficulties, and talipes equinovarus. Mutations of RNA-binding motif protein 10 (RBM10) have been recognized as the causative factor, likely due to the loss of its splicing function for essential developmental regulators, thereby hindering normal palate development^[91,92].

Nager syndrome (MIM # 154400) exemplifies preaxial acrofacial dysostosis, distinguished by craniofacial skeleton and limb malformations. Rodriguez syndrome has been proposed as a potentially severe variant. SF3B4 haploinsufficiency is confirmed as the primary etiology. Nager syndrome involves mutations that alter SF3B4 synthesis, leading to defective mRNA splicing and decreased expression of target genes in growth plate chondrocytes. The heterozygous mutant mouse (Sf3b4+/-) was initially noted to exhibit diminished body size, homeotic posteriorization of vertebral morphology, and flattened calvaria^[93-96].

Craniofacial microsomia 1 (MIM #164210) is the second most common congenital facial anomaly, characterized by microtia, mandibular hypoplasia, and preauricular tags. The predominant genetic etiology involves haplo-insufficient variants in splicing factor 3B subunit 2 (SF3B2), present in ~3% of sporadic and ~25% of familial cases. In Xenopus embryos, injecting sf3b2 morpholinos leads to improper formation of cranial neural crest precursors and defective craniofacial cartilage^[97,98]. Furthermore, an increase in cell death and a decrease in proliferative capacity have been observed in the differentiation of patient-derived induced pluripotent stem cells (iPSCs) into neural crest cells. Mechanistically, loss-of-function variants in the SF3B2 gene induce extensive mRNA splicing disruption across the transcriptome, especially affecting TP53-dependent cell death, and contribute to these craniofacial features^[99].

Robin sequence with cleft mandible and limb anomalies (MIM #268305) (known as Richieri-Costa-Pereira syndrome) is an autosomal-recessive and unique acrofacial dysostosis type caused by mutations in non-coding expansions in the 5’UTR of the eukaryotic translation initiation factor 4A3 (EIF4A3) gene. It features a median cleft in the mandible, accompanied by craniofacial anomalies and considerable limb defects. In zebrafish, loss of eif4a3 leads to underdevelopment of multiple craniofacial cartilage and bone structures. Moreover, impaired neural crest cell development clarifies craniofacial abnormalities identified in patient-derived iPSCs and conditional cre-mediated Eif4a3 mouse models^[100,101]. However, the mechanism by which the EIF4A3 mutation affects the specific downstream targets remains unclear.

Paget disease of bone (PDB) (MIM #602080) is a common metabolic bone disorder characterized by increased bone resorption and disorganized bone formation, leading to abnormal focal bone remodeling. Mutations in the m6A reader heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1) were detected in patients with PDB, who frequently experience bone deformities, pain, and fragility fractures at the pagetic site^[102]. Preliminary investigation into degenerative diseases indicates that these mutations may create more potent steric zippers within the prion-like domain of hnRNPA2B1, consequently modifying RNA granule assembly, impairing RNA metabolism, and accelerating fibrillization^[103].

Thrombocytopenia-Absent Radius (TAR) syndrome (MIM # 274000) is a hematological and skeletal disorder characterized by low platelet counts and the absence of one forearm bone. TAR syndrome is generally caused by mutations in RNA-binding motif protein 8A (RBM8A), with haploinsufficiency resulting in severe microcephaly and impaired neurogenesis in mouse models^[104,105]. Recent research using the hypomorphic rbm8a zebrafish has demonstrated that the absence of RBM8A inhibits non-canonical Wnt/PCP signaling, resulting in abnormal lateral plate mesoderm patterning and hematopoietic defects^[106].

Verheij syndrome (MIM #615583) (8q24.3 microdeletion syndrome) is a rare craniofacial spliceosomopathy characterized by craniofacial dysmorphism, multiple congenital anomalies, and variable neurodevelopmental delay. Pathogenic variants in poly(U) binding splicing factor 60 (PUF60) have been recognized as causative for Verheij syndrome^[107]. Although an increasing number of novel PUF60 variants have been identified in the literature, the underlying mechanisms remain a mystery.

Haploinsufficiency-inducing deletions or mutations in the spliceosome factor elongation factor Tu GTP binding domain containing 2 (EFTUD2, a GTPase and a core component of the U5 snRNP) were identified in patients with mandibulofacial dysostosis with microcephaly (MFDM, MIM #610536). Affected individuals display microcephaly, craniofacial anomalies, auditory impairment, and dysmorphic features. Conditional knockout models and mechanistic studies have demonstrated that EFTUD2 deficiency impairs cortical histogenesis and drives cortical neuronal attrition. By modulating the splicing patterns of the apoptosis regulators Caspase3 and apoptosis-inducing factor mitochondria-associated 1 (Aifm1), it activates programmed cell death^[108,109].

