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FB2026_01
,
released March 12, 2026
This report describes inclusion body myopathy with early-onset Paget disease with or without frontotemporal dementia 2 (IBMPFD2), which is a subtype of inclusion body myopathy with Paget disease of bone and frontotemporal dementia (IBMPFD); IBMPFD2 exhibits autosomal dominant inheritance. The human gene implicated in this disease is HNRNPA2B1, which encodes two proteins through alternative splicing: heterogeneous nuclear ribonucleoprotein A2 and heterogeneous nuclear ribonucleoprotein B1. There are two high-scoring fly orthologs: Hrb98DE, for which RNAi targeting constructs, alleles caused by insertional mutagenesis, and classical amorphic alleles have been generated; and Hrb87F, for which RNAi targeting constructs and classical amorphic alleles have been generated. Only Hrb98DE has been analyzed in the context of IBMPFD in flies. An orthologous gene in human, HNRNPA1, is implicated in inclusion body myopathy with early-onset Paget disease without frontotemporal dementia 3 (IBMPFD2) and amyotrophic lateral sclerosis 20 (ALS20); see FBhh0000299.
Multiple UAS constructs of the human Hsap\HNRNPA2B1 gene have been introduced into flies, including wild-type HNRNPA2B1 and a gene carrying a mutational lesion implicated in IBMPFD2. Transgenic expression of mutant constructs recapitulates some of the key phenotypes of IBMPFD, including degeneration of muscle fibers and the formation of cytoplasmic inclusions of mutant protein. Heterologous rescue of mutations in the Drosophila ortholog by expression of HNRNPA2B1 has not been tested.
A variant implicated in IBMPFD2 has been assessed using a transgenic construct of the human gene and as the analogous mutation in the fly gene; see the 'Disease-Implicated Variants' table below.
A UAS construct of Dmel\Hrb98DE bearing a mutation that corresponds to both the mutational lesion in HNRNPA1 that is implicated in IBMPFD3 and the mutational lesion in HNRNPA2B1 that is implicated in IBMPFD2 has been introduced into flies; it recapitulates some of the key phenotypes of ALS20 and IBMPFD, including degeneration of muscle fibers and the formation of cytoplasmic inclusions of mutant protein.
[updated Jul. 2021 by FlyBase; FBrf0222196]
Inclusion body myopathy associated with Paget disease of bone (PDB) and/or frontotemporal dementia (IBMPFD) is characterized by adult-onset proximal and distal muscle weakness (clinically resembling a limb-girdle muscular dystrophy syndrome), early-onset PDB, and premature frontotemporal dementia (FTD). Muscle weakness progresses to involve other limb and respiratory muscles. Cardiac failure and cardiomyopathy have been observed in later stages. PDB involves focal areas of increased bone turnover that typically lead to spine and/or hip pain and localized enlargement and deformity of the long bones; pathologic fractures occur on occasion. Early stages of FTD are characterized by dysnomia, dyscalculia, comprehension deficits, paraphasic errors, and relative preservation of memory, and later stages by inability to speak, auditory comprehension deficits for even one-step commands, alexia, and agraphia. Mean age at diagnosis for muscle disease and PDB is 42 years; for FTD, 55 years. [from GeneReviews, Inclusion Body Myopathy with Paget Disease of Bone and/or Frontotemporal Dementia, pubmed:20301649 2016.06.03]
IBMPFD is a autosomal dominant disorder linked to multiple genes, characterized by incomplete penetrance of 3 main features: disabling muscle weakness (in 90%), osteolytic bone lesions consistent with Paget disease (in 51%), and frontotemporal dementia (in 32%). Muscle weakness is an isolated symptom in about 30% of patients and the presenting symptom in greater than half of patients, suggesting that IBMPFD may commonly be seen in a neuromuscular clinic without its other syndromic features (review by Weihl et al., 2009, pubmed:19380227). [From MIM:167320, 2016.06.03]
[INCLUSION BODY MYOPATHY WITH EARLY-ONSET PAGET DISEASE WITH OR WITHOUT FRONTOTEMPORAL DEMENTIA 2; IBMPFD2](https://omim.org/entry/615422)
[HETEROGENEOUS NUCLEAR RIBONUCLEOPROTEIN A2/B1; HNRNPA2B1](https://omim.org/entry/600124)
