Mutations in GMPPA cause a glycosylation disorder characterized by intellectual disability and autonomic dysfunction

Biallelic NDC1 variants that interfere with ALADIN binding are associated with neuropathy and triple A-like syndrome

Nuclear pore complexes (NPCs) regulate nucleocytoplasmic transport and are anchored in the nuclear envelope by the transmembrane nucleoporin NDC1. NDC1 is essential for post-mitotic NPC assembly and the recruitment of ALADIN to the nuclear envelope. While no human disorder has been associated to one of the three transmembrane nucleoporins, biallelic variants in AAAS, encoding ALADIN, cause triple A syndrome (Allgrove syndrome). Triple A syndrome, characterized by alacrima, achalasia, and adrenal insufficiency, often includes progressive demyelinating polyneuropathy and other neurological complaints. In this report, diagnostic exome and/or RNA sequencing was performed in seven individuals from four unrelated consanguineous families with AAAS-negative triple A syndrome. Molecular and clinical studies followed to elucidate the pathogenic mechanism. The affected individuals presented with intellectual disability, motor impairment, severe demyelinating with secondary axonal polyneuropathy, alacrima, and achalasia. None of the affected individuals has adrenal insufficiency. All individuals presented with biallelic NDC1 in-frame deletions or missense variants that affect amino acids and protein domains required for ALADIN binding. No other significant variants associated with the phenotypic features were reported. Skin fibroblasts derived from affected individuals show decreased recruitment of ALADIN to the NE and decreased post-mitotic NPC insertion, confirming pathogenicity of the variants. Taken together, our results implicate biallelic NDC1 variants in the pathogenesis of polyneuropathy and a triple A-like disorder without adrenal insufficiency, by interfering with physiological NDC1 functions, including the recruitment of ALADIN to the NPC.

Manipulating mannose metabolism as a potential anticancer strategy

Cancer cells acquire metabolic advantages over their normal counterparts regarding the use of nutrients for sustained cell proliferation and cell survival in the tumor microenvironment. Notable among the metabolic traits in cancer cells is the Warburg effect, which is a reprogrammed form of glycolysis that favors the rapid generation of ATP from glucose and the production of biological macromolecules by diverting glucose into various metabolic intermediates. Meanwhile, mannose, which is the C-2 epimer of glucose, has the ability to dampen the Warburg effect, resulting in slow-cycling cancer cells that are highly susceptible to chemotherapy. This anticancer effect of mannose appears when its catabolism is compromised in cancer cells. Moreover, de novo synthesis of mannose within cancer cells has also been identified as a potential target for enhancing chemosensitivity through targeting glycosylation pathways. The underlying mechanisms by which alterations in mannose metabolism induce cancer cell vulnerability are just beginning to emerge. This review summarizes the current state of our knowledge of mannose metabolism and provides insights into its manipulation as a potential anticancer strategy.

Ubiquitination contributes to the regulation of GDP-mannose pyrophosphorylase B activity

GDP-mannose pyrophosphorylase B (GMPPB) loss-of-function is associated with muscular dystrophy and variable additional neurological symptoms. GMPPB facilitates the catalytic conversion of mannose-1-phosphate and GTP to GDP-mannose, which serves as a mannose donor for glycosylation. The activity of GMPPB is regulated by its non-catalytic paralogue GMPPA, which can bind GDP-mannose and interact with GMPPB, thereby acting as an allosteric feedback inhibitor of GMPPB. Using pulldown, immunoprecipitation, turnover experiments as well as immunolabeling and enzyme activity assays, we provide first direct evidence that GMPPB activity is regulated by ubiquitination. We further show that the E3 ubiquitin ligase TRIM67 interacts with GMPPB and that knockdown of TRM67 reduces ubiquitination of GMPPB, thus reflecting a candidate E3 ligase for the ubiquitination of GMPPB. While the inhibition of GMPPB ubiquitination decreases its enzymatic activity, its ubiquitination neither affects its interaction with GMPPA nor its turnover. Taken together, we show that the ubiquitination of GMPPB represents another level of regulation of GDP-mannose supply.

