A novel Met116Thr mutation in the GDAP1 gene in a Polish family with the axonal recessive Charcot–Marie–Tooth type 4 disease

Homozygous Mutations in GDAP1 and MFN2 Genes Resulted in Autosomal Recessive Forms of Charcot-Marie-Tooth Disease in Consanguineous Pakistani Families

Charcot-Marie-Tooth disease (CMT) is a heritable neurodegenerative disease of peripheral nervous system diseases in which more than 100 genes and their mutations are associated. Two consanguineous families Dera Ghazi Khan (PAK-CMT1-DG KHAN) and Layyah (PAK-CMT2-LAYYAH) with multiple CMT-affected subjects were enrolled from Punjab province in Pakistan. Basic epidemiological data were collected for the subjects. Nerve conduction study (NCS) and electromyography (EMG) were performed for the patients. Whole-exome sequencing (WES) followed by Sanger sequencing was applied to report the genetic basic of CMT. The NCS findings revealed that sensory and motor nerve conduction velocities for both families were <38 m/s. EMG presented denervation, neuropathic motor unit potential, and reduced interference pattern of peripheral nerves. WES identified that a novel nonsense mutation (c. 226 G>T) in GADP1 gene and a previously known missense mutation in MFN2 gene (c. 334 G>A) cause CMT4A (Charcot-Marie-Tooth disease type 4A) in the PAK-CMT1-DG KHAN family and CMT2A (Charcot-Marie-Tooth disease type 2A) in the PAK-CMT2-LAYYAH family, respectively. Mutations followed Mendelian pattern with autosomal recessive mode of inheritance. Multiple sequence alignment by Clustal Omega indicated that mutation-containing domain in both genes is highly conserved, and in situ analysis revealed that both mutations are likely to be pathogenic. We reported that a novel nonsense mutation and a previously known missense mutation in GAPD1 gene and MFN2 gene, respectively, cause CMT in consanguineous Pakistani families.

CMT-linked loss-of-function mutations in GDAP1 impair store-operated Ca2+ entry-stimulated respiration

GDAP1 is an outer mitochondrial membrane protein involved in Charcot-Marie-Tooth (CMT) disease. Lack of GDAP1 gives rise to altered mitochondrial networks and endoplasmic reticulum (ER)-mitochondrial interactions resulting in a decreased ER-Ca²⁺ levels along with a defect on store-operated calcium entry (SOCE) related to a misallocation of mitochondria to subplasmalemmal sites. The defect on SOCE is mimicked by MCU silencing or mitochondrial depolarization, which prevent mitochondrial calcium uptake. Ca²⁺ release from de ER and Ca²⁺ inflow through SOCE in neuroblastoma cells result in a Ca²⁺-dependent upregulation of respiration which is blunted in GDAP1 silenced cells. Reduced SOCE in cells with CMT recessive missense mutations in the α-loop of GDAP1, but not dominant mutations, was associated with smaller SOCE-stimulated respiration. These cases of GDAP1 deficiency also resulted in a decreased ER-Ca²⁺ levels which may have pathological implications. The results suggest that CMT neurons may be under energetic constraints upon stimulation by Ca²⁺ mobilization agonists and point to a potential role of perturbed mitochondria-ER interaction related to energy metabolism in forms of CMT caused by some of the recessive or null mutations of GDAP1.

Mitochondria and peripheral neuropathies

There has been considerable progress during the past 24 years in the molecular genetics of mitochondrial DNA and related nuclear DNA mutations, and more than 100 nerve biopsies from hereditary neuropathies related to mitochondrial cytopathy have been accurately examined. Neuropathies were first reported in diseases related to point mutations of mitochondrial DNA, but they proved to be a prominent feature of the phenotype in mitochondrial disorders caused by defects in nuclear DNA, particularly in 3 genes: polymerase gamma 1 (POLG1), mitofusin 2 (MFN2), and ganglioside-induced differentiation-associated protein 1 (GDAP1). Most patients have sensory-motor neuropathy, sometimes associated with ophthalmoplegia, ataxia, seizures, parkinsonism, myopathy, or visceral disorders. Some cases are caused by consanguinity, but most are sporadic with various phenotypes mimicking a wide range of other etiologies. Histochemistry on muscle biopsy, as well as identification of crystalloid inclusions at electron microscopy, may provide a diagnostic clue to mitochondriopathy, but nerve biopsy is often less informative. Nevertheless, enlarged mitochondria containing distorted or amputated cristae are highly suggestive, particularly when located in the Schwann cell cytoplasm. Also noticeable are clusters of regenerating myelinated fibers surrounded by concentric Schwann cell processes, and such onion bulb-like formations are frequently observed in neuropathies caused by GDAP1 mutations.

The homozygous ganglioside-induced differentiation-associated protein 1 mutation c.373C > T causes a very early-onset neuropathy: case report and literature review

Mutations in the ganglioside-induced differentiation-associated protein 1 (GDAP1) gene may cause severe early-onset inherited neuropathies. Here, the authors report a clinical and neurophysiological follow-up of a Pakistani child with a very early-onset neuropathy carrying a novel homozygous mutation in the GDAP1gene. They discuss the relationship between the several forms of Charcot-Marie-Tooth disease presenting in the first months of life and focus on the literature of GDAP1-associated early-onset neuropathy. This case further expands on the clinical spectrum and the genetic heterogeneity of early-onset inherited neuropathy due to GDAP1 gene mutations.

L239F founder mutation in GDAP1 is associated with a mild Charcot-Marie-Tooth type 4C4 (CMT4C4) phenotype

Over 40 mutations in the GDAP1 gene have been shown to segregate with Charcot-Marie-Tooth disease (CMT). Among these, only two mutations, i.e., S194X and Q163X have been reported in a sufficient number of CMT families to allow for the construction of reliable phenotype-genotype correlations. Both the S194X and Q163X mutations have been shown to segregate with an early-onset and severe neuropathy resulting in loss of ambulance at the beginning of the second decade of life. In this study, we identified the L239F mutation in the GDAP1 gene in one Bulgarian and five Polish families. We hypothesized that the L239F mutation may result from a founder effect in the European population since this mutation has previously been reported in Belgian, Czech, and Polish patients. In fact, we detected a common disease-associated haplotype within the 8q13-q21 region in the Polish, German, Italian, Czech, and Bulgarian CMT families. Like the previously detected "regional" S194X and Q163X mutations, respectively present in Maghreb countries and in patients of Spanish descent, the L239F mutation seems to be the most common GDAP1 pathogenic variant in the Central and Eastern European population. Given the likely presence of a common ancestor harboring the L239F mutation, we decided to compare the phenotypes of the CMT (L239F) patients collected in this study with those of previously reported cases. In contrast to CMT4A caused by the S194X and Q163X mutations, the CMT phenotype resulting from the L239F substitution represents a milder clinical entity with a long-preserved period of ambulance at least until the end of the second decade of life.