# RBMX Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- RBMX, an X-chromosome-encoded RNA-binding protein, is a multifunctional factor involved in pre-mRNA splicing, transcriptional regulation, DNA damage response, and m6A RNA modification reading, with critical roles in neurodevelopment and genome stability.
- Germline loss-of-function mutations in RBMX cause X-linked intellectual disability syndromes (e.g., Shashi syndrome) and Gustavson syndrome, highlighting its essential role in neurodevelopment, with phenotypic variability influenced by the functional retrocopy RBMXL1.
- RBMX acts as both a tumor suppressor and oncogene across various malignancies, including head and neck squamous cell carcinoma, acute myeloid leukemia, and hepatocellular carcinoma, where its dysregulation impacts prognosis and therapeutic response, often through m6A-dependent pathways.
- Somatic mutations and altered expression of RBMX are implicated in leukemogenesis (AML) by maintaining heterochromatin and promoting transcriptional activity, and in pancreatic ductal adenocarcinoma (PDAC) where its silencing reduces proliferation and induces apoptosis.
- RBMX is a key component of the m6A RNA modification machinery, functioning as an m6A reader that influences mRNA stability, splicing, and translation, with its dysregulation contributing to diseases like ALS and various cancers.
- Aberrant RBMX function is linked to reproductive disorders such as non-obstructive azoospermia and sperm morphological abnormalities, and it plays a role in the pathogenesis of conditions like polycystic ovary syndrome and asthma.

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## Executive Summary & Key Metadata

The RNA-binding motif protein X-linked (RBMX) gene, encoding the heterogeneous nuclear ribonucleoprotein G (hnRNP G), is a ubiquitously expressed, X-chromosome-encoded member of the hnRNP family. RBMX is a multifunctional nuclear protein that participates in pre-mRNA splicing, transcriptional regulation, DNA damage response, sister chromatid cohesion, and N6-methyladenosine (m6A) RNA modification reading. Its functional significance spans neurodevelopment, spermatogenesis, genome stability, and a broad spectrum of malignancies, where it acts as both a tumor suppressor and an oncogene depending on cellular context. The gene has been implicated in X-linked intellectual disability syndromes, Gustavson syndrome, and various cancers, including head and neck squamous cell carcinoma, acute myeloid leukemia, hepatocellular carcinoma, and pancreatic ductal adenocarcinoma. RBMX also interacts with viral proteins and long non-coding RNAs, modulating host-pathogen dynamics. This reference manual provides a comprehensive, biophysically detailed analysis of RBMX, covering its genomic architecture, protein domain organization, molecular pathways, clinical mutations, and therapeutic implications.

| **Attribute** | **Value** |
|---|---|
| **HGNC Symbol** | RBMX |
| **UniProt Accession** | P38159 |
| **Representative PDB ID** | True (homology models available; no high-resolution crystal structure yet) |
| **Chromosomal Locus** | Xq26.3 |
| **Primary Molecular Function** | RNA binding, pre-mRNA splicing regulation, transcriptional co-regulation, m6A reader |
| **Disease & Pathology Associations** | X-linked intellectual disability (Shashi syndrome), Gustavson syndrome, head and neck cancer, acute myeloid leukemia, hepatocellular carcinoma, pancreatic ductal adenocarcinoma, breast cancer, amyotrophic lateral sclerosis, polycystic ovary syndrome, renal ischemia/reperfusion injury |

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## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The RBMX gene is located on the long arm of the X chromosome at cytogenetic band Xq26.3. The genomic coordinates (GRCh38/hg38) span approximately chrX: 136,146,481–136,158,647 (reverse strand), encompassing roughly 12.2 kb of genomic DNA. The gene comprises 20 exons and 19 introns, with the open reading frame (ORF) spanning exons 2 through 20. The coding sequence is 1,197 nucleotides in length, encoding a protein of 391 amino acids with a predicted molecular mass of approximately 42.3 kDa.

The RBMX locus is embedded within a gene-dense region of Xq26.3, flanked by the *VGLL1* gene telomerically and the *GPR101* gene centromerically. Notably, this region is a known hotspot for genomic rearrangements, particularly tandem duplications associated with X-linked acrogigantism (X-LAG). While the X-LAG critical region typically involves *GPR101* and its enhancer elements, the proximity of RBMX to these topological associating domain (TAD) boundaries has implications for chromatin architecture and long-range regulatory interactions. The invariant TAD border at Xq26.3, which is disrupted in X-LAG duplications, may also influence RBMX expression in pituitary tissues, although direct evidence remains limited.

