A 63-year-old woman presented with gait disturbance, progressive hearing loss, and sensory dominant polyneuropathy. Brain MRI mainly revealed midbrain tegmental atrophy (hummingbird sign). Genetic testing identified a heterozygous DNMT1 exon 21 mutation, previously linked to autosomal dominant cerebellar ataxia, deafness, and narcolepsy (ADCA-DN), although her clinical presentation resembled hereditary sensory and autonomic neuropathy type 1E (HSAN1E). A review of previously published cases with the same mutation revealed variability in sensory involvement, cerebellar signs, and sleep disorders, supporting the existence of a wide disease spectrum of DNMT1-related disorders. This case illustrates the phenotypic and radiological overlap within DNMT1-related disorders and supports the concept of a disease spectrum, rather than discrete syndromes, highlighting the diagnostic value of combining neuroimaging with genetic analysis.

DNA methylation is essential for transcriptional regulation and the maintenance of chromosome stability, and its precise inheritance upon DNA replication is indispensable for cellular homeostasis. The DNMT1/UHRF1 complex is critical in copying DNA methylation with accessory proteins, including CDCA7 and HELLS. The DNMT1/UHRF1 complex is also crucial for maintaining DNA methylation at imprinting control regions during preimplantation development against genome-wide DNA demethylation, an essential process for early embryos to acquire totipotency. Pathogenic variants in the genes involved in the mechanism of DNA methylation maintenance result in immunodeficiency, centromeric instability, and facial anomalies (ICF) syndrome, multilocus imprinting disturbance (MLID), autosomal dominant cerebellar ataxia, deafness, and narcolepsy (ADCADN), neuropathy, hereditary sensory, type 1E (HSN1E), Kleefstra syndrome 1 (KLEFS1) and immunodeficiency 96 (IMD96). This review discusses recent progress in understanding the molecular pathogenesis of these diseases, with a particular focus on ICF syndrome and MLID.

Cerebellar ataxia-deafness-narcolepsy syndrome (ADCA-DN) is a very rare polymorphic subtype of autosomal dominant ataxia type 1 (ADCA type 1). It begins in adulthood, and may also be associated with other variable symptoms as optic atrophy, cataracts, psychosis, depression or sensory neuropathy. We present a family diagnosed from the first generation, its phenotypic description and genetic study. We describe three members of a family with ADCA sydrome. Patient II-2 started at 51 years, with ataxia, tremor, epileptic seizures, cerebellar atrophy, narcolepsy without cataplexy, hearing loss, moderate cognitive impairment. Patient III-1 started at 42 years with narcolepsy with cataplexy, hearing loss and tremor, and patient IV-1 started at 4 years, with mild intellectual disability, and narcolepsy without cataplexy. Genetic test showed a mutation in DNMT1 gene: variant c.1709 C > T; p.Ala570Val in the DNMT1 gene. ADCA syndrome has a variable phenotype in a same family. In our experience, this type of ataxia develops narcolepsy as the first sympton in all three cases, with tremor and cognitive impairment, together with tremor and cognitive impairment. Ataxia, despite being a cardinal symptom, does not appear at the onset in younger patients, and hearing loss also seems to develop over the years. Sensory neuropathy is not present in any of the cases studied.

Hereditary sensory neuropathy (HSN) 1E is a neurodegenerative disorder caused by pathogenic variants in DNA methyltransferase 1 (DNMT1). It is characterised by sensorineural deafness, sensory neuropathy and cognitive decline. Variants in DNMT1 are also associated with autosomal dominant cerebellar ataxia, deafness and narcolepsy. A 42-year-old man presented with imbalance, lancinating pain, numerous paucisymptomatic injuries, progressive deafness since his mid-20s, mild cognitive decline and apathy. Examination revealed abnormalities of eye movements, distal sensory loss to all modalities, areflexia without weakness and lower limb ataxia. MRI brain and FDG-PET scan demonstrated biparietal and cerebellar atrophy/hypometabolism. Whole exome sequencing detected a heterozygous likely pathogenic missense variant in DNMT1, c.1289G > A, p.Cys430Tyr. Cochlear implant was performed at 44 years for the bilateral high frequency sensorineural hearing loss with improvement in hearing and day-to-day function. We describe a novel variant in DNMT1 and confirm that an overlapping HSN1E-cerebellar phenotype can occur. Only one prior case of cochlear implant in HSN1E has been reported to date but this case adds to that literature, suggesting that cochlear implant can be successful in such patients. We further explore the clinical and radiological signature of the cognitive syndrome associated with this disorder.

DNAJC3, a co-chaperone of BiP, is a member of the heat shock protein family. These proteins are produced in the endoplasmic reticulum (ER) to counter cell stress resulting from healthy functional protein processing. Dysregulation of unfolded proteins within the ER is implicated as a mechanism of genetic disease. Examples include Marinesco-Sjogren and Wolcott-Rallison syndromes that share similar clinical features, manifesting neurodegenerative disease and endocrine dysfunction. Recently, loss of function mutations in DNAJC3 was associated with syndromic diabetes mellitus in three families. The full phenotype included neurodegeneration, ataxia, deafness, neuropathy, adolescent-onset diabetes mellitus, growth hormone deficiency and hypothyroidism. A subsequent report of two unrelated individuals extended the phenotype to include early-onset hyperinsulinaemic hypoglycaemia. Here, we describe two siblings that recapitulate this extended phenotype in association with a homozygous novel mutation in the final exon of DNAJC3 [c.1367_1370delAGAA (p.Lys456SerfsTer85)] resulting in protein elongation predicted to abrogate the functional J domain. This report confirms DNAJC3 as a cause of syndromic congenital hyperinsulinaemic hypoglycaemia. Currently, PanelApp only includes this gene on diabetes mellitus panels. We propose DNAJC3 should be promoted from a red to a green gene on a wider number of panels to improve the diagnosis of this rare condition.