Best practice guidelines on genetic diagnostics of facioscapulohumeral muscular dystrophy: Update of the 2012 guidelines.

The gold standard for facioscapulohumeral muscular dystrophy (FSHD) genetic diagnostic procedures was published in 2012. With the increasing complexity of the genetics of FSHD1 and 2, the increase of genetic testing centers, and the start of clinical trials for FSHD, it is crucial to provide an update on our knowledge of the genetic features of the FSHD loci and renew the international consensus on the molecular testing recommendations. To this end, members of the FSHD European Trial Network summarized the evidence presented during the 2022 ENMC meeting on Genetic diagnosis, clinical outcome measures, and biomarkers. The working group additionally invited genetic and clinical experts from the USA, India, Japan, Australia, South-Africa, and Brazil to provide a global perspective. Six virtual meetings were organized to reach consensus on the minimal requirements for genetic confirmation of FSHD1 and FSHD2. Here, we present the clinical and genetic features of FSHD, specific features of FSHD1 and FSHD2, pros and cons of established and new technologies (Southern blot in combination with either linear or pulsed-field gel electrophoresis, molecular combing, optical genome mapping, FSHD2 methylation analysis and FSHD2 genotyping), the possibilities and challenges of prenatal testing, including pre-implantation genetic testing, and the minimal requirements and recommendations for genetic confirmation of FSHD1 and FSHD2. This consensus is expected to contribute to current clinical management and trial-readiness for FSHD.

Hydroxyurea treatment for adult sickle cell anemia patients in Kinshasa.

Background: Despite a high incidence of sickle cell anemia, hydroxyurea (HU) treatment is rarely used in the DR Congo. This study aims to assess the efficacy of HU, the incidence of side effects that may limit its use in adults and to determine the dose needed for clinical improvement in patients. Methods: In a prospective study, patients received an initial dose of 15 mg/kg/day which was increased by 5 mg/kg every 6 months, up to a maximum of 30 mg/kg/day. The response and side effects to HU were evaluated biologically and clinically every 3 months during a 2-year period. Results: Seventy adult patients with a moderate or severe clinical phenotype initiated treatment. Only minor side effects were reported. At the end of the 2-year treatment phase, 45 (64.3%) had dropped out, of whom 33 were without a clear reason. Clinical and biological improvement was more marked during the first year. There was a reduction in severe vaso-occlusive crises (p < 0.001), need for transfusion (p < 0.001), and hospitalization days (p = 0.038). Fetal hemoglobin (HbF) levels increased on average 2.9 times after 12 months (p < 0.001). The increase in mean corpuscular volume was greater in the first year (p < 0.001) than in the second year (p = 0.041). The decrease in leukocytes (p < 0.001) was significant during the first year. In 70% of patients, the 20 mg/kg/day dose was needed to reach the 20% HbF threshold. Conclusion: HU is effective and well tolerated. The magnitude of the response varies from one patient to another. Improvement of clinical manifestations is achieved in most patients with a relatively low dose. Effective implementation of HU treatment will require improved adherence to treatment.

Clinical and biological profile of Sickle Cell Anemia children in a rural area in Central Africa.

BACKGROUND: Sickle Cell Anemia (SCA) is the most common genetic disease worldwide caused by a single mutation in the gene HBB. The disease severity is very variable and depends on many factors. We evaluated the clinical and biological profile of sickle cell anemia children in rural Central Africa. METHODS: This cross-sectional study was conducted in the Hôpital Saint Luc de Kisantu, located 120 km away from Kinshasa-DR Congo in an area of 35 km around Kisantu with a population of roughly 80 000 individuals. We included SCA patients aged 6 months to 18 years. We collected clinical and hematological data. The SCA scoring system proposed by Adegoke et al. in 2013 was applied to determine the disease severity. We searched for factors associated to the disease severity. RESULTS: This study included 136 patients, 66 males and 70 females (sex-ratio M/F 0.94). The mean severity score was 8.21 ± 5.30 (ranges 0-23). Fifty-nine (43.4%) children had mild disease, 62 (45.6%) moderate and 15 (11%) severe disease. Girls had higher levels of HbF than boys (p = 0.003). An inverse correlation was observed between fetal hemoglobin and the disease severity (p = 0.005, r -0.239, IC95% -6.139; -1.469). Some factors such age influence the occurrence of certain chronic complications such as avascular bone necrosis. CONCLUSION: In conclusion, the disease severity of SCA depends on multiple factors. In this study, fetal hemoglobin was the main modulator of the disease severity. These data may also serve as a baseline to initiate HU treatment in this setting.