Child Neurology: Allgrove Syndrome: An Intriguing Etiology of Motor Neuron Disease in Children

Motor neuron diseases are a rare group of neurodegenerative disorders with considerable phenotypic heterogeneity and a multitude of etiologies in the pediatric population. In this study, we report 2 unrelated adolescents (a boy and a girl) who presented with 4-6 years of progressive difficulty in walking, thinning of limbs, and gradually progressive darkening of the skin. Examination revealed generalized hyperpigmentation of skin and features suggestive of motor neuron involvement such as tongue atrophy, wasting of distal extremities, and brisk deep tendon reflexes. On detailed exploration for systemic involvement, history of dysphagia, inability to produce tears, and Addisonian crises were evident. An etiologic diagnosis of Allgrove syndrome, which is characterized by a triad of achalasia, alacrimia, and adrenal insufficiency was considered. Next-generation sequencing revealed pathogenic variants in the AAAS gene, confirming the diagnosis. Steroid replacement therapy was initiated along with relevant multidisciplinary referrals. The disease stabilized in the boy and a significant improvement was noted in the girl. These cases highlight the value of non-neurologic cues in navigating the etiologic complexities of motor neuron diseases in children and adolescents. It is imperative for neurologists to develop awareness of the diverse neurologic manifestations associated with Allgrove syndrome because they are often the first to be approached. A multidisciplinary team of experts including neurologists, endocrinologists, gastroenterologists, ophthalmologists, and dermatologists is essential for planning comprehensive care for these patients.

Mannose: A Promising Player in Clinical and Biomedical Applications

Mannose, an isomer of glucose, exhibits a distinct molecular structure with the same formula but a different atom arrangement, contributing to its specific biological functions. Widely distributed in body fluids and tissues, particularly in the nervous system, skin, testes, and retinas, mannose plays a crucial role as a direct precursor for glycoprotein synthesis. Glycoproteins, essential for immune regulation and glycosylation processes, underscore the significance of mannose in these physiological activities. The clinical and biomedical applications of mannose are diverse, encompassing its anti-inflammatory properties, potential to inhibit bacterial infections, role in metabolism regulation, and suggested involvement in alleviating diabetes and obesity. Additionally, mannose shows promise in antitumor effects, immune modulation, and the construction of drug carriers, indicating a broad spectrum of therapeutic potential. The article aims to present a comprehensive review of mannose, focusing on its molecular structure, metabolic pathways, and clinical and biomedical applications, and also to emphasize its status as a promising therapeutic agent.

Silencing GMPPB Inhibits the Proliferation and Invasion of GBM via Hippo/MMP3 Pathways

Glioblastoma multiforme (GBM) is a highly aggressive malignancy and represents the most common brain tumor in adults. To better understand its biology for new and effective therapies, we examined the role of GDP-mannose pyrophosphorylase B (GMPPB), a key unit of the GDP-mannose pyrophosphorylase (GDP-MP) that catalyzes the formation of GDP-mannose. Impaired GMPPB function will reduce the amount of GDP-mannose available for O-mannosylation. Abnormal O-mannosylation of alpha dystroglycan (α-DG) has been reported to be involved in cancer metastasis and arenavirus entry. Here, we found that GMPPB is highly expressed in a panel of GBM cell lines and clinical samples and that expression of GMPPB is positively correlated with the WHO grade of gliomas. Additionally, expression of GMPPB was negatively correlated with the prognosis of GBM patients. We demonstrate that silencing GMPPB inhibits the proliferation, migration, and invasion of GBM cells both in vitro and in vivo and that overexpression of GMPPB exhibits the opposite effects. Consequently, targeting GMPPB in GBM cells results in impaired GBM tumor growth and invasion. Finally, we identify that the Hippo/MMP3 axis is essential for GMPPB-promoted GBM aggressiveness. These findings indicate that GMPPB represents a potential novel target for GBM treatment.

Impact of Hypermannosylation on the Structure and Functionality of the ER and the Golgi Complex

Proteins of the secretory pathway undergo glycosylation in the endoplasmic reticulum (ER) and the Golgi apparatus. Altered protein glycosylation can manifest in serious, sometimes fatal malfunctions. We recently showed that mutations in GDP-mannose pyrophosphorylase A (GMPPA) can cause a syndrome characterized by alacrima, achalasia, mental retardation, and myopathic alterations (AAMR syndrome). GMPPA acts as a feedback inhibitor of GDP-mannose pyrophosphorylase B (GMPPB), which provides GDP-mannose as a substrate for protein glycosylation. Loss of GMPPA thus enhances the incorporation of mannose into glycochains of various proteins, including α-dystroglycan (α-DG), a protein that links the extracellular matrix with the cytoskeleton. Here, we further characterized the consequences of loss of GMPPA for the secretory pathway. This includes a fragmentation of the Golgi apparatus, which comes along with a regulation of the abundance of several ER- and Golgi-resident proteins. We further show that the activity of the Golgi-associated endoprotease furin is reduced. Moreover, the fraction of α-DG, which is retained in the ER, is increased. Notably, WT cells cultured at a high mannose concentration display similar changes with increased retention of α-DG, altered structure of the Golgi apparatus, and a decrease in furin activity. In summary, our data underline the importance of a balanced mannose homeostasis for the secretory pathway.

