Distribution and genotype-phenotype correlation of GDAP1 mutations in Spain

Mutations in the GDAP1 gene can cause Charcot-Marie-Tooth disease. These mutations are quite rare in most Western countries but not so in certain regions of Spain or other Mediterranean countries. This cross-sectional retrospective multicenter study analyzed the clinical and genetic characteristics of patients with GDAP1 mutations across Spain. 99 patients were identified, which were distributed across most of Spain, but especially in the Northwest and Mediterranean regions. The most common genotypes were p.R120W (in 81% of patients with autosomal dominant inheritance) and p.Q163X (in 73% of autosomal recessive patients). Patients with recessively inherited mutations had a more severe phenotype, and certain clinical features, like dysphonia or respiratory dysfunction, were exclusively detected in this group. Dominantly inherited mutations had prominent clinical variability regarding severity, including 29% of patients who were asymptomatic. There were minor clinical differences between patients harboring specific mutations but not when grouped according to localization or type of mutation. This is the largest clinical series to date of patients with GDAP1 mutations, and it contributes to define the genetic distribution and genotype-phenotype correlation in this rare form of CMT.


Genotype. The genetic information is recorded in
. Thirteen different mutations were detected, 3 inherited in an autosomal dominant fashion and the other 10 were recessive. All had been previously reported. The two most common mutations were AD p.R120W, and AR p.Q163X. The type of mutations was quite diverse including 6 missense, 4 nonsense, 2 frameshift mutations and one splice-site variant. They were localized along exons 2-6 and affected the 2 GST domains and the α-loop predominantly. A schematic diagram of the GDAP1 protein and the mutations found in this series is reproduced in Fig. 1.
The AD p.R120W was detected predominantly in families near the Mediterranean coast, while p.R226del was found only in two families in the region of Galicia, in the Northwest of Spain (Fig. 2). The AR patients were distributed throughout Spain, but there existed a cluster of affected individuals in the North of Spain, especially in the Basque country and neighboring regions.
Clinical characteristics. The clinical features of the series are summarized in Table 2. Patients were separated between AD and AR inherited mutations because all the severity scores and certain clinical features were different between the groups.
Patients with AD inherited mutations had a mild-moderate neuropathy with variable age of onset (4-65 years). The first symptoms most commonly reported were distal lower leg weakness and cramps (83%). Foot deformities (pes cavus, Achilles tendon contracture or hammer toes) were quite common (85.2%), but only one patient presented scoliosis and none dysphonia or respiratory failure. Severity scores were usually between the mild-moderate range, and 28.6% considered themselves asymptomatic. There was important clinical variability regarding severity scores even between family members as can be seen in the pedigree of Fig. 3. Three patients died during follow-up, but the cause of death was not related with the neuropathy.
Patients with AR inherited mutations suffered a severe early onset neuropathy (0-12 years) causing a great disability. Most of the patients were wheelchair-bound before age 20, and only three patients older than 40 years remained ambulant. Dysphonia was present in more than half of the AR patients and appeared usually in the second decade (9-35 years). Respiratory dysfunction was also quite common, and 19.5% of the patients required non-invasive ventilation (29-47 years). Intra-familiar variability was not prominent, although most were sporadic patients. Six patients died, two because of cardiovascular events, one of hepatic cirrhosis, and three due to infections that could be related to the severe disability inherent to the disease course. Nerve conduction studies. The electrophysiological findings are detailed in Table 3. In AD patients there was a decrease of CMAP and SNAP amplitudes with normal conduction velocities, corresponding to a mild axonal motor and sensory neuropathy. Of the 12 clinically asymptomatic patients that underwent nerve conduction studies, in 8 we detected a decrease in SNAP of the sural nerve and/or CMAP of the peroneal nerve. The other 4 asymptomatic patients had normal nerve conduction studies, but 3 of them had detectable abnormalities in needle electromyography (large polyphasic motor unit action potentials in distal lower limb muscles with no spontaneous muscular activity), while it was not performed in the other patient.
In AR patients the decrease in CMAP and SNAP was much more marked, as were the number of nerves that were unexcitable when explored. These findings were more severe in the lower limb nerves; in fact all patients over 25 years had unexcitable sural and peroneal nerves. Conduction velocities were generally not reduced except in nerves with very low amplitudes (Fig. 4b). In these cases, when conduction velocity was measured to proximal muscles, it was always within the normal range.