### 1.2 Promoter Architecture and Transcriptional Regulation

The RBMX promoter region lacks a canonical TATA box but contains a high-density CpG island spanning the transcription start site (TSS) and first exon, characteristic of housekeeping genes with broad tissue expression. The promoter is regulated by multiple transcription factor binding sites, including SP1, E2F, and members of the KLF family. Chromatin immunoprecipitation sequencing (ChIP-seq) data from ENCODE reveal that the RBMX promoter is marked by H3K4me3 (active promoter) and H3K27ac (active enhancer) in most cell types, consistent with its ubiquitous expression.

Transcriptional regulation of RBMX is context-dependent. In hepatocellular carcinoma, the transcription factor SREBP-1c (sterol regulatory element-binding protein 1c) directly binds to the RBMX promoter and activates its transcription in response to high-fructose diets. This regulatory loop is physiologically significant because RBMX, in turn, regulates SREBP-1c promoter activity, creating a positive feedback circuit that amplifies lipogenic gene expression in the liver. The RBMX-SREBP-1c axis is further modulated by the scaffold protein SAFB1, which binds to RBMX and facilitates its recruitment to the SREBP-1c promoter.

Epigenetic regulation of RBMX expression involves the polycomb repressive complex 2 (PRC2). In renal ischemia/reperfusion injury, EZH2, the catalytic subunit of PRC2, mediates H3K27me3 deposition at the RBMX promoter, leading to transcriptional repression. This repression attenuates pyroptosis via the SIRT3/NLRP3 inflammasome pathway, highlighting a protective role for RBMX downregulation in acute kidney injury.

### 1.3 Alternative Splicing and Isoform Diversity

RBMX undergoes alternative splicing to generate multiple transcript variants. The major transcript (NM_002139.3) encodes the full-length 391-amino acid protein. Alternative splicing events include:

- **Exon 4 skipping**: Produces a shorter isoform lacking part of the RNA recognition motif (RRM), which may exhibit altered RNA-binding specificity.
- **Alternative 3' splice site in exon 8**: Generates an isoform with a modified linker region between the RRM and the C-terminal domain.
- **Retention of intron 12**: Produces a transcript subject to nonsense-mediated decay (NMD), representing a potential regulatory mechanism for RBMX dosage control.

The 5' untranslated region (UTR) of RBMX is also subject to alternative splicing, particularly in the RBMX-like paralog RBMXL1. In activated B lymphocytes, alternative splicing of the RBMXL1 5' UTR induces upstream open reading frame (uORF)-mediated translational control, suggesting that similar mechanisms may regulate RBMX expression in specific cellular contexts.

### 1.4 Processed Pseudogenes and Retrocopies

RBMX has given rise to multiple processed pseudogenes and functional retrocopies during mammalian evolution. The most significant of these is RBMXL1, located on chromosome 1p31.3, which encodes a protein with 97% amino acid identity to RBMX. RBMXL1 is an intronless retrocopy that retains functionality and is expressed in a tissue-specific manner, particularly in the brain and testis. The existence of RBMXL1 provides a functional buffer against RBMX loss-of-function mutations, as demonstrated by the observation that RBMX hemizygous deletions in humans are viable, whereas complete loss of both RBMX and RBMXL1 is embryonic lethal in mice.

Additional retrocopies include RBMXL2 on chromosome 11p15.4 and RBMXL9 on chromosome 9p24.1. RBMXL2 is a germ cell-specific paralog essential for male meiosis, where it represses cryptic splicing patterns during the transcriptionally permissive environment of meiotic sex chromosome inactivation. RBMXL9 has been associated with non-obstructive azoospermia, suggesting a role in spermatogenesis. The evolutionary conservation of these retrocopies underscores the functional importance of the RBMX protein family in reproduction and neurodevelopment.

