Recent advances in CGG repeat diseases and a proposal of fragile X-associated tremor/ataxia syndrome, neuronal intranuclear inclusion disease, and oculophryngodistal myopathy (FNOP) spectrum disorder

Advances on the Mechanisms and Therapeutic Strategies in Non-coding CGG Repeat Expansion Diseases

Non-coding CGG repeat expansions within the 5' untranslated region are implicated in a range of neurological disorders, including fragile X-associated tremor/ataxia syndrome, oculopharyngeal myopathy with leukodystrophy, and oculopharyngodistal myopathy. This review outlined the general characteristics of diseases associated with non-coding CGG repeat expansions, detailing their clinical manifestations and neuroimaging patterns, which often overlap and indicate shared pathophysiological traits. We summarized the underlying molecular mechanisms of these disorders, providing new insights into the roles that DNA, RNA, and toxic proteins play. Understanding these mechanisms is crucial for the development of targeted therapeutic strategies. These strategies include a range of approaches, such as antisense oligonucleotides, RNA interference, genomic DNA editing, small molecule interventions, and other treatments aimed at correcting the dysregulated processes inherent in these disorders. A deeper understanding of the shared mechanisms among non-coding CGG repeat expansion disorders may hold the potential to catalyze the development of innovative therapies, ultimately offering relief to individuals grappling with these debilitating neurological conditions.

Sequential development of parkinsonism in two patients with oculopharyngodistal type myopathy in GIPC1-related repeat expansion disorder

A heterozygous CGG repeat expansion in 5' untranslated region (5' UTR) of GIPC1 is one of the causative factors of oculopharyngodistal myopathy (OPDM), an adult-onset hereditary muscle disease characterized by progressive ptosis, ophthalmoplegia, and facial, bulbar, and distal limb muscle weakness. CGG repeat expansion in GIPC1 has also been reported to be associated with Parkinson's disease, but these patients did not exhibit myopathic symptoms. We experienced two unrelated cases of oculopharyngeal type myopathy with CGG repeat expansion in GIPC1 presenting parkinsonism after exhibiting myopathic symptoms. Both cases showed p62-positive intranuclear inclusions in the skin, similar to those in NOTCH2NLC-related disorders. Our cases suggest that GIPC1-related repeat expansions may be associated with a broad spectrum and tissue-differential neuromuscular manifestations, indicating a common mechanism between OPDM2 and other CGG-repeat expansion diseases. It is important to note OPDM2 patients' central neurological symptoms, as myopathic symptoms may obscure central nervous system manifestations.

The predominance of "astrocytic" intranuclear inclusions in neuronal intranuclear inclusion disease manifesting encephalopathy-like symptoms: A case series with brain biopsy

Neuronal intranuclear inclusion disease (NIID) is a neurodegenerative disorder represented by eosinophilic intranuclear inclusions (EIIs) and GGC/CGG repeat expansion in the NOTCH2NLC gene. We report here two adult cases of NIID, genetically confirmed, with manifestation of encephalopathy-like symptoms and address the histopathologic findings obtained by brain biopsies, with a focus on "astrocytic" intranuclear inclusions (AIIs). Case 1 presented with paroxysmal restlessness, vertigo, or fever and was later involved in severe dementia and tetraparesis. Case 2 presented with forgetfulness and then with paroxysmal fever and headache. In both cases, delimited areas with gadolinium enhancement on magnetic resonance imaging and corresponding hyperperfusion were detected, leading to brain biopsies of the cortex. On histology, Case 1 showed an abnormal lamination, where the thickness of layers was different from usual. Both neurons and astrocytes showed some dysmorphologic features. Notably, astrocytes rather than neurons harbored EIIs. Case 2 showed a cortex, where neurons tended to be arrayed in a columnar fashion. Astrocytes showed some dysmorphologic features. Notably, much more astrocytes than neurons harbored EIIs. By a double-labeling immunofluorescence study for p62/NeuN and p62/glial fibrillary acidic protein, the predominance of AIIs was confirmed in both cases. Considering the physiological functions of astrocytes for the development and maintenance of the cortex, the encephalopathy-like symptoms, dynamic change of cerebral blood flow, and cortical dysmorphology can reasonably be explained by the dysfunction of EII-bearing astrocytes rather than EII-bearing neurons. This study suggests the presence of a subtype of NIID where AIIs rather than "neuronal" intranuclear inclusions are likely a key player in the pathogenesis of NIID, particularly in cases with encephalopathy-like symptoms. The importance of AIIs ("gliopathy") should be more appreciated in future studies of NIID.

Neurological disorders caused by novel non-coding repeat expansions: clinical features and differential diagnosis

Nucleotide repeat expansions in the human genome are a well-known cause of neurological disease. In the past decade, advances in DNA sequencing technologies have led to a better understanding of the role of non-coding DNA, that is, the DNA that is not transcribed into proteins. These techniques have also enabled the identification of pathogenic non-coding repeat expansions that cause neurological disorders. Mounting evidence shows that adult patients with familial or sporadic presentations of epilepsy, cognitive dysfunction, myopathy, neuropathy, ataxia, or movement disorders can be carriers of non-coding repeat expansions. The description of the clinical, epidemiological, and molecular features of these recently identified non-coding repeat expansion disorders should guide clinicians in the diagnosis and management of these patients, and help in the genetic counselling for patients and their families.

A GGC-repeat expansion in ZFHX3 encoding polyglycine causes spinocerebellar ataxia type 4 and impairs autophagy

Despite linkage to chromosome 16q in 1996, the mutation causing spinocerebellar ataxia type 4 (SCA4), a late-onset sensory and cerebellar ataxia, remained unknown. Here, using long-read single-strand whole-genome sequencing (LR-GS), we identified a heterozygous GGC-repeat expansion in a large Utah pedigree encoding polyglycine (polyG) in zinc finger homeobox protein 3 (ZFHX3), also known as AT-binding transcription factor 1 (ATBF1). We queried 6,495 genome sequencing datasets and identified the repeat expansion in seven additional pedigrees. Ultrarare DNA variants near the repeat expansion indicate a common distant founder event in Sweden. Intranuclear ZFHX3-p62-ubiquitin aggregates were abundant in SCA4 basis pontis neurons. In fibroblasts and induced pluripotent stem cells, the GGC expansion led to increased ZFHX3 protein levels and abnormal autophagy, which were normalized with small interfering RNA-mediated ZFHX3 knockdown in both cell types. Improving autophagy points to a therapeutic avenue for this novel polyG disease. The coding GGC-repeat expansion in an extremely G+C-rich region was not detectable by short-read whole-exome sequencing, which demonstrates the power of LR-GS for variant discovery.