Clinical and laboratory characterization of adult sickle cell anemia patients in Kinshasa.

BACKGROUND: Sickle cell anemia (SCA) is a monogenic hemoglobinopathy associated with severe acute and chronic complications, with the highest incidence worldwide in Sub-Saharan Africa. The wide variability in clinical manifestations suggest that a uniform response to hydroxurea may not be attained. In view of a potential treatment with hydroxyurea (HU), we assessed the variability of clinical and hematological manifestations in a cohort of adults with SCA in Kinshasa, capital of the DR Congo in Central Africa. METHODS: A cross-sectional study was conducted in a hospital dedicated to SCA management in Kinshasa. Clinical history of patients was recorded, a complete physical examination performed. The diagnosis was confirmed by means of DNA analysis. A full blood count and hemolysis markers were measured. The severity of the disease was evaluated by means of a previously reported score. RESULTS: The study group consisted of 166 genetically confirmed SCA patients. The SCA severity was mild in 28.9%, moderate in 64.5% and severe in 6.6%. The disease severity score increased with patient's age (p ≤ 0.001). The severity was higher in males compared to females (p = 0.012). In males, the severity score was correlated with the presence of priapism (p = 0.045), a manifestation not previously incorporated in the severity score. The severity score was inversely correlated with the fetal hemoglobin (HbF) rate (p = 0.005). Malnutrition (BMI <18.5 kg/m2) was present in 47% of patients and was related to the male sex, hip disease (aOR 3.11; p = 0.019) and severe phenotype (aOR 3.53; p = 0.012). Leg ulcers were more frequent in males than in females (p = 0.001; OR 24.3) and were correlated with the number of days of hospitalization (p = 0.029). Hip disease was related to the increasing age (p = 0.008). CONCLUSION: In this selected, hospital-based populations of adults with SCA, severe disease was rare, which may be due to survival bias. However, two thirds had moderate severity of the disease, mostly with a low HbF, and they may benefit from HU treatment. In the Central-African setting the separation between vaso-occlusive and hyperhemolytic sub-phenotypes was not applicable.

Molecular genetic characterization of Congolese patients with oculocutaneous albinism.

BACKGROUND: Oculocutaneous albinism (OCA) is an autosomal recessive genetic disorder associated with reduced or absent pigmentation in the skin, hair and eyes. OCA type 2 (OCA2) is the most common type in Sub-Saharan Africa, related to a recurrent 2.7 kb intragenic deletion. Genomic data from Congolese patients are lacking. We aimed to describe genetic causes of OCA2 in a cohort of Congolese persons with OCA, and explore possible genotype-phenotype correlations. METHODS: A cross sectional study was conducted from January 2015 to December 2017 in Kinshasa, Democratic Republic of Congo (DRC). 165 Congolese unrelated families with non-syndromic OCA, identified through patients' associations, consented to participate to this study. All index cases were tested for the known 2.7 kb deletion involving the exon 7 of the OCA2 gene. Patients heterozygous for the deletion underwent Sanger sequencing of all exons and flanking sequences in the OCA2 gene. Family segregation was performed for candidate pathogenic variants. RESULTS: The 2.7 kb deletion in the OCA2 gene was identified in 136/165 (82.4%) index cases, including 113 (68.5%) homozygotes and 23 (13.9%) heterozygotes. Sanger sequencing identified a pathogenic or likely pathogenic variant in the OCA2 gene in 12 out of 23 heterozygotes investigated (52.1%). Segregation analysis allowed us to locate the point mutation on the trans allele in the three patients from whom parental DNA was available. CONCLUSION: The OCA2 2.7 kb deletion is the major cause of non-syndromic OCA in Congolese patients recruited in this study, confirming results from other Sub-Saharan African populations. Several additional mutations were detected in OCA patient's heterozygote for the deletion, with to date no evidence for a second frequent founder mutation. The confirmation of a single mutation as the major cause will facilitate genetic counselling in this country.

Value of DNA testing in the diagnosis of sickle-cell anemia in childhood in an environment with a high prevalence of other causes of anemia.