Altered mannose metabolism in chronic stress and depression is rapidly reversed by vitamin B12

GDP-Mannose Pyrophosphorylase B (GMPPB) is a key enzyme for glycosylation. Previous studies suggested a dysregulation of GMPBB and mannose in depression. Evidence, however, was sporadic and interventions to reverse these changes are unknown. Here, we show that GMPPB protein, but not RNA abundance is increased in the postmortem prefrontal cortex (PFC) of depressed patients and the chronic variable stress (CVS) mouse-model. This is accompanied by higher plasma mannose levels. Importantly, a single dose of intraperitoneally administered vitamin B12, which has previously been shown to rapidly reverse behavioral symptoms and molecular signatures of chronic stress in mice, normalized GMPPB plasma mannose levels and elevated GDP-mannose abundance. In summary, these data underline metabolic dysregulation in chronic stress and depression and provide further support for rapid effects of vitamin B12 on chronic stress.

Hypolacrimia and Alacrimia as Diagnostic Features for Genetic or Congenital Conditions

As part of the lacrimal apparatus, the lacrimal gland participates in the maintenance of a healthy eye surface by producing the aqueous part of the tear film. Alacrimia and hypolacrimia, which are relatively rare during childhood or young adulthood, have their origin in a number of mechanisms which include agenesia, aplasia, hypoplasia, or incorrect maturation of the gland. Moreover, impaired innervation of the gland and/or the cornea and alterations of protein secretion pathways can lead to a defective tear film. In most conditions leading to alacrimia or hypolacrimia, however, the altered tear film is only one of numerous defects that arise and therefore is commonly disregarded. Here, we have systematically reviewed all of those genetic conditions or congenital disorders that have alacrimia or hypolacrimia as a feature. Where it is known, we describe the mechanism of the defect in question. It has been possible to clearly establish the physiopathology of only a minority of these conditions. As hypolacrimia and alacrimia are rare features, this review could be used as a tool in clinical genetics to perform a quick diagnosis, necessary for appropriate care and counseling.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

A rare cause of infantile achalasia: GMPPA-congenital disorder of glycosylation with two novel compound heterozygous variants

Achalasia is rare in the pediatric population and should prompt clinicians to consider genetic disorders associated with this condition. While AAA syndrome (also known as Allgrove or Triple A syndrome) is commonly considered, GMPPA‐congenital disorder of glycosylation (CDG) should also be in the differential diagnosis. We report a 9‐month‐old female born to nonconsanguineous parents with achalasia and alacrima found to have two novel compound heterozygous variants in the GMPPA gene associated with GMPPA‐CDG. This rare disorder is commonly associated with developmental delay and intellectual disability. We discuss management of this disorder including the importance of confirming a genetic diagnosis and summarize reported cases.

AAMR syndrome in a 22-month-old and literature review

PURPOSE

Alacrima is characterized by severely decreased or deficient tear production. It can be associated with systemic findings; most commonly Triple-A Syndrome with alacrima, achalasia, and adrenal dysfunction.

METHODS

A case report and review of the literature.

RESULTS

A 22-month-old boy with cognitive delay presented with a lack of tearing. MRI brain and orbits showed a bilateral absence of lacrimal glands. Biochemical testing revealed normal adrenal function. Genetic testing showed no abnormalities in the AAAS gene. Mutations in the AAAS gene are associated with Triple-A syndrome. Whole-exome sequencing did reveal compound heterozygosity for the GMPPA gene, and mutations in the GMPPA gene have been associated with AAMR syndrome with alacrima, achalasia, and mental retardation. A literature review revealed all AAMR syndrome patients with consanguineous parents. This is the first reported case of AAMR syndrome in non-consanguineous parents.

