Clinical features of homozygous FIG4-p.Ile41Thr Charcot-Marie-Tooth 4J patients

Clinical and genetic features of patients suffering from CMT4J

Mutations in the FIG4 gene have been identified in various diseases, including amyotrophic lateral sclerosis, Parkinson's disease, and Charcot-Marie-Tooth 4 J (CMT4J), with a wide range of phenotypic manifestations. We present eight cases of CMT4J patients carrying the p.Ile41Thr mutation of FIG4. The patients were categorized according to their phenotype. Six patients had a pure CMT; whereas, two patients had a CMT associated with parkinsonism. Three patients had an early onset and exhibited more severe forms of the disease. Three others experienced symptoms in their teenage years and had milder forms. Two patients had a late onset in adulthood. Four patients showed electrophysiological evidence of conduction blocks, typically associated with acquired neuropathies. Consequently, two of them received intravenous immunoglobulin treatment without a significant objective response. Interestingly, two heterozygous patients with the same mutations exhibited contrasting phenotypes, one having a severe early-onset form and the other experiencing a slow disease progression starting at the age of 49. Notably, although 7 out of 8 patients in this study were compound heterozygous for the p.Ile41Thr mutation, only one individual was found to be homozygous for this genetic variant and exhibited an early-onset, severe form of the disease. Additionally, one patient who developed the disease in his youth was also diagnosed with hereditary neuropathy with pressure palsies. Our findings provide insights into the CMT4J subtype by reporting on eight heterogeneous patient cases and highlight the potential for misdiagnosis when conduction blocks or asymmetrical nerve conduction study results are observed in patients with FIG4 mutations.

Altered phenotypes due to genetic interaction between the mouse phosphoinositide biosynthesis genes Fig4 and Pip4k2c

Loss-of-function mutations of FIG4 are responsible for neurological disorders in human and mouse that result from reduced abundance of the signaling lipid PI(3,5)P2. In contrast, loss-of-function mutations of the phosphoinositide kinase PIP4K2C result in elevated abundance of PI(3,5)P2. These opposing effects on PI(3,5)P2 suggested that we might be able to compensate for deficiency of FIG4 by reducing expression of PIP4K2C. To test this hypothesis in a whole animal model, we generated triallelic mice with genotype Fig 4-/-, Pip4k2c+/-; these mice are null for Fig 4 and haploinsufficient for Pip4k2c. The neonatal lethality of Fig 4 null mice in the C57BL/6J strain background was rescued by reduced expression of Pip4k2c. The lysosome enlargement characteristic of Fig 4 null cells was also reduced by heterozygous loss of Pip4k2c. The data demonstrate interaction between these two genes, and suggest that inhibition of the kinase PIPK4C2 could be a target for treatment of FIG4 deficiency disorders such as Charcot-Marie-Tooth Type 4J and Yunis-Varón Syndrome.

Chloroquine corrects enlarged lysosomes in FIG4 null cells and reduces neurodegeneration in Fig4 null mice

Loss-of-function mutations of FIG4 impair the biosynthesis of PI(3,5)P₂ and are responsible for rare genetic disorders including Yunis-Varón Syndrome and Charcot-Marie-Tooth Disease Type 4 J. Cultured cells deficient in FIG4 accumulate enlarged lysosomes with hyperacidic pH, due in part to impaired regulation of lysosomal ion channels and elevated intra-lysosomal osmotic pressure. We evaluated the effects of the FDA approved drug chloroquine, which is known to reduce lysosome acidity, on FIG4 deficient cell culture and on a mouse model. Chloroquine corrected the enlarged lysosomes in FIG4 null cells. In null mice, addition of chloroquine to the drinking water slowed progression of the disorder. Growth and mobility were dramatically improved during the first month of life, and spongiform degeneration of the nervous system was reduced. The median survival of Fig4 null mice was increased from 4 weeks for untreated mutants to 8 weeks with chloroquine treatment (p < 0.009). Chloroquine thus corrects the lysosomal swelling in cultured cells and ameliorates Fig4 deficiency in vivo. The improved phenotype of mice with complete loss of Fig4 suggests that chloroquine could be beneficial FIG2 in partial loss-of-function disorders such as Charcot-Marie-Tooth Type 4 J.