Muscle magnetic resonance imaging. Lower limb muscle magnetic resonance imaging findings are detailed in Table 4, and representative images are shown in the supplementary Figure S1. It was performed in 22 patients, 19 of them harboring the AD p.R120W mutation. In those patients the findings were in concordance with that previously reported including fatty infiltration with a distal to proximal gradient 16 . In 3/5 clinically asymptomatic patients, there was detectable fatty substitution in the intrinsic foot muscles and in 2/5 also in the   muscles of the calf. In the 2 asymptomatic patients with normal muscle MRI, the muscles of the feet were not studied. In the calf, the muscles of the posterior compartment (soleus > gastrocnemius) were affected earlier and more severely than those of the anterolateral compartment. MRI was performed in two patients with the AD p.R226del mutation. In one symptomatic patient there were detectable abnormalities consisting in fatty infiltration with a distal to proximal gradient, and the same posterior > anterolateral pattern in the calf. The other was asymptomatic, although minor abnormalities could be detected in the clinical examination. The muscle MRI in this patient was normal in the calf and thigh muscles, but the feet were not studied.    One patient with an AR p.Q163X/p.L344R mutation also underwent muscle MRI testing. He was the AR patient with the less severe phenotype (CMT neuropathy score -CMTNS: 12, Functional disability score -FDS: 2, age 49 at the time of the MRI) and there was complete fatty substitution of the muscles in the feet and posterior compartment of the calf, but in the anterolateral muscles of the calf and the thigh muscles, fatty infiltration was only partial.
Nerve pathology. Sural nerve biopsy was performed in 15 patients, 8 with AR and 7 with AD inherited mutations. Representative images are shown in the supplementary Figure S1. The pathologic characteristics of 7 of these patients had been previously been reported, and are similar to the other 8 patients 1,16,18 . The main findings were concordant with an axonal neuropathy (depletion of myelinated fibers, myelin thinning, and rather frequent regenerative clusters) with minor myelin abnormalities (abnormal myelin foldings and occasional onion bulb-like formations). The fiber loss was especially prominent in AR patients.
Genotype-phenotype correlation. The most important genetic factor that influenced the phenotype was the mode of inheritance. Certain clinical features like the presence of dysphonia and respiratory dysfunction were only present in patients with AR inheritance and scoliosis was more frequent in patients with AR inheritance (p < 0.001) ( Table 2). In this group, 75% of the patients were wheelchair-bound at a median age of 15 years. By contrast none in the group of AD inheritance lost ambulation with a median follow-up of nearly 7 years. The presence of any kind of neuropathic symptoms was more frequent in patients with AR inheritance, as were higher severity scores in any of the scales employed (p < 0.05). In Fig. 4a  Regarding the patients with AD inheritance, we detected minor clinical differences between the two most frequent mutations. The members of the two families with the p.R226del mutation had a slightly milder disease course. Also, in the p.R120W mutation ankle extensors and flexors were affected to a similar degree (3.9 and 3.7 mean MRC scores respectively) while in those with the p.R226del mutation weakness was more marked in ankle extension (4.3 and 4.7 mean MRC scores).
In the patients with AR inherited mutations no clinical differences could be detected when grouping patients according to localization of the mutation or mutation type. Neither could we detect any statistically significant difference in age of onset, years to wheelchair, or other severity scores between patients with 'truncating' mutations and those with two missense mutations. In patients with the same mutation, clinical variability regarding severity was not prominent. Regarding specific mutations, the only two patients with the homozygous p.R282C mutation seemed to have a milder clinical course, as did the only patient compound heterozygous for the p.Q163X and p.L344R mutations.