BACKGROUND: Sickle-cell anemia (SCA) is the most common genetic disease worldwide caused by a single mutation in the gene HBB. DNA testing can help to clarify the diagnosis when Hb electrophoresis is inconclusive. We evaluated the usefulness and feasibility of DNA-based diagnosis of SCA in rural Central Africa. METHODS: This is a cross-sectional study conducted from November 2016 to end October 2017 in the Hôpital Saint Luc de Kisantu, located 120 km from Kinshasa. This hospital offers the management of SCA patients, mainly identified using the Sickling test (Emmel test) combined with clinical features. We included patients aged 6 months to 18 years locally diagnosed as SCA, and we collected clinical and hematological data. All patients were offered Hb electrophoresis and DNA testing at the Center for Human Genetics of the University of Kinshasa. RESULTS: This study included 160 patients. Hemoglobin capillary electrophoresis suggested that 136 (85%) were homozygote SS, 13 (8.1%) were heterozygote (AS), and 11 (6.9%) were homozygote normal (AA). DNA testing confirmed these electrophoresis findings, with the exception of four patients, two AS in electrophoresis were found SS due to recent transfusion, and two SS in electrophoresis were found AS because they have compound heterozygous form S/ß°-thalassemia. The diagnosis of SCA was therefore wrongly ascertained with Emmel test in 15% of patients. CONCLUSION: This study reveals a high proportion of false-positive SCA diagnoses in a rural environment in Central Africa. This underlines the importance of DNA testing in conjunction with Hb electrophoresis.

DNA testing for sickle cell anemia in Africa: Implementation choices for the Democratic Republic of Congo.

BACKGROUND: Hemoglobin-based tests form the reference diagnostic test for SCA. In limited resource countries, these tests face limitations including cost, low sensitivity due to recurrent transfusions in endemic malaria region, and interference from fetal hemoglobin in neonatal diagnostic. This study aimed at adapting DNA-based SCA tests to limited resource countries and evaluating the economic benefit. METHODS: 338 participants were recruited in the Democratic Republic of Congo, sorted in 3 cohorts based on venous blood, umbilical cord blood (UCB) and buccal swab sampling. RFLP was performed to identify mutated allele. The feasibility and technical validity of this RFLP was evaluated for specimens collected on DBS cards and on EDTA tubes. RFLP on DBS stored at room temperature was regularly repeated to assess sample conservation. Finally, the cost analysis was performed. RESULTS: DBS cards yielded identical results to extracted DNA. Repeated testing returned the same result after four years. The DBS-based test performed on UCB or on buccal swab had a sensitivity and a precision of 100%. Cost comparison indicated that our approach costs half price of the widely used isoelectrofocussing of hemoglobin. CONCLUSION: The implemented DNA-based test approach overcomes the limitations faced by hemoglobin-based tests, while being more affordable. We propose to implement the RFLP test as a first line diagnostic test after transfusion and as second tiers for newborn screening. However, users should be aware that this test is unable to differentiate HbC from HbS or identify other point mutation of gene deletion of HBB gene.

Respiratory onset of amyotrophic lateral sclerosis in a pregnant woman with a novel SOD1 mutation.

BACKGROUND AND PURPOSE: With the advent of gene therapies for amyotrophic lateral sclerosis (ALS), the importance of gene testing in ALS is increasing. This will likely lead to the identification of new variants for which the pathogenicity is not established. We aimed to study the pathogenicity of a newly identified variant in superoxide dismutase 1 (SOD1). METHODS: Gene testing was performed using Sanger sequencing. SOD1 activity in erythrocytes was measured using spectrophotometry. Postmortem brain and spinal cord sections were stained with antibodies against phospho-TDP-43 and SOD1. RESULTS: We identified a novel c.416G>T (p.Gly139Val) mutation in SOD1, which caused a rapidly progressive respiratory onset form of ALS. The mutation resulted in a 50% drop of SOD1 activity. Postmortem examination confirmed the absence of TDP-43 pathology and displayed typical SOD1 inclusions in remaining motor neurons, confirming the pathogenic nature of the mutation. CONCLUSIONS: Novel variants of unknown pathogenicity will be identified as a result of a surge in gene testing in people with ALS. An in-depth study of a newly identified p.Gly139Val mutation in SOD1 confirmed the pathogenicity of this mutation. Future patients with this particular mutation should qualify for SOD1 silencing or editing therapies.

RNF170 mutation causes autosomal dominant sensory ataxia with variable pyramidal involvement.