CONCLUSIONS

In children older than 6 months old with decreased or deficient tear production, systemic work-up should include radiographic, biochemical, and genetic testing. Conditions associated with alacrima include familial dysautonomia, anhidrotic ectodermal dysplasia, Triple-A syndrome, and AAMR syndrome. One key clinical difference between the two syndromes is that patients with normal adrenal function have AAMR syndrome rather than Triple-A syndrome. After a child has a diagnosis with alacrima, these patients should be evaluated closely by a pediatric multidisciplinary team. Treatment for alacrima depends on the severity of ocular symptoms and ranges from the use of lubricant tears and ointment to moisture chambers.

Achalasia

Achalasia is a rare disorder of the oesophageal smooth muscle characterized by impaired relaxation of the lower oesophageal sphincter (LES) and absent or spastic contractions in the oesophageal body. The key pathophysiological mechanism is loss of inhibitory nerve function that probably results from an autoimmune attack targeting oesophageal myenteric nerves through cell-mediated and, possibly, antibody-mediated mechanisms. Achalasia incidence and prevalence increase with age, but the disorder can affect all ages and both sexes. Cardinal symptoms consist of dysphagia, regurgitation, chest pain and weight loss. Several years can pass between symptom onset and an achalasia diagnosis. Evaluation starts with endoscopy to rule out structural causes, followed by high-resolution manometry and/or barium radiography. Functional lumen imaging probe can provide complementary evidence. Achalasia subtypes have management and prognostic implications. Although symptom questionnaires are not useful for diagnosis, the Eckardt score is a simple symptom scoring scale that helps to quantify symptom response to therapy. Oral pharmacotherapy is not particularly effective. Botulinum toxin injection into the LES can temporize symptoms and function as a bridge to definitive therapy. Pneumatic dilation, per-oral endoscopic myotomy and laparoscopic Heller myotomy can provide durable symptom benefit. End-stage achalasia with a dilated, non-functioning oesophagus may require oesophagectomy or enteral feeding into the stomach. Long-term complications can, rarely, include oesophageal cancer, but surveillance recommendations have not been established.

Congenital alacrima

The congenital absence of tear production or alacrima is a distinctively unusual clinical sign that harbors a wide variety of etiologies. While alacrima can be only isolated to the lacrimal system, it is more often associated with progressive multisystem involvement from underlying genetic disorders. Recognizing the subtle ocular signs in these diseases will promote a timely diagnosis and management before potential life-threatening consequences occur. Hence, the current article will review the ophthalmological findings, systemic manifestations, genetic associations, and differential diagnosis of congenital alacrima.

Dissecting the transcriptional program of phosphomannomutase 2 deficient cells: B-LCL as a valuable model for congenital disorders of glycosylation studies

Congenital disorders of glycosylation (CDG) include 150 disorders constituting in genetically and clinically heterogeneous diseases, showing significant glycoprotein hypoglycosylation that leads to pathological consequences on multiple organs and systems which underlying mechanisms are not yet understood. A few cellular and animal models have been used to study specific CDG characteristics although they have given limited information due to the few CDG mutations tested and the still missing comprehensive molecular and cellular basic research. Here we provide specific gene expression profiles, based on RNA microarray analysis, together with some biochemical and cellular characteristics of a total of 9 control EBV-transformed lymphoblastoid B cell lines (B-LCL) and 13 CDG B-LCL from patients carrying severe mutations in the PMM2 gene, strong serum protein hypoglycosylation and neurological symptoms. Significantly dysregulated genes in PMM2-CDG cells included those regulating stress responses, transcription factors, glycosylation, motility, cell junction and, importantly, those related to development and neuronal differentiation and synapse such as CA2 and ADAM23. PMM2-CDG associated biological consequences involved the unfolded protein response, RNA metabolism and the endoplasmic reticulum, Golgi apparatus and mitochondria components. Changes in transcriptional and CA2 protein levels are consistent with CDG physiopathology. These results demonstrate the global transcriptional impact in phosphomannomutase 2 deficient cells, reveal CA2 as a potential cellular biomarker and confirm B-LCL as an advantageous model for CDG studies.