Discussion
This multicentric cross-sectional study provides information about 99 patients with CMT caused by GDAP1 mutations. In Spain the relative frequency of GDAP1 mutations has been reported to be as high as 13% of the genetically defined CMT in certain regions 7 . Patients in this series were distributed throughout Spain, and in most centers comprised the first cause of CMT2, excluding GJB1 mutations. This contrasts with the scarcity of these mutations in most Western Countries, accounting for less than 2% of the genetically defined patients in series from the United Kingdom, Germany, United States, and also in the cohort of patients from the Inherited Neuropathies Consortium 5,6,19,20 . On the other hand, there have been reports of regional Italian clinical CMT series that found frequencies of GDAP1 mutations of > 7%, and after analyzing a group of patients referred to the Medical Genetics Unit of the University of Genoa, the authors conclude that GDAP1 mutations should be the first genetic diagnosis to be considered in an Italian patient with CMT2 8,9 . Further information about the relative frequency of GDAP1 mutations in the South of France and other countries of the Mediterranean coastline (Greece, Turkey, Morocco, etc.) will be needed to clarify the geographical distribution of these mutations.   In Spain, the high frequency of GDAP1 mutations among CMT2 patients is in part due to the high frequency of two mutations. The p.R120W substitution accounted for 81% of AD patients and was detected mostly in the Mediterranean basin, while p.Q163X was detected either as a homozygous or compound heterozygous mutation in 73.2% of AR patients, especially in the North/Northwest of Spain. Both of these mutations had been studied previously with haplotype analysis and postulated as having a founder effect in the region, although they have been detected in families in other European and American countries 1,16,21 .
Regarding the phenotype, the most important factor that influences the clinical characteristics of these patients is the mode of inheritance ( Table 2). The patients with AR inherited mutations have a more severe course and certain clinical features like dysphonia and respiratory dysfunction were exclusively found in this group 13,18 . In fact when the CMTNS scores of the series were modeled with a logistic regression analysis, the only factor that could independently predict higher CMTNS scores apart from disease evolution was AR inheritance. Although AR patients had a more severe course as a group, there existed a certain overlap between the severity scores when grouped by the inheritance pattern (Fig. 4a).
For example, there were two AR patients with a milder phenotype than the rest of the cohort (CMTNS scores of 12 and 16 at ages 49 and 46 respectively). The latter is still ambulant with crutches at 56 years of age, and harbors the homozygous missense p.R282C change. This mutation has only been detected in him and his sister, who has a slightly more severe phenotype and became wheelchair-bound only at age 52. The other patient was a compound heterozygous for p.Q163X and p.L344R, and remained ambulant with orthosis until his death due to hepatic cirrhosis at age 55. Even taking into account these outliers, when the clinical characteristics or severity scores were compared according to the localization of the mutation or mutation type, no differences could be established. It has been reported that patients with two 'truncating' mutations have a more severe phenotype than those with two missense mutations, but when we compared age of onset, years to wheelchair or other impairment scores we could not replicate these findings 14 .
In patients with AD inheritance, phenotypic variability regarding severity was prominent, even within affected members of the same family. There are three AD patients with a severe neuropathy according to CMTNS, one of them needing a walker for ambulation; while 28.6% considered themselves asymptomatic, two of them older than 70 years. The factors that influence this variability are largely unknown, but the search for genetic modifiers in this disease led to the detection of a concomitant junctophilin-1 (JPH1) change which modified GDAP1 function in one of the families with the p.R120W mutation included in this series 22 . JPH1 substitutions have been detected in two other families, one of them is represented in Fig. 3 where the moderately affected siblings have inherited a change in JPH1 from their mother, and the p.R120W GDAP1 mutation from their practically asymptomatic father (in press). In any case, important clinical variability can be found in at least 6 other families with the p.R120W mutation and no concomitant changes in JPH1 have been detected.
Minor differences could be established between the phenotype in the p.R226del and p.R120W mutations but these have to be interpreted in the context of the clinical variability inherent to this disease. In the p.R120W mutation ankle flexion weakness could be detected as soon as ankle extension weakness, and fatty substitution predominated in the posterior compartment of the calf, while in the p.R226del mutation the MRI findings were similar in the symptomatic patient, but in the clinical examination ankle extension weakness predominated. In any case the description of more patients will be necessary to confirm these findings.
Autosomal dominant GDAP1 mutations have been described as having an incomplete penetrance, but in this series all asymptomatic patients had detectable abnormalities in clinical examination or ancillary testing except one. Of the 16 asymptomatic patients 9 had abnormalities in the examination, 11/12 in the electrophysiological studies and 3/6 in the MRI. The only patient that had normal examination and ancillary tests did not undergo needle electromyography or feet MRI, which are the most sensitive tests in this series. This is in keeping with the findings in the Italian p.Arg120Gly mutation where the phenotypic variability mimicked a reduced penetrance, but electrophysiological studies were unequivocally abnormal in affected patients 23 .
