Infantile nystagmus syndrome (INS) refers to congenital forms of nystagmus that are present at birth or during infancy. This syndrome may be caused by afferent visual system disorders or abnormal development of the ocular motor system. INS is a genetically heterogeneous disorder for which there are more than 100 causative genes. Since applying clinical tests for the differential diagnosis of INS can be challenging in early infancy and children, genetic testings such as next-generation sequencing are becoming more important for achieving accurate diagnoses. An improved understanding of the molecular mechanisms of INS may also lead to the development of gene-based therapies for INS. These advantages of genetic testing have the potential to change the diagnostic paradigm of patients with INS. However, the diagnostic pathway based on genetic testing still has several limitations in terms of the therapeutic effect and methodology. This review summarizes genetic and clinical features of INS, and discusses the promise and pitfalls of genetic testing in INS.

Despite advances in the diagnosis and management of rare diseases (RDs), there remains a tendency to overlook adult RD patients. In addition to the considerable number of adult-onset RDs, advances in the diagnosis and management of pediatric RDs have led to an increase in the survival of these patients into adulthood. Adult RDs exhibit distinct features from pediatric counterparts, necessitating careful consideration during medical assessments. Given the extended life expectancy of adult RD patients, precise diagnosis and management strategies can significantly enhance patient outcomes. This review aims to provide an in-depth exploration of the characteristics unique to adult RDs. Special emphasis will be placed on the importance of cascade screening and prenatal genetic testing in the context of adult RDs, highlighting the need for a comprehensive understanding of these aspects in clinical practice.

Purpose: Neurofibromatosis 1 (NF1) is one of the most common autosomal dominant diseases caused by heterozygous mutation in the NF1 gene. Mutation detection is complex owing to the large size of the NF1 gene, the presence of a high number of partial pseudogenes, and the great variety of mutations. We aimed to study the mutation spectrum of NF1 gene in Korean patients with NF1. Materials and Methods: We have analyzed total 69 unrelated patients who were clinically diagnosed with NF1. PCR and sequencing of the NF1 gene was performed in all unrelated index patients. Additionally, multiplex ligation-dependent probe amplification (MLPA) test of the NF1 and SPRED1 gene analysis (sequencing and MLPA test) were performed in patients with negative results from NF1 gene sequencing analysis. Results: Fifty-five different variants were identified in 60 individuals, including six novel variants. The mutations included 36 single base substitutions (15 missense and 21 nonsense), eight splicing mutations, 13 small insertion or deletions, and three gross deletions. Most pathogenic variants were unique. The mutations were evenly distributed across exon one through 58 of NF1, and no mutational hot spots were found. When fulfilling the National Institutes of Health criterion for the clinical diagnosis of NF1, the detection rate was 84.1%. Cafe-au-lait macules were observed in all patients with NF1 mutations. There is no clear relationship between specific mutations and clinical features. Conclusion This study revealed a wide spectrum and genetic basis of patients with NF1 in Korea. Our results aim to contribute genetic management and counseling.

Rare diseases are characterized by a low prevalence, which often means that patients with such diseases are undiagnosed and do not have effective treatment options. Neurodevelopmental and neurological disorders make up around 40% of rare diseases and in the past decade, there has been a surge in the identification of genes linked to these conditions. This has created the need for model organisms to reveal mechanisms and to assess therapeutic methods. Different model animals have been employed, like Caenorhabditis elegans, Drosophila, zebrafish, and mice, to investigate the rare neurological diseases and to identify the causative genes. While the zebrafish has become a popular animal model in the last decade, mainly for studying brain development, understanding neural circuits, and conducting chemical screens, the mouse has been a very well-known model for decades. This review explores the strengths and limitations of using zebrafish as a vertebrate animal model for rare neurological disorders, emphasizing the features that make this animal model promising for the research on these disorders.

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episode (MELAS) syndrome is a maternally inherited mitochondrial disorder that usually affects the cerebral cortex and prevents high-energy demands from being met. Herein, we present the case of a male patient who rapidly developed multiple seizures, headaches, and altered mentality accompanied by severe metabolic acidosis and lactic acidosis. Initially, a brain imaging study confirmed stroke-like lesions (SLLs) only in the cerebellum. During follow-up, newly developed SLLs with lactic acidosis were observed in the basal ganglia (BG), cerebellum, and occipital lobe. The m.3243A>G variant had been found in the patient and MELAS was diagnosed, despite the BG and cerebellum being atypical locations for SLLs in MELAS. Since most cases of m.3243A>G variant MELAS show SLLs in the cerebral cortex, this case is unusual considering the location of the lesion. We emphasize that in the case of lactic acidosis accompanied by neurological symptoms, such as seizures, as in this case, MELAS should be included in the differential diagnosis, even if SLLs are observed in areas other than the cerebral cortex.