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## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Structure and Domain Organization

The RBMX protein (hnRNP G) is a 391-amino acid polypeptide with a modular domain architecture that reflects its multifunctional nature. From the N-terminus to the C-terminus, the protein contains:

1. **RNA Recognition Motif (RRM)**: Residues 1–90
2. **Central Glycine-Rich Region**: Residues 91–170
3. **C-Terminal Domain (CTD)**: Residues 171–391, containing:
   - **Nuclear Localization Signal (NLS)**: Residues 171–180
   - **SR-like Domain**: Residues 200–280
   - **Glycine-Arginine Rich (GAR) Motif**: Residues 300–391

### 2.2 RNA Recognition Motif (RRM)

The RRM domain (residues 1–90) adopts the canonical β1-α1-β2-β3-α2-β4 fold, consisting of a four-stranded antiparallel β-sheet backed by two α-helices. The β-sheet surface presents the conserved ribonucleoprotein consensus sequences RNP1 (residues 13–20: KGFGFVTY) and RNP2 (residues 3–8: IVYSNL), which mediate sequence-specific RNA binding. The RNP1 and RNP2 motifs contact the RNA backbone and base-stack with aromatic residues (Phe15, Phe17, Tyr20), providing the energetic basis for nucleic acid recognition.

Structural homology modeling against the RRM of hnRNP A1 (PDB: 1HA1) predicts that RBMX's RRM binds single-stranded RNA with a preference for poly(G) and poly(U) sequences. The RNA-binding specificity of RBMX is modulated by phosphorylation of serine residues within the RRM, which alters the electrostatic surface potential and reduces RNA affinity. This phosphorylation-dependent regulation is critical for the dynamic exchange of RBMX between splicing complexes and its role in alternative splicing decisions.

### 2.3 Central Glycine-Rich Region

The glycine-rich region (residues 91–170) is predicted to be intrinsically disordered, lacking stable secondary structure. This region serves as a flexible linker that permits conformational sampling of the RRM relative to the C-terminal domain. The glycine-rich region contains multiple RG/RGG repeats, which are sites for arginine methylation by protein arginine methyltransferases (PRMTs). Asymmetric dimethylation of arginine residues within this region modulates RBMX's interaction with Tudor domain-containing proteins and affects its subnuclear localization.

### 2.4 C-Terminal Domain and SR-like Motifs

The C-terminal domain (residues 171–391) is the most functionally diverse region of RBMX. It contains:

- **Nuclear Localization Signal (NLS)**: The sequence KRRR (residues 171–174) constitutes a classical bipartite NLS that mediates importin-α/β-dependent nuclear import. Mutation of these basic residues results in cytoplasmic mislocalization and loss of splicing regulatory function.

- **SR-like Domain**: Residues 200–280 contain multiple serine-arginine (SR) dipeptide repeats, characteristic of SR proteins that are essential splicing factors. This domain mediates protein-protein interactions with components of the spliceosome, including U1 snRNP and SR proteins such as Tra2β. The SR-like domain is phosphorylated by SR protein kinases (SRPK1/2) and CLK kinases, which regulate RBMX's recruitment to active transcription sites and its splicing activity.

- **GAR Motif**: The C-terminal glycine-arginine-rich region (residues 300–391) contains RGG repeats that contribute to RNA binding and mediate interactions with other hnRNP proteins. This region also contains a putative SH3 domain-binding motif (PxxP), which is disrupted in Gustavson syndrome-associated in-frame deletions, leading to disturbed SH3 domain interactions.

### 2.5 Quaternary Structure and Protein-Protein Interactions

RBMX functions as part of larger ribonucleoprotein complexes. It self-associates to form homodimers through interactions between the RRM and C-terminal domains. The protein also heterodimerizes with other hnRNP family members, including hnRNP A1, hnRNP C, and hnRNP K, to form the core hnRNP complex that packages nascent pre-mRNA.

Key protein-protein interaction partners identified through affinity purification and yeast two-hybrid screens include:

- **Tra2β (SRSF10)**: Interacts with the SR-like domain of RBMX and modulates alternative splicing of target genes.
- **SAFB1**: Binds to RBMX and facilitates its transcriptional regulatory function at the SREBP-1c promoter.
- **ARTS-1 (ERAP1)**: Associates with RBMX to regulate extracellular TNFR1 release, linking RBMX to inflammatory signaling.
- **EZH2**: RBMX interacts with the polycomb repressive complex component EZH2, and this interaction is modulated by the lncRNA LINC01615 in colorectal cancer.
- **YTHDC1**: RBMX regulates YTHDC1 nuclear condensates to promote nascent transcription in acute myeloid leukemia.
- **HP1α (CBX5)**: RBMX controls the transcriptional activity of HP1α, maintaining chromatin state in myeloid leukemia.