These findings enhance the complexity of the clinical phenotype and of predicting the clinical course in GDAP1 mutations. This last can be especially challenging, but is necessary for the development of future clinical trials. In this regard there are several considerations that are important in this CMT subtype. In AR patients the severity of the phenotype determined that the scales employed could not adequately assess disease progression except in the first or second decade, as afterwards there was a clear ceiling effect in both the CMTNS and FDS scales. In these patients ancillary testing could contribute scarcely as MRI of the lower limbs was affected to a great degree and the rate of change in these patients would be difficult to interpret. In AD patients, apart from the intra-familiar variability, the analysis of the available longitudinal information shows that the change rate of CMTNS per year in one same patient can be quite variable (data not shown). The responsiveness to change probably could be improved if the CMTNS version 2, or the Rasch-wheighted CMTNS version 2 scores had been used, but this was impossible as radial nerve conduction studies were available for only a minority of the patients 24 . In any case, taking into account the rarity of this CMT subtype, and the intra-familiar variability, more objective measures of disease progression will be necessary, like the MRI biomarkers that have already been developed for CMT1A 25 .
This is the largest clinical series of CMT patients with GDAP1 mutations and it contributes to expand the knowledge of the genetic distribution and genotype-phenotype correlation of this disease. Mutational analysis. Mutation analysis of the GDAP1 gene (NM_018972.2) was performed mostly in the same center as previously described using an Applied Biosystems 3730xl DNA analyzer (Foster City, CA, USA) 1 .
Clinical assessments. A standardized history and symptom questionnaire was employed to collect the basal information. Clinical examinations included strength using the standard Medical Research Council (MRC) scale, pinprick and vibratory sensory loss, reflexes, as well as a general and neurologic examination. Severity of the neuropathy was evaluated with the CMT neuropathy score version 1 (CMTNSv1) as a significant proportion of these patients had not undergone the neurophysiologic radial nerve testing which is mandatory for the CMTNS version 2 26,27 . The CMT examination score (CMTES) was used for patients without nerve conduction studies. The functional disability scale (FDS), which grades the functional impairment from 0-8, was used to measure the disability status 28 . Respiratory dysfunction was defined by standard spirometry values in each center.
Nerve conduction studies. Motor and sensory nerve conduction velocities (NCVs) were performed using standard techniques. Temperature was controlled during the procedure and kept at more than 32 °C. Compound muscle action potential amplitudes (CMAP), sensory nerve action potential amplitudes (SNAP), conduction velocities and distal latencies were recorded.
Muscle magnetic resonance imaging (MRI). MRI was performed on the feet and distal legs. The protocol employed was the same as described previously, and fatty substitution was graded from 0 to 4 as follows: 0, no fat signal in muscle; 1, some fatty streaks; 2, fat occupying a minor part of muscle; 3, similar amount of fat and muscle tissue; 4, fat occupying the greater part of muscle 16,29 . The muscles in the calf were grouped according to 4 anatomical compartments: anterior compartment (tibialis anterior, extensor hallucis longus, and extensor digitorum longus), lateral compartment (peronei longus and brevis), superficial posterior compartment (soleus and gastrocnemius), and deep posterior compartment (tibialis posterior, flexor digitorum longus, and flexor hallucis longus).
SCIENTIFIC REPORTS | 7: 6677 | DOI:10.1038/s41598-017-06894-6 Nerve pathology. Sural nerve biopsy was performed before the genetic diagnosis was confirmed, or for investigational reasons after specific informed consent. Sections were evaluated under a light microscope, and morphometric data was analyzed when available 18 . Statistical studies. Patient characteristics, data from clinical examination, electrophysiological, examination and physical disability were analyzed using descriptive statistics with the program R (version 3.3.3), the ordinal package (version 2015.6-28) and brglm (0. [5][6][7][8][9]. A logistic regression model was developed to determine the co-variables that could predict a higher CMTNS score. Data Availability. The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.