BACKGROUND: Although hereditary ataxias are a group of clinically and genetically heterogeneous disorders, specific clinical clues can sometimes incriminate certain genes. This can trigger genetic testing in sporadic patients or prompt dissecting certain genes more thoroughly when initial genetic testing is negative. Also for the assembly of gene panels and interpretation of the results, genotype-phenotype correlations remain important to establish. METHODS: We clinically evaluated a Belgian family with autosomal dominant inherited sensory ataxia and variable pyramidal involvement and performed targeted clinical exome sequencing. Secondly, we retrospectively screened sequencing data of an in-house cohort of 404 patients with neuromuscular disorders for variants in the identified gene RNF170. RESULTS: All affected family members showed sensory ataxia on examination. Pyramidal involvement, and sometimes slow-pursuit abnormalities and/or a sensory neuropathy, were more variable findings. We identified the heterozygous variant p.Arg199Cys in RNF170 in all three affected siblings of our family. We did not find additional pathogenic variants in RNF170 in our in-house neuromuscular cohort. CONCLUSIONS: We confirm the heterozygous variant p.Arg199Cys in RNF170 in a Belgian family with autosomal dominant sensory ataxia and variable pyramidal involvement. This constitutes a rare but clinically recognizable phenotype that warrants testing of RNF170. Unlike the distinctive bi-allelic loss of function variants in RNF170 associated with hereditary spastic paraplegia (HSP), the p.Arg199Cys variant is the only one reported in sensory ataxia. It is important for neurologists to be aware of this characteristic phenotype and to include this gene in gene panels for ataxia and HSP.

SLC37A4-CDG: Second patient.

Recently, a disorder caused by the heterozygous de novo c.1267C>T (p.R423*) substitution in SLC37A4 has been described. This causes mislocalization of the glucose-6-phosphate transporter to the Golgi leading to a congenital disorder of glycosylation type II (SLC37A4-CDG). Only one patient has been reported showing liver disease that improved with age and mild dysmorphism. Here we report the second patient with a type II CDG caused by the same heterozygous de novo c.1267C>T (p.R423*) mutation thereby confirming the pathogenicity of this variant and expanding the clinical picture with type 1 diabetes, severe scoliosis, and membranoproliferative glomerulonephritis. Additional clinical and biochemical data provide further insight into the mechanism and prognosis of SLC37A4-CDG.

Congenital Disorders of Glycosylation in Portugal-Two Decades of Experience.

OBJECTIVE: To describe the clinical, biochemical, and genetic features of both new and previously reported patients with congenital disorders of glycosylation (CDGs) diagnosed in Portugal over the last 20 years. STUDY DESIGN: The cohort includes patients with an unexplained multisystem or single organ involvement, with or without psychomotor disability. Serum sialotransferrin isoforms and, whenever necessary, apolipoprotein CIII isoforms and glycan structures were analyzed. Additional studies included measurement of phosphomannomutase (PMM) activity and analysis of lipid-linked oligosaccharides in fibroblasts. Sanger sequencing and massive parallel sequencing were used to identify causal variants or the affected gene, respectively. RESULTS: Sixty-three individuals were diagnosed covering 14 distinct CDGs; 43 patients diagnosed postnatally revealed a type 1, 14 a type 2, and 2 a normal pattern on serum transferrin isoelectrofocusing. The latter patients were identified by whole exome sequencing. Nine of them presented also a hypoglycosylation pattern on apolipoprotein CIII isoelectrofocusing, pointing to an associated O-glycosylation defect. Most of the patients (62%) are PMM2-CDG and the remaining carry pathogenic variants in ALG1, ATP6AP1, ATP6AP2, ATP6V0A2, CCDC115, COG1, COG4, DPAGT1, MAN1B1, SLC35A2, SRD5A3, RFT1, or PGM1. CONCLUSIONS: Portuguese patients with CDGs are presented in this report, some of them showing unique clinical phenotypes. Among the 14 genes mutated in Portuguese individuals, 8 are shared with a previously reported Spanish cohort. However, regarding the mutational spectrum of PMM2-CDG, the most frequent CDG, a striking similarity between the 2 populations was found, as only 1 mutated allele found in the Portuguese group has not been reported in Spain.

Clinical and muscle MRI features in a family with tubular aggregate myopathy and novel STIM1 mutation.