### 2.6 Structural Insights from Cryo-EM and Crosslinking Studies

While no high-resolution crystal structure of full-length RBMX exists, recent cryo-electron microscopy (cryo-EM) studies of spliceosomal complexes have provided structural context for RBMX's function. In the human spliceosome, RBMX is observed as a peripheral component of the B complex, where it stabilizes the interaction between U1 snRNP and the 5' splice site. Crosslinking and immunoprecipitation (CLIP-seq) studies have mapped RBMX binding sites to exonic splicing enhancers (ESEs) and intronic splicing silencers (ISSs), consistent with its role as a context-dependent splicing regulator.

The intrinsically disordered regions of RBMX, particularly the glycine-rich and GAR motifs, are predicted to undergo liquid-liquid phase separation (LLPS). This property is shared with other hnRNP proteins and is critical for the formation of nuclear condensates that concentrate splicing factors at sites of active transcription. In acute myeloid leukemia, RBMX regulates the formation of YTHDC1-containing nuclear condensates, linking phase separation to m6A-dependent transcriptional regulation.

> **[Interactive 3D Protein Visualizer: Load RBMX (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=P38159)**
>
> The interactive visualizer provides a predicted 3D structure of RBMX based on homology modeling against known RRM-containing proteins. Users can explore the RRM domain (residues 1–90), the disordered glycine-rich region (residues 91–170), and the C-terminal SR-like and GAR domains (residues 171–391). The visualizer includes surface electrostatic potential maps, predicted phosphorylation sites, and annotated interaction interfaces.

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## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Pre-mRNA Splicing Regulation

RBMX is a bona fide splicing factor that regulates both constitutive and alternative splicing of a large cohort of target genes. High-throughput RNA-seq following RBMX knockdown has identified hundreds of splicing events that are dependent on RBMX, with a particular enrichment for ultra-long exons (>200 nucleotides). RBMX functions by repressing cryptic splice sites within these ultra-long exons, thereby preventing non-productive splicing and maintaining transcript integrity.

The mechanism of cryptic splice site repression involves RBMX binding to intronic sequences flanking the cryptic sites and recruiting the U1 snRNP to the correct 5' splice site. This activity is particularly important for genes involved in genome stability, including *BRCA1*, *ATM*, and *FANCD2*. Loss of RBMX leads to aberrant splicing of these genes, resulting in reduced protein expression and compromised DNA damage repair capacity.

RBMX also regulates alternative splicing of specific disease-relevant genes:

- **α-Synuclein (SNCA)**: In Parkinson's disease models, RBMX regulates the alternative splicing of SNCA, promoting the inclusion of exon 5. Loss of RBMX leads to increased production of the 112-synuclein isoform, which lacks exon 5 and exhibits enhanced aggregation propensity.
- **SREBP-1c**: RBMX regulates the promoter activity of SREBP-1c through a transcriptional mechanism, in addition to its splicing functions.
- **Tau (MAPT)**: RBMX is part of the splicing regulatory network that controls tau exon 10 inclusion, which is misregulated in Alzheimer's disease and other tauopathies.

### 3.2 Transcriptional Regulation

Beyond splicing, RBMX functions as a transcriptional co-regulator. The protein shuttles between splicing factor compartments and active transcription sites, where it interacts with RNA polymerase II and chromatin modifiers. RBMX has been shown to:

- **Regulate SREBP-1c transcription**: RBMX binds to the SREBP-1c promoter and recruits the histone acetyltransferase p300, leading to increased histone acetylation and transcriptional activation. This function is enhanced by SAFB1, which acts as a scaffold to stabilize the RBMX-p300 interaction.

- **Maintain heterochromatin state**: In myeloid leukemia cells, RBMX and its paralog RBMXL1 regulate the transcription of HP1α (CBX5). RBMX binds to the CBX5 promoter and maintains H3K9me3 marks at heterochromatic regions, thereby preserving genome stability and preventing aberrant expression of alternate fate genes.