Disorders of the enteric nervous system - a holistic view

The enteric nervous system (ENS) is the largest division of the peripheral nervous system and closely resembles components and functions of the central nervous system. Although the central role of the ENS in congenital enteric neuropathic disorders, including Hirschsprung disease and inflammatory and functional bowel diseases, is well acknowledged, its role in systemic diseases is less understood. Evidence of a disordered ENS has accumulated in neurodegenerative diseases ranging from amyotrophic lateral sclerosis, Alzheimer disease and multiple sclerosis to Parkinson disease as well as neurodevelopmental disorders such as autism. The ENS is a key modulator of gut barrier function and a regulator of enteric homeostasis. A 'leaky gut' represents the gateway for bacterial and toxin translocation that might initiate downstream processes. Data indicate that changes in the gut microbiome acting in concert with the individual genetic background can modify the ENS, central nervous system and the immune system, impair barrier function, and contribute to various disorders such as irritable bowel syndrome, inflammatory bowel disease or neurodegeneration. Here, we summarize the current knowledge on the role of the ENS in gastrointestinal and systemic diseases, highlighting its interaction with various key players involved in shaping the phenotypes. Finally, current flaws and pitfalls related to ENS research in addition to future perspectives are also addressed.

Altered Glycosylation in the Aging Heart

Cardiovascular disease is one of the leading causes of death in developed countries. Because the incidence increases exponentially in the aging population, aging is a major risk factor for cardiovascular disease. Cardiac hypertrophy, fibrosis and inflammation are typical hallmarks of the aged heart. The molecular mechanisms, however, are poorly understood. Because glycosylation is one of the most common post-translational protein modifications and can affect biological properties and functions of proteins, we here provide the first analysis of the cardiac glycoproteome of mice at different ages. Western blot as well as MALDI-TOF based glycome analysis suggest that high-mannose N-glycans increase with age. In agreement, we found an age-related regulation of GMPPB, the enzyme, which facilitates the supply of the sugar-donor GDP-mannose. Glycoprotein pull-downs from heart lysates of young, middle-aged and old mice in combination with quantitative mass spectrometry bolster widespread alterations of the cardiac glycoproteome. Major hits are glycoproteins related to the extracellular matrix and Ca²⁺-binding proteins of the endoplasmic reticulum. We propose that changes in the heart glycoproteome likely contribute to the age-related functional decline of the cardiovascular system.

More evidence on TRIO missense mutations in the spectrin repeat domain causing severe developmental delay and recognizable facial dysmorphism with macrocephaly

TRIO is a Dbl family guanine nucleotide exchange factor (GEF) and an important regulator of neuronal development. Most truncating and missense variants affecting the Dbl homology domain of TRIO are associated with a neurodevelopmental disorder with microcephaly (MIM617061). Recently, de novo missense variants affecting the spectrin repeat region of TRIO were associated with a novel phenotype comprising severe developmental delay and macrocephaly (MIM618825). Here, we provide more evidence on this new TRIO-associated phenotype by reporting two severely affected probands with de novo missense variants in TRIO affecting the spectrin repeat region upstream of the typically affected GEF1 domain of the protein.

GMPPA defects cause a neuromuscular disorder with α-dystroglycan hyperglycosylation

GDP-mannose-pyrophosphorylase-B (GMPPB) facilitates the generation of GDP-mannose, a sugar donor required for glycosylation. GMPPB defects cause muscle disease due to hypoglycosylation of α-dystroglycan (α-DG). Alpha-DG is part of a protein complex, which links the extracellular matrix with the cytoskeleton, thus stabilizing myofibers. Mutations of the catalytically inactive homolog GMPPA cause alacrima, achalasia, and mental retardation syndrome (AAMR syndrome), which also involves muscle weakness. Here, we showed that Gmppa-KO mice recapitulated cognitive and motor deficits. As structural correlates, we found cortical layering defects, progressive neuron loss, and myopathic alterations. Increased GDP-mannose levels in skeletal muscle and in vitro assays identified GMPPA as an allosteric feedback inhibitor of GMPPB. Thus, its disruption enhanced mannose incorporation into glycoproteins, including α-DG in mice and humans. This increased α-DG turnover and thereby lowered α-DG abundance. In mice, dietary mannose restriction beginning after weaning corrected α-DG hyperglycosylation and abundance, normalized skeletal muscle morphology, and prevented neuron degeneration and the development of motor deficits. Cortical layering and cognitive performance, however, were not improved. We thus identified GMPPA defects as the first congenital disorder of glycosylation characterized by α-DG hyperglycosylation, to our knowledge, and we have unraveled underlying disease mechanisms and identified potential dietary treatment options.