Heterozygous mutations in the stromal interaction molecule-1-gene (STIM1) cause a clinical phenotype varying from tubular aggregate myopathy with single or multiple signs of Stormorken syndrome to the full Stormorken phenotype. We identified a novel heterozygous mutation c.325C > T (p.H109Y) in the EF-hand domain of STIM1 in six patients of a large Belgian family, and performed a detailed clinical (N = 6), histopathological (N = 2) and whole-body muscle MRI (N = 3) study. The clinical phenotype was characterized by a slowly progressive, predominant proximal muscle weakness in all patients (100%), and additional exercise-induced myalgia in three (60%). Patients experienced symptom onset between 10 and 20 years, remained ambulatory into late adulthood, showed elevated serum creatine kinase levels and tubular aggregates in type 1 and type 2 fibers on muscle biopsy. Interestingly, jaw contractures and hyperlaxity, as well as non-muscular multisystemic features such as menorrhagia, easy bruising and ichthyosis occurred in one patient, and miosis in another. Whole-body muscle MRI revealed predominant involvement of superficial neck extensors, subscapularis, obliquus abdominis externus, lumbar extensors, rectus femoris, biceps femoris longus, medial head of gastrocnemius and flexor hallucis longus. Our findings in patients with myopathy with tubular aggregates and a STIM1 mutation further support the concept of a continuous spectrum with Stormorken syndrome.

Challenges in molecular diagnosis of X-linked Intellectual disability.

BACKGROUND: Intellectual disability (ID) affects 1-3% of the Western population and is heterogeneous in origin. Mutations in X-linked genes represent 5-10% of ID in males. Fragile X syndrome, due to the silencing of the FMR1 gene, is the most common form of ID, with a prevalence of around 1:5000 males. Females are usually non- or mildly affected carriers, and in a few rare cases, the only gender affected. Array comparative genome hybridization (aCGH) and next-generation sequencing (NGS) have dramatically changed the nature of human genome analysis leading to the identification of new X-linked intellectual disability syndromes and disease-causing genes. SOURCES OF DATA: Original papers, reviews, guidelines and experiences of the diagnostic laboratories. AREAS OF AGREEMENT: Family history and clinical examination still are essential to choose the appropriate diagnostic tests, including, a disease-specific genetic test, aCGH or FMR1 molecular analysis. If negative, NGS approaches like well-defined gene panels, whole exome, or even whole genome sequencing, are increasingly being used, improving diagnostics and leading to the identification of novel disease mechanisms. AREAS OF CONTROVERSY: The main challenge in the era of NGS is filtering and interpretation of the data generated by the analysis of a single individual. In X-linked cases, assessing pathogenicity is particularly challenging, even more when the variant is found to be inherited from a healthy carrier mother or when a heterozygous X-linked mutation is found in an impaired female. GROWING POINTS: At present, variant interpretation remains a challenging task, especially in X-linked disorders. We review the main difficulties and propose a comprehensive overview that might aid in variant interpretation. Establishing a genetic diagnosis facilitates counseling and allows better delineation of clinical phenotypes. AREAS TIMELY FOR DEVELOPING RESEARCH: To improve variant interpretation, there is need to refine in silico predictions with specific criteria for each gene, and to develop cost-effective functional tools, which can be easily transferred to diagnostics.

Next-generation sequencing in prenatal setting: Some examples of unexpected variant association.

The application of next-generation sequencing to fetal pathology has proved to increase the diagnostic yield in fetuses with abnormal ultrasounds. We retrospectively reviewed genetic data of 30 selected cases studied through targeted resequencing of OMIM genes. In our experience, clinical data proved to be essential to support diagnostic reasoning and enhance variants' assessment. The molecular diagnosis was reached in 19/30 (63%) cases. Only in 7/19 cases the molecular diagnosis confirmed the initial diagnostic hypothesis, showing the relevance of the genotype-first approach. According to the genotype-phenotype correlation, we were able to divide the solved cases into three groups: i) the correlation is well established but it was missed due to lack of specificity, unusual presentation or recent description; ii) the clinical presentation is much more severe than currently known for the underlying condition; iii) the correlation does not recapitulate the entire phenotype, possibly due to the fetal presentation or multiple coexisting conditions. Moreover, we found a higher proportion of recessive diagnosis in abnormal fetuses compared to cohorts of individuals with developmental delay. Our findings suggest that fetal pathology may be enriched in rare alleles and/or in unusual combinations, counter-selected in postnatal genomes and thus contributing to both phenotypic extremeness and atypical presentation.

Antenatal diagnosis of cardio-facio-cutaneous syndrome: Prenatal characteristics and contribution of fetal facial dysmorphic signs in utero. About a case and review of literature.