- **Modulate HIV-1 latency**: RBMX maintains the trimethylation of histone H3 lysine 9 (H3K9me3) at the downstream region of the HIV-1 5' long terminal repeat (LTR), contributing to proviral latency. Knockdown of RBMX reactivates HIV-1 transcription, suggesting a role in viral persistence.

### 3.3 N6-Methyladenosine (m6A) RNA Modification

RBMX is classified as an m6A "reader" protein, recognizing N6-methyladenosine modifications on mRNA and modulating their downstream effects. m6A is the most abundant internal mRNA modification in eukaryotes and is dynamically regulated by writers (METTL3/METTL14 complex), erasers (FTO, ALKBH5), and readers (YTHDF1-3, YTHDC1-2, IGF2BP1-3, RBMX).

RBMX recognizes m6A modifications through its RRM domain, which has been shown to bind methylated RNA with higher affinity than unmethylated RNA. The functional consequences of RBMX-m6A interactions include:

- **mRNA stability regulation**: RBMX binding to m6A-modified mRNAs can either stabilize or destabilize the transcript, depending on the context and the presence of co-factors.
- **Alternative splicing regulation**: m6A modifications within pre-mRNA influence splicing decisions, and RBMX acts as a bridge between the m6A writer complex and the spliceosome.
- **Translation regulation**: RBMX can recruit translation initiation factors to m6A-modified mRNAs, enhancing their translation efficiency.

The clinical significance of RBMX as an m6A reader is evident across multiple cancer types. In oral squamous cell carcinoma (OSCC), RBMX expression correlates with poor prognosis and is part of an m6A-related gene signature that predicts patient outcomes. In esophageal cancer, RBMX expression is associated with tumor invasion, metastasis, and unfavorable prognosis. In hepatocellular carcinoma, RBMX is part of a prognostic model based on m6A regulatory factors and cancer stemness.

### 3.4 DNA Damage Response and Genome Stability

RBMX plays a critical role in maintaining genome stability through multiple mechanisms:

- **Sister chromatid cohesion**: RBMX is required for the establishment of sister chromatid cohesion during S phase. Depletion of RBMX leads to premature sister chromatid separation and chromosomal instability.

- **DNA repair**: RBMX is recruited to sites of DNA double-strand breaks (DSBs) and facilitates homologous recombination repair. This function is mediated through its interaction with the MRE11-RAD50-NBS1 (MRN) complex and the regulation of BRCA1 splicing.

- **Telomere maintenance**: RBMX binds to telomeric repeat-containing RNA (TERRA) and regulates telomere length homeostasis. Loss of RBMX results in telomere shortening and accelerated cellular senescence.

### 3.5 Regulation of Long Non-Coding RNAs (lncRNAs)

RBMX interacts with multiple lncRNAs, forming regulatory complexes that influence gene expression:

- **NORAD**: The lncRNA NORAD (non-coding RNA activated by DNA damage) assembles a topoisomerase complex that is critical for genome stability. RBMX is part of the NORAD-associated ribonucleoprotein complex, and this interaction is required for NORAD's function in maintaining genomic integrity.

- **LINC01615**: In colorectal cancer, cancer-derived exosomal LINC01615 induces M2 polarization of tumor-associated macrophages through the RBMX-EZH2 axis. LINC01615 binds to RBMX and facilitates its interaction with EZH2, leading to epigenetic reprogramming of macrophages.

- **LUCAT1**: The lncRNA LUCAT1 promotes immune gene expression in human macrophages, and RBMX is among the RNA-binding proteins that associate with LUCAT1 to mediate its effects.

### 3.6 Protein-Protein Interaction Networks

The RBMX interaction network, as curated in BioGRID and STRING databases, includes over 100 high-confidence interactors. Key functional modules within this network include:

1. **Spliceosome module**: U1-70K, U2AF65, SF3B1, PRPF8, and other core spliceosomal components.
2. **m6A modification module**: METTL3, METTL14, WTAP, YTHDC1, YTHDF2, and ALKBH5.
3. **Chromatin remodeling module**: EZH2, SUZ12, HP1α, HDAC1, and p300.
4. **DNA damage response module**: MRE11, RAD50, NBS1, BRCA1, and ATM.
5. **Transcriptional regulation module**: SREBP-1c, SAFB1, RNA Pol II, and TBP.

The connectivity of RBMX within these modules positions it as a hub protein that integrates RNA processing with chromatin state and transcriptional output.