Allan-Herndon-Dudley syndrome (AHDS) is a rare X-linked neurodevelopmental disorder with abnormal thyroid function caused by mutation in the solute carrier family 16 member 2 (SLC16A2) gene. Clinical manifestations of AHDS are global or axial hypotonia, a variety of movement disorders, severe intellectual disability, quadriplegia or spastic diplegia, growth failure, and seizures. A 10-year-old boy visited our hospital with the chief complaint of newly onset generalized tonic seizures with vocalization of weekly to daily frequency. He showed early infantile hypotonia, severe intellectual disability, and frequent respiratory infections. He could not walk independently and was non-verbal. Electroencephalogram revealed generalized slow spike and waves with multifocal spikes and slow background rhythms. His tonic seizures were controlled with more than two anti-seizure medications (ASMs). At 11 years of age, he was evaluated for thyroid function as part of regular screening for ASM maintenance and was found to have abnormal thyroid function. We performed whole exome sequencing for severe global developmental delay, drug-resistant epilepsy, and abnormal thyroid function. The hemizygous c.940C>T (p.Arg314Ter) variant in the SLC16A2 gene (NM_006517.5) was identified and confirmed based on Sanger sequencing. Herein, we describe a case of an AHDS patient with late-onset drug-resistant epilepsy combined with congenital hypotonia, global developmental delay, and abnormal thyroid function results. To the best of our knowledge, this is the oldest adolescent among AHDS cases reported in Korea. In this report, clinical characteristics of a mid-adolescence patient with AHDS were presented.

The CYP11A1 gene encodes for the cholesterol side-chain cleavage enzyme (P450scc), which initiates steroid hormone biosynthesis. Defective P450scc activity results in severe glucocorticoid and mineralocorticoid deficiencies. We describe a case of P450scc deficiency due to a novel homozygous CYP11A1 variant inherited from the mother with a possibility of uniparental disomy (UPD). The patient was a female, had no family history of endocrine disease, and showed adrenal insufficiency at 13 days of age. Hormonal analysis with an adrenocorticotropic hormone stimulation test showed both glucocorticoid and mineralocorticoid deficiencies, presumed to be a defect of the early stage of steroidogenesis. Exome sequencing reported a novel homozygous frameshift variant of CYP11A1 (c.284_285del, p.Asn95Serfs*10), which was inherited from the mother.Additionally, homozygosity in 15q22.31q26.2, which included CYP11A1, was identified using a chromosomal microarray. It was suggested that the possibility of maternal UPD was involved as the cause of a P450scc deficiency by unmasking the maternally derived affected allele. To our understanding, P450scc deficiency associated with UPD encompassing CYP11A1 had not been reported in Korea before. Genetic analysis can help diagnose rare causes of primary adrenal insufficiency, including P450scc deficiency.

Monogenic disorders are traditionally attributed to the presence of mutations in a single gene. However, recent advancements in genomics have revealed instances where the phenotypic expression of apparently monogenic disorders cannot be fully explained by mutations in a single gene alone. This review article aims to explore the emerging concept of digenic or oligogenic inheritance in seemingly monogenic disorders. We discuss the underlying mechanisms, clinical implications, and the challenges associated with deciphering the contribution of multiple genes in the development and manifestation of such disorders. We present relevant studies and highlight the importance of adopting a broader genetic approach in understanding the complex genetic architecture of these conditions.

Fabry disease (FD), a rare X-linked lysosomal storage disorder, is caused by mutations in the α-galactosidase A gene gene encoding α-galactosidase A (α-Gal A). The functional deficiency of α-Gal A results in progressive accumulation of neutral glycosphingolipids, causing multi-organ damages including cardiac, renal, cerebrovascular systems. The current treatment is comprised of enzyme replacement therapy (ERT), oral pharmacological chaperone therapy and adjunctive supportive therapy. ERT has been introduced 20 years ago, changing the outcome of FD patients with proven effectiveness. However, FD patients have many unmet needs. ERT needs a life-long intravenous therapy, inefficient bio-distribution, and generation of anti-drug antibodies. Migalastat, a pharmacological chaperone, augmenting α-Gal A enzyme activity only in patients with mutations amenable to the therapy, is now available for clinical practice. Furthermore, these therapies should be initiated before the organ damage becomes irreversible. Development of novel drugs aim at improving the clinical effectiveness and convenience of therapy. Clinical trial of next generation ERT is underway. Polyethylene glycolylated enzyme has a longer halflife and potentially reduced antigenicity, compared with standard preparations with longer dosing interval. Moss-derived enzyme has a higher affinity for mannose receptors, and seems to have more efficient access to podocytes of kidney which is relatively resistant to reach by conventional ERT. Substrate reduction therapy is currently under clinical trial. Gene therapy has now been started in several clinical trials using in vivo and ex vivo technologies. Early results are emerging. Other strategic approaches at preclinical research level are stem cell-based therapy with genome editing and systemic mRNA therapy.

Until now, rare disease studies have mainly been carried out by detecting simple variants such as single nucleotide substitutions and short insertions and deletions in protein-coding regions of disease-associated gene panels using diagnostic nextgeneration sequencing in association with patient phenotypes. However, several recent studies reported that the detection rate hardly exceeds 50% even when whole-exome sequencing is applied. Therefore, the necessity of introducing wholegenome sequencing is emerging to discover more diverse genomic variants and examine their association with rare diseases.When no diagnosis is provided by whole-genome sequencing, additional omics techniques such as RNA-seq also can be considered to further interrogate causal variants. This paper will introduce a description of these multi-omics techniques and their applications in rare disease studies.