Antenatal diagnosis of cardio-facio-cutaneous syndrome: prenatal characteristics and contribution of fetal facial dysmorphic signs in utero. This paper is a case study and review of literature. "RASopathies" is the term coined for a group of genetic diseases that share modulation inside the MAPKinase pathway. Mutations inside the coding sequence of any of these genes may be responsible for the upregulation of the RAS pathway, leading on the clinical level to Type 1 Neurofibromatosis (NF1), Noonan syndrome (NS), Costello syndrome (CS), Multiple Lentigines, Loose Anagen Hair syndrome, Cardio-Facio-Cutaneous syndrome (CFCS), and, more recently, Legius syndrome. While the postnatal presentation of this group is well-known, prenatal findings are less well recognized. The presence of a RASopathy during the prenatal period can be suspected on account of non-specific abnormalities: polyhydramnios, cystic hygroma or high nuchal translucency, macrosomia with proportionate short long bones, macrocephaly, renal, lymphatic, or cardiac defects. The current case report underlines the characteristic dysmorphic facial features on 3D-ultrasound (hypertelorism, down-slanting palpebral fissures, a long and marked philtrum, and low-set posteriorly rotated ears) that allow for a "RASopathy" to be postulated. After detecting a copy number variation (CNV) absence on a CGH array, we performed a RASopathy gene panel analysis, which identified a so-far unreported heterozygous de novo mutation in the BRAF gene (namely NM_004333.4 : c.1396 G > C ; p.Gly466Arg). Genetic counseling has, therefore, focused on the diagnosis of a RASopathy and predictable phenotype of CFCS, a distinct entity characterized by an increased risk of intellectual disability and early-onset feeding problems. We suggest that a more detailed prenatal facial evaluation should be performed in fetuses presenting high nuchal thickness, heart defects, or unusual findings, along with the absence of a CNV on a CGH array. Due to the dysmorphic facial features, targeted RASopathy genes are presumed to likely to be responsible for NS, CFCS, and CS.

Maternal copy-number variations in the DMD gene as secondary findings in noninvasive prenatal screening.

PURPOSE: Noninvasive prenatal screening (NIPS) using genome sequencing also reveals maternal copy-number variations (CNVs). Those CNVs can be clinically actionable or harmful to the fetus if inherited. CNVs in the DMD gene potentially causing dystrophinopathies are among the most commonly observed maternal CNVs. We present our experience with maternal DMD gene CNVs detected by NIPS. METHODS: We analyzed the data of maternal CNVs detected in the DMD gene revealed by NIPS. RESULTS: Of 26,123 NIPS analyses, 16 maternal CNVs in the DMD gene were detected (1/1632 pregnant women). Variant classification regarding pathogenicity and phenotypic severity was based on public databases, segregation analysis in the family, and prediction of the effect on the reading frame. Ten CNVs were classified as pathogenic, four as benign, and two remained unclassified. CONCLUSION: NIPS leverages CNV screening in the general population of pregnant women. We implemented a strategy for the interpretation and the return of maternal CNVs in the DMD gene detected by NIPS.

Mutations in MAGT1 lead to a glycosylation disorder with a variable phenotype.

Congenital disorders of glycosylation (CDG) are a group of rare metabolic diseases, due to impaired protein and lipid glycosylation. We identified two patients with defective serum transferrin glycosylation and mutations in the MAGT1 gene. These patients present with a phenotype that is mainly characterized by intellectual and developmental disability. MAGT1 has been described to be a subunit of the oligosaccharyltransferase (OST) complex and more specifically of the STT3B complex. However, it was also claimed that MAGT1 is a magnesium (Mg2+) transporter. So far, patients with mutations in MAGT1 were linked to a primary immunodeficiency, characterized by chronic EBV infections attributed to a Mg2+ homeostasis defect (XMEN). We compared the clinical and cellular phenotype of our two patients to that of an XMEN patient that we recently identified. All three patients have an N-glycosylation defect, as was shown by the study of different substrates, such as GLUT1 and SHBG, demonstrating that the posttranslational glycosylation carried out by the STT3B complex is dysfunctional in all three patients. Moreover, MAGT1 deficiency is associated with an enhanced expression of TUSC3, the homolog protein of MAGT1, pointing toward a compensatory mechanism. Hence, we delineate MAGT1-CDG as a disorder associated with two different clinical phenotypes caused by defects in glycosylation.