### 3.7 Signaling Pathways in Disease Contexts

#### 3.7.1 CDH4/UBA1/RBMX Axis in Polycystic Ovary Syndrome (PCOS)

In PCOS, the CDH4/UBA1/RBMX axis promotes disease progression through YAP1 activation. RBMX is upregulated in granulosa cells of PCOS patients and interacts with UBA1 (ubiquitin-like modifier activating enzyme 1) to regulate YAP1 expression. This pathway contributes to the abnormal follicle development characteristic of PCOS.

#### 3.7.2 SOCS5-RBMX Axis in Hepatocellular Carcinoma

In steatotic HCC with HBV-related cirrhosis, the SOCS5-RBMX axis stimulates SREBP1-mediated lipogenesis to promote metastasis. SOCS5 (suppressor of cytokine signaling 5) binds to RBMX and enhances its transcriptional activity at the SREBP1 promoter, leading to increased lipid synthesis and tumor progression.

#### 3.7.3 RBMX in Renal Ischemia/Reperfusion Injury

RBMX is transcriptionally repressed by EZH2-associated H3K27me3 modification in renal ischemia/reperfusion injury. This repression attenuates pyroptosis by regulating SIRT3/NLRP3 inflammasome activation, suggesting a protective role for RBMX downregulation in acute kidney injury.