Cryo-EM structures of human GMPPA-GMPPB complex reveal how cells maintain GDP-mannose homeostasis

GDP-mannose (GDP-Man) is a key metabolite essential for protein glycosylation and glycophosphatidylinositol anchor synthesis, and aberrant cellular GDP-Man levels have been associated with multiple human diseases. How cells maintain homeostasis of GDP-Man is unknown. Here, we report the cryo-EM structures of human GMPPA-GMPPB complex, the protein machinery responsible for GDP-Man synthesis, in complex with GDP-Man or GTP. Unexpectedly, we find that the catalytically inactive subunit GMPPA displays a much higher affinity to GDP-Man than the active subunit GMPPB and, subsequently, inhibits the catalytic activity of GMPPB through a unique C-terminal loop of GMPPA. Importantly, disruption of the interactions between GMPPA and GMPPB or the binding of GDP-Man to GMPPA in zebrafish leads to abnormal brain development and muscle abnormality, analogous to phenotypes observed in individuals carrying GMPPA or GMPPB mutations. We conclude that GMPPA acts as a cellular sensor to maintain mannose homeostasis through allosterically regulating GMPPB.

Homozygous mutation in murine retrovirus integration site 1 gene associated with a non-syndromic form of isolated familial achalasia

BACKGROUND

Achalasia is a condition characterized by impaired function of esophageal motility and incomplete relaxation of the lower esophagus sphincter, causing dysphagia and regurgitation. Rare cases of early-onset achalasia appear often in combination with further symptoms in a syndromic form as an inherited disease.

METHODS

Whole genome sequencing was used to investigate the genetic basis of isolated achalasia in a family of Tunisian origin. We analyzed the function of the affected protein with immunofluorescence and affinity chromatography study.

KEY RESULTS

A homozygous nonsense mutation was detected in murine retrovirus integration site 1 (MRVI1) gene (Human Genome Organisation Gene Nomenclature Committee (HGNC) approved gene symbol: IRAG1) encoding the inositol 1,4,5-trisphosphate receptor 1 (IP₃ R1)-associated cyclic guanosine monophosphate (cGMP) kinase substrate (IRAG). Sanger sequencing confirmed co-segregation of the mutation with the disease. Sequencing of the entire MRVI1 gene in 35 additional patients with a syndromic form of achalasia did not uncover further cases with MRVI1 mutations. Immunofluorescence analysis of transfected COS7 cells revealed GFP-IRAG with the truncating mutation p.Arg112* (transcript variant 1) or p.Arg121* (transcript variant 2) to be mislocalized in the cytoplasm and the nucleus. Co-transfection with cGMP-dependent protein kinase 1 isoform β (cGK1β) depicted a partial mislocalization of cGK1β due to mislocalized truncated IRAG. Isolation of protein complexes revealed that the truncation of this protein causes the loss of the interaction domain of IRAG with cGK1β.

CONCLUSIONS & INFERENCES

In individuals with an early onset of achalasia without further accompanying symptoms, MRVI1 mutations should be considered as the disease-causing defect.

Molecular basis of a new ovine model for human 3M syndrome-2

Background

Brachygnathia, cardiomegaly and renal hypoplasia syndrome (BCRHS, OMIA 001595–9940) is a previously reported recessively inherited disorder in Australian Poll Merino/Merino sheep. Affected lambs are stillborn with various congenital defects as reflected in the name of the disease, as well as short stature, a short and broad cranium, a small thoracic cavity, thin ribs and brachysternum. The BCRHS phenotype shows similarity to certain human short stature syndromes, in particular the human 3M syndrome-2. Here we report the identification of a likely disease-causing variant and propose an ovine model for human 3M syndrome-2.

Results

Eight positional candidate genes were identified among the 39 genes in the approximately 1 Mb interval to which the disease was mapped previously. Obscurin like cytoskeletal adaptor 1 (OBSL1) was selected as a strong positional candidate gene based on gene function and the resulting phenotypes observed in humans with mutations in this gene. Whole genome sequencing of an affected lamb (BCRHS3) identified a likely causal variant ENSOARG00000020239:g.220472248delC within OBSL1. Sanger sequencing of seven affected, six obligate carrier, two phenotypically unaffected animals from the original flock and one unrelated control animal validated the variant. A genotyping assay was developed to genotype 583 animals from the original flock, giving an estimated allele frequency of 5%.