Roles of PIKfyve in multiple cellular pathways

Phosphoinositide signaling lipids are crucial for eukaryotes and regulate many aspects of cell function. These signaling molecules are difficult to study because they are extremely low abundance. Here, we focus on two of the lowest abundance phosphoinositides, PI(3,5)P2 and PI(5)P, which play critical roles in cellular homeostasis, membrane trafficking and transcription. Their levels are tightly regulated by a protein complex that includes PIKfyve, Fig4 and Vac14. Importantly, mutations in this complex that decrease PI(3,5)P2 and PI(5)P are linked to human diseases, especially those of the nervous system. Paradoxically, PIKfyve inhibitors which decrease PI(3,5)P2 and PI(5)P, are currently being tested for some neurodegenerative diseases, as well as other diverse diseases including some cancers, and as a treatment for SARS-CoV2 infection. A more comprehensive picture of the pathways that are regulated by PIKfyve will be critical to understand the roles of PI(3,5)P2 and PI(5)P in normal human physiology and in disease.

Clinical and Genetic Analysis of a Patient with CMT4J

We report the clinical and genetic analysis of a patient with a rare form of an autosomal recessive genetic neuropathy, Charcot Marie Tooth (CMT) disease type 4J. She presented at age 62 years with signs and symptoms consistent with a mild neuropathy. The onset of symptoms began approximately ten years earlier. Electrophysiological testing confirmed a demyelinating neuropathy and a comprehensive neuropathy screening for common causes of neuropathy was unrevealing. She underwent commercial whole exome sequencing, analyzing more than eighty genes known to cause neuropathy. Two mutations were detected, c.122T > C, p.Ile41Thr and c.2247dupC, p.Ser750GlnX10 in the FIG4 gene. The p.Ile41Thr mutation, which is paternally inherited, is a recurrent mutation reported in a number of unrelated families of European descent. The patient’s father, also of European descent, provides further evidence supporting a founder effect for this mutation. In most patients carrying the p.Ile41Thr mutation, the neuropathy, unlike our patient, is often severe with early onset. The second mutation, c.2247dupC, p.Ser750GlnX10 is maternally inherited and not previously reported. Furthermore, based upon our protein modeling analysis, c.2247dupC is disease producing, representing a novel pathogenic mutation. Our study of this patient expands the clinical and genetic spectrum of patients with CMT 4J.

Case report and literature review: Novel compound heterozygous FIG4 variants causing both of peripheral and central nervous system defects

BACKGROUND

Pathogenic variants in the FIG4 gene have been described to be associated with a diverse spectrum of syndromes, such as autosomal recessive bilateral temporooccipital polymicrogyria (OMIM 612691), autosomal dominant amyotrophic lateral sclerosis-11 (ALS11; OMIM 612577), autosomal recessive Charcot-Marie-Tooth disease, type 4J (CMT4J; OMIM 611228), and autosomal recessive Yunis-Varon syndrome (YVS; OMIM 216340). Heterozygous FIG4 variants are responsible for ALS11 characterized by progressive muscular weakness, atrophy, and bulbar palsy. CMT4J is a disorder of peripheral nervous system defects mainly presenting with a highly variable onset of proximal and/or distal muscle weakness. YVS is a disorder of severe neurological involvement with central nervous system (CNS) dysfunction and extensive skeletal anomalies.

CASE PRESENTATION

We reported two Chinese siblings born with a weakness in all limbs. They experienced rapidly progressive weakness in distal limbs. At the age of 6 years, the elder brother presented with severe scoliosis and cervical kyphosis. They both had global developmental delay and a CNS involvement with cognitive deficits and swallowing problems. Genetic screening in the patients' family for inherited diseases was recommended. Novel compound heterozygous variants in the FIG4 gene (c.2148delTinsAA and c.317A > G) were found by whole-exome sequencing in the patients. These variants were confirmed by Sanger sequencing in family members.

CONCLUSIONS

Herein, we reported two Chinese male patients with CMT4J who presented with abnormal CNS features. CMT4J with CNS involvement has been very rarely reported. We hoped this study could expand the phenotypic and genetic spectrum of FIG4-related diseases. And we helped physicians to understand the genotype-phenotype correlation.