IBMPFD2 has been reported in a family in which autosomal dominant early-onset Paget disease of bone was associated with a scapuloperoneal type of muscular dystrophy (SPMD). Muscle histology was nonspecific. Creatine phosphokinase (CPK) levels were elevated in active forms of the disorder. Paget disease of bone in this family presented primarily in the long bones and progressed to the spine and other bones (Waggoner et al., 2002, pubmed:11891683). A subsequent study of this family, patients manifested dominantly inherited degeneration affecting muscle, bone, brain, and motor neurons that was clinically indistinguishable from IBMPFD caused by mutation in the VCP gene (MIM:601023). The father in this pedigree had behavioral changes, weakness, muscle atrophy, and progressive skeletal abnormalities. At autopsy he was diagnosed with frontotemporal dementia, inclusion body myopathy, and Paget disease of the bone. He had eight children, four of whom were affected. All had myopathy and Paget disease of the bone, with onset of slowly progressive weakness and skeletal abnormalities in their twenties. Two had cognitive impairment and two had motor neuron dysfunction (Kim, et al., 2013, pubmed:23455423). [From MIM:615422, 2016.06.01]
IBMPFD2 is caused by heterozygous mutation in the HNRNPA2B1 gene, which encodes two proteins, HNRNPA2 and HNRNPB1, through alternative splicing. [From MIM:615422, 2016.06.01]
Muscle biopsies from an affected patient showed nuclear clearance and cytoplasmic inclusions of HNRNPA2B1 protein in 10% of muscle fibres.
Muscle biopsies from a patient with IBMFD2 manifesting disease in all organ systems showed atrophic fibers, central nuclei, and rimmed vacuoles characteristic of inclusion body myopathy. Whereas in normal muscle HNRNPA2B1 expression is exclusively nuclear, analysis of muscle tissue from this patient showed that the HNRNPA2B1 cleared from many nuclei and accumulated in cytoplasmic inclusions in approximately 10% of fibers. This patient also showed TDP43 (MIM:605078) pathology consisting of nuclear clearance and cytoplasmic inclusions, consistent with observations in VCP-related and sporadic inclusion body myopathy.(Kim, et al., 2013, pubmed:23455423).[From MIM:615422, 2016.06.01]
Disease-associated mutations are localized to the prion-like domains (PrLD) of HNRNPA1 and HNRNPA2B1. In vitro, disease-associated mutations greatly accelerate HNRNPA1 and HNRNPA2B1 fibrillization, and directly promote nucleation of wild-type HNRNPA1 and HNRNPA2B1 into fibrils.
HNRNPA2B1 belongs to the A/B subfamily of ubiquitously expressed heterogeneous nuclear ribonucleoproteins (hnRNPs). The hnRNPs are RNA binding proteins and they complex with heterogeneous nuclear RNA (hnRNA). These proteins are associated with pre-mRNAs in the nucleus and appear to influence pre-mRNA processing and other aspects of mRNA metabolism and transport. While all of the hnRNPs are present in the nucleus, some seem to shuttle between the nucleus and the cytoplasm. The hnRNP proteins have distinct nucleic acid binding properties. The protein encoded by this gene has two repeats of quasi-RRM domains that bind to RNAs. This gene has been described to generate two alternatively spliced transcript variants which encode different isoforms. [provided by RefSeq, Jul 2008]
The HNRNPA2B1 gene encodes two major proteins, HNRNPA2 and HNRNPB1, through alternative splicing. HNRNPA/B proteins, such as HNRNPA2 and HNRNPB1, are involved in packaging nascent mRNA, in alternative splicing, and in cytoplasmic RNA trafficking, translation, and stabilization. HNRNPA2 and HNRNPB1 also appear to function in telomere maintenance, cell proliferation and differentiation, and glucose transport (Moran-Jones et al., 2005, pubmed:15659580; Iwanaga et al., 2005, pubmed:15964549). [From MIM:600124, 2016.06.01]
Many to many: 5 human to 4 Drosophila.
Ortholog of human HNRNPA2B1, HNRNPA3, HNRNPA1, HNRNPA1L2, and HNRNPA0 (4 Drosophila to 5 human; additional more distantly related gene(s) in both species). Dmel\Hrb98DE shares 50% identity and 64% similarity with human HNRNPA2B1, 50% identity and 62% similarity with human HNRNPA3, 49% identity and 63% similarity with human HNRNPA1L2, 48% identity and 64% similarity with human HNRNPA1, and 44% identity and 59% similarity with human HNRNPA0.