Alazami syndrome (MIM #615071) is an autosomal recessive microcephalic primordial dwarfism characterized by notable facial dysmorphisms and severe intellectual disability. The observed phenotypes may be ascribed to the depletion or loss of functional variants in La-related protein 7 (LARP7), resulting in splicing modification in affected patients, accompanied by diminished 2’-O-methylation of U6 small nuclear RNA^[110,111].

Spinal muscular atrophy with congenital bone fractures 2 (SMABF2) (MIM #616867) is a rare autosomal recessive neuromuscular disorder resulting from biallelic variants in activating signal co-integrator complex 1 (ASCC1). It is defined by congenital arthrogryposis multiplex and prenatal fractures of the long bones. Multi-omics analyses of patient-derived fibroblasts demonstrate that the downregulation of ASCC1 inhibits the TGF-β/SMAD signaling pathway, consequently suppressing osteoblastogenesis^[112,113].

Au-Kline Syndrome (AKS) (MIM #616580) was initially characterized in 2015 as a multiple malformation syndrome distinguished by remarkable deformities in the limbs and spine. De novo missense or intronic variants in heterogeneous nuclear ribonucleoprotein K (HNRNPK) have been associated with AKS^[114]. Recent studies demonstrated that depletion of hnRNPK during chondrogenesis using Col2a1-Cre results in dwarfism, marked by impaired survival and premature differentiation of growth plate chondrocytes. Mechanistically, hnRNPK deficiency extends the half-life of Hif1a mRNA, consequently activating the Hif-1 signaling pathway^[115]. Recently, the RNA-binding motif protein 42 (RBM42) C-terminal A438T variant disrupted its interaction with hnRNP K, leading to a multisystem disease with musculoskeletal involvement^[116].

Cornelia de Lange Syndrome (CdLS) (MIM #122470) is the most common cohesinopathy, characterized by intellectual disability, growth retardation, limb malformations, and unique facial features. Heterozygous loss-of-function mutations in Nipped-B-Like protein (NIPBL) account for approximately 70% of CdLS cases^[117,118]. Surviving Nipbl+/- mice exhibit significant reductions in body size, notable craniofacial changes, and anteriorization of thoracic vertebrae, with left-right asymmetry accompanied by collective changes in osteogenesis-regulatory pathways^[119-121].

Spondyloepimetaphyseal dysplasia, Guo-Campeau type (SEMDGC) (MIM #620663) is defined by severe bone dysplasia affecting the spine and long tubular bones, leading to substantial short stature with disproportionate extremities. Research has revealed that pathogenic variants in the exoribonuclease 1 (ERI1) gene are responsible for this novel form of bone dysplasia in patients from seven affected families. Mechanistically, missense mutations in the conserved 3’-5’ RNA exonuclease impair enzyme activity, failing to catalyze 5.8S rRNA processing and affecting histone mRNA decay. Furthermore, Eri1 knockout mice exhibit skeletal dysplasia that partially resembles human SEMDGC^[122].

Cherubism (MIM #118400) is an autosomal dominant condition associated with severe craniofacial developmental anomalies, notably excessive jaw-bone destruction. Gain-of-function missense variants in c-Abl-SH3 domain binding protein-2 (SH3BP2) have been identified as being responsible for the condition in patients from 12 affected families^[123]. In OCs, SH3BP2 phosphorylation seems to activate phospholipase C gamma 2 (PLCG2), resulting in excessive OC activity. Consistently, in the homozygous cherubism knock-in (KI) mouse model (Sh3bp2 KI/+), systemic bone destruction has been observed owing to increased OC formation. Additionally, spleen tyrosine kinase (SYK) inhibitor, entospletinib, abolishes all observed cherubism phenotypes^[124,125].

Craniofacial malformations rank among the most common congenital disabilities. Considering the role of RBPs in modifying craniofacial disorders has recently been extensively reviewed^[126,127], we presented fundamental research on craniofacial development. A homozygous frameshift variant in hnRNP UL1 was detected in patients exhibiting craniofacial and limb anomalies, consistent with the developmental role of hnRNP UL1 observed in zebrafish^[128]. Studies utilizing animal models highlight the essential function of Serine-rich splicing factor 3 (SRSF3), nucleolin, RNA-binding Fox-1 homolog 2 (RBFOX2), and RBMS3 in regulating craniofacial development^[129-132].