#### 3.7.4 RBMX in Dengue Virus Infection

During dengue virus infection, there is a dynamic interplay between miR-133a and RBMX. Viral infection modulates the expression of both RBMX and miR-133a, with miR-133a targeting RBMX mRNA for degradation. This interaction affects viral replication and the host antiviral response.

```mermaid
sequenceDiagram
    participant RBMX as "RBMX Protein"
    participant RNA as "Pre-mRNA Target"
    participant SPL as "Spliceosome"
    participant m6A as "m6A Writer Complex"
    participant CHR as "Chromatin Modifiers"
    participant DNA as "DNA Damage Response"
    RBMX->>RNA: Binds ESE/ISS sequences
    RBMX->>SPL: Recruits U1 snRNP
    SPL-->>RNA: Correct splicing of ultra-long exons
    RBMX->>m6A: Reads m6A modifications
    m6A-->>RNA: Regulates splicing/stability
    RBMX->>CHR: Recruits EZH2/HP1α
    CHR-->>DNA: Maintains heterochromatin
    RBMX->>DNA: Recruits MRN complex
    DNA-->>RNA: Facilitates homologous recombination
    Note over RBMX,DNA: Loss of RBMX leads to cryptic splicing, genomic instability, and disease
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Mutations and Neurodevelopmental Disorders

#### 4.1.1 Shashi X-Linked Intellectual Disability Syndrome

RBMX was identified as a candidate gene for Shashi X-linked intellectual disability syndrome, a condition characterized by moderate to severe intellectual disability, speech delay, and distinctive facial features. Whole-exome sequencing in affected families identified hemizygous loss-of-function mutations in RBMX, including nonsense mutations and frameshift deletions that result in premature termination codons and nonsense-mediated decay of the mutant transcript.

The clinical phenotype of RBMX-associated intellectual disability includes:
- Moderate to severe intellectual disability
- Delayed speech and language development
- Behavioral abnormalities, including hyperactivity and anxiety
- Mild dysmorphic facial features
- Seizures in some affected individuals

The penetrance of RBMX mutations is incomplete, and there is significant phenotypic variability even within families carrying the same mutation. This variability is attributed to the functional redundancy provided by the autosomal retrocopy RBMXL1, which can partially compensate for RBMX loss.

#### 4.1.2 Gustavson Syndrome

Gustavson syndrome is a severe X-linked neurodevelopmental disorder characterized by profound intellectual disability, epilepsy, optic atrophy, and early death. The syndrome is caused by an in-frame deletion in RBMX that removes a critical portion of the C-terminal domain, including the putative SH3 domain-binding motif. This deletion disrupts RBMX's interaction with SH3 domain-containing proteins, leading to aberrant signaling and neuronal dysfunction.

The in-frame deletion in Gustavson syndrome is located within the GAR motif (residues 300–391) and removes a PxxP motif that is predicted to bind SH3 domains. Structural modeling suggests that this deletion alters the conformation of the C-terminal domain, impairing protein-protein interactions without affecting RNA binding. This genotype-phenotype correlation highlights the importance of RBMX's protein interaction network in neurodevelopment.

#### 4.1.3 Brain Development and Neuronal Function

RBMX is essential for brain development across species. In zebrafish, RBMX is required for normal brain development, and morpholino-mediated knockdown results in severe brain malformations. In Xenopus, RBMX is necessary for neural and muscle development, with knockdown leading to defects in neural plate patterning and tadpole morphology. In mice, conditional knockout of RBMX in the brain results in cortical thinning, reduced neuronal density, and behavioral abnormalities.

The functional retrocopy RBMXL1 safeguards brain development in a species-dependent context. In humans, RBMXL1 is expressed in the developing brain and can compensate for RBMX loss, whereas in mice, RBMXL1 expression is more restricted, leading to more severe phenotypes upon RBMX deletion. This species-specific difference has important implications for modeling RBMX-associated disorders in animals.

#### 4.1.4 Amyotrophic Lateral Sclerosis (ALS)

Loss-of-function variants in RBMX have been identified in patients with amyotrophic lateral sclerosis (ALS). Whole-exome sequencing of ALS cohorts identified rare deleterious variants in RBMX, including missense mutations in the RRM domain and splice-site variants. Functional studies demonstrated that these variants impair RBMX's RNA-binding activity and lead to neuronal defects, including reduced neurite outgrowth and increased susceptibility to oxidative stress.

The role of RBMX in ALS is linked to its function as an m6A reader. m6A modifications are enriched in ALS-affected motor neurons, and dysregulation of m6A-related genes, including RBMX, contributes to disease pathogenesis. RBMX loss leads to aberrant splicing of genes involved in neuronal survival and synaptic function, including *TARDBP* (TDP-43) and *FUS*.

### 4.2 Somatic Mutations and Cancer

#### 4.2.1 Head and Neck Squamous Cell Carcinoma (HNSCC)

RBMX expression is dysregulated in head and neck cancer, including oral squamous cell carcinoma (OSCC). The splicing factor SRSF3, which is overexpressed in OSCC, regulates RBMX expression. Underexpression of both SRSF3 and RBMX predicts good prognosis in HNSCC patients, suggesting that the SRSF3-RBMX axis is an oncogenic driver.

In Chinese OSCC patients, RBMX is part of an m6A-related gene signature that predicts prognosis and disease progression. High RBMX expression is associated with advanced tumor stage, lymph node metastasis, and poor overall survival. The prognostic value of RBMX in OSCC is independent of other clinical variables, making it a potential biomarker for patient stratification.

#### 4.2.2 Acute Myeloid Leukemia (AML)

RBMX is upregulated in acute myeloid leukemia and is essential for myeloid leukemogenesis. In murine MLL-AF9-driven leukemia models, shRNA-mediated knockdown of RBMX impairs leukemia cell proliferation and induces differentiation. RBMX maintains the transcriptional activity of HP1α, preserving the heterochromatin state required for leukemia stem cell self-renewal.

RBMX also regulates YTHDC1 nuclear condensates to promote nascent transcription in AML. YTHDC1 is an m6A reader that forms phase-separated condensates at sites of active transcription. RBMX is required for the formation and maintenance of these condensates, and its loss leads to reduced transcriptional output and impaired leukemia cell growth.

#### 4.2.3 Hepatocellular Carcinoma (HCC)

RBMX plays a dual role in hepatocellular carcinoma depending on the etiological context. In steatotic HCC with HBV-related cirrhosis, the SOCS5-RBMX axis stimulates SREBP1-mediated lipogenesis to promote metastasis. High RBMX expression in this subtype is associated with poor prognosis and increased metastatic potential.

In contrast, RBMX is part of a prognostic model based on m6A regulatory factors and stemness in HCC. The model, which includes RBMX along with other m6A regulators, predicts patient outcomes and may guide therapeutic decisions. RBMX expression in HCC is also associated with immune infiltration and response to immunotherapy.

#### 4.2.4 Pancreatic Ductal Adenocarcinoma (PDAC)

Spliceosomic dysregulation in pancreatic cancer has identified RBMX as a novel candidate actionable target. RBMX is overexpressed in PDAC tissues compared to normal pancreas, and high expression correlates with poor survival. Silencing of RBMX in PDAC cell lines reduces cell proliferation and induces apoptosis, suggesting that RBMX is a potential therapeutic target.

#### 4.2.5 Breast Cancer

RBMX is expressed in mammary carcinoma and shows a positive correlation with the proapoptotic Bax gene. This correlation suggests that RBMX may have a proapoptotic function in breast cancer, potentially acting as a tumor suppressor. However, the relationship between RBMX expression and breast cancer prognosis is complex, with some studies showing favorable outcomes associated with high RBMX expression and others showing the opposite.

In triple-negative breast cancer (TNBC), RBMX has been identified as a pathogenic variant in pathway analysis of the endogamous Mizo tribe population. These variants may contribute to the high incidence of TNBC in this population.

#### 4.2.6 Colorectal Cancer

In colorectal cancer, RBMX is part of the m6A regulatory network that influences tumor subtypes and clinical outcomes. Comprehensive analysis of m6A modification factors has identified RBMX as a key player in colorectal cancer pathogenesis. Additionally, cancer-derived exosomal LINC01615 induces M2 polarization of tumor-associated macrophages via the RBMX-EZH2 axis, promoting colorectal cancer progression.

#### 4.2.7 Renal Cell Carcinoma and PEComa

A novel RBMX-TFE3 gene fusion has been identified in a highly aggressive pediatric renal PEComa (perivascular epithelioid cell tumor). This fusion results from a paracentric X chromosome inversion, inv(X)(p11;q26), and produces a chimeric protein that retains the RRM domain of RBMX fused to the activation domain of TFE3. The RBMX-TFE3 fusion acts as an oncogenic transcription factor, driving the expression of TFE3 target genes.

This fusion is diagnostically significant, as it may be missed by standard TFE3 break-apart FISH assays. The identification of RBMX-TFE3 expands the spectrum of TFE3-rearranged renal cell carcinomas and highlights the importance of comprehensive molecular testing in MiTF family-altered tumors.

### 4.3 Mutations in Reproductive Disorders

#### 4.3.1 Non-Obstructive Azoospermia (NOA)

RBMX and its autosomal homolog RBMXL9 have been investigated as candidate genes for non-obstructive azoospermia. Genetic association studies have identified polymorphisms in RBMX and RBMXL9 that are associated with NOA susceptibility, suggesting that these genes play a role in spermatogenesis. The Y-chromosome paralog RBMY, which evolved from RBMX, is a probable spermatogenesis factor, and deletions of RBMY are associated with azoospermia.

#### 4.3.2 Sperm Morphological Abnormalities

RBMX is among the genes implicated in the genetic etiology of sperm morphological abnormalities. DNA methylation patterns at the RBMX locus differ between patients with asthenospermia and healthy controls, suggesting that epigenetic dysregulation of RBMX contributes to sperm quality.

### 4.4 Mutations in Other Diseases

#### 4.4.1 Polycystic Ovary Syndrome (PCOS)

The CDH4/UBA1/RBMX axis promotes PCOS progression through YAP1 activation. RBMX is upregulated in granulosa cells of PCOS patients, and its expression correlates with disease severity.

#### 4.4.2 Asthma

RBMX is among the m6A-related genes associated with asthma. Coordinated DNA methylation and gene expression analysis identified RBMX as a critical m6A gene in asthma pathogenesis. RBMX expression is also altered in asthma, and it may serve as a biomarker for disease diagnosis and treatment response.

#### 4.4.3 Ischemic Stroke

RBMX is a significant m6A regulator in ischemic stroke, and its expression is associated with the immune microenvironment. RBMX may serve as a diagnostic biomarker for ischemic stroke.

#### 4.4.4 Spinal Cord Injury

RBMX is among the m6A-related genes that are potential therapeutic targets for spinal cord injury. Integrative network analysis has identified RBMX as a hub gene in the response to spinal cord injury. RBMX expression is also altered after spinal cord injury in rats, suggesting a role in the injury response.

#### 4.4.5 Retinal Damage

RBMX is expressed in the retina and is upregulated in response to light-induced damage in rats. This suggests a role for RBMX in the retinal stress response.

#### 4.4.6 Type 2 Diabetes Mellitus

Differences in splicing factor expression, including RBMX, may predict type 2 diabetes remission in the CORDIOPREV study. RBMX expression is associated with metabolic improvements following dietary

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