Conclusions

The identification of a likely disease-causing variant resulting in a frameshift (p.(Val573Trpfs*119)) in the OBSL1 protein has enabled improved breeding management of the implicated flock. The opportunity for an ovine model for human 3M syndrome and ensuing therapeutic research is promising given the availability of carrier ram semen for BCRHS.

A broad range of symptoms in allgrove syndrome: single center experience in Southeast Anatolia

BACKGROUND

Allgrove syndrome (OMIM 231550) is a rare autosomal recessive disease characterized by non-CAH primary adrenal insufficiency (non-CAH PAI), alacrima, and achalasia. It is caused by mutations in the AAAS gene. The syndrome is also associated with variable progressive neurological impairment and dermatological abnormalities.

METHODS AND RESULTS

We diagnosed 23 patients from 14 families with Allgrove syndrome, based on the presence of at least two characteristic symptoms, usually adrenal insufficiency and alacrima, between 2008 and 2018. A previously described nonsense variant of AAAS was detected in 19 patients from 12 families at homozygous state. Another novel homozygous mutation (c.394-397delCTGT) in AAAS was detected in four patients from two families. Presenting symptoms were alacrima (23/23; 100%), adrenal insufficiency (18/23; 78%), achalasia (13/23; 57%), short stature/growth retardation (16/23; 70%), hyperreflexia (15/23; 65%), palmoplantar hyperkeratosis (13/23; 57%), hyperpigmentation of the skin (10/23; 43%), hypoglycemia-induced convulsion (7/23; 30%), swallowing difficulty and vomiting (6/23; 26%). Serum DHEAS concentrations were low in all patients (23/23; 100%).

CONCLUSIONS

Clinical symptoms vary even among patients carrying the same mutation. Triple A syndrome should be considered in the etiology of non-CAH PAI in Arab populations and in Southeast Turkey. Any child with non-CAH PAI should be evaluated for the presence of alacrima and/or achalasia or family history of alacrima and/or achalasia. Children with alacrima and/or achalasia should also be investigated for adrenal insufficiency. Definitive molecular diagnosis is essential for early diagnosis and management of adrenal insufficiency, neurological symptoms, and growth retardation in patients and early diagnosis of as yet asymptomatic cases in the family, together with genetic counseling.

Evidence of GMPPA founder mutation in indigenous Guatemalan population associated with alacrima, achalasia, and mental retardation syndrome

Congenital disorders of glycosylation (CDG) are a heterogeneous group of inborn errors of metabolism mostly causing multisystem disease. In 2013, biallelic mutations in the GMPPA gene were described in association with one such CDG known as alacrima, achalasia, and mental retardation syndrome (AAMR). To date, 18 patients have been reported, nearly all displaying the same pathognomonic triad of symptoms described in the name. This condition shares considerable phenotypic overlap with Triple-A syndrome caused by biallelic mutations in the AAAS gene; however, AAMR lacks the characteristic adrenocortical findings associated with Triple-A syndrome. We report three patients from two unrelated families with the same homozygous GMPPA mutation (c.265dup, p.L89fs). Notably, both families reported indigenous Maya-Mam heritage and originated from the town of Concepción Chiquirichapa in Quezaltenango, Guatemala. Our cases help to expand the AAMR phenotype by outlining dysmorphic features not well described in the prior cases. Additionally, we encourage all providers with patients presenting with this unique triad of symptoms to consider sequencing of the GMPPA gene. Special consideration should be given to families of Guatemalan Maya-Mam ancestry who may also have this identified founder mutation. Finally, this condition may indeed be underdiagnosed based on a review of the literature.

A novel homozygous nonsense mutation of VPS13B associated with previously unreported features of Cohen syndrome

Cohen syndrome (CS) is a rare autosomal recessive disorder associated with mutations in the vacuolar protein sorting 13 homolog B (VPS13B; formerly COH1) gene. The core clinical phenotype comprises a characteristic facial gestalt, marked developmental delay, and myopia. Additional, nonobligatory features include obesity, microcephaly, short stature, muscular hypotonia, scoliosis, narrow hands and feet, progressive retinopathy, as well as neutropenia. Here we report a novel homozygous nonsense mutation in the VPS13B gene and previously undescribed clinical features in a 19-year-old woman with developmental delay, intellectual disability, and a particular facial appearance. The patient showed several features consistent with CS. In addition, the parents observed congenital alacrima and anhidrosis persisting until onset of puberty. The diagnosis was not established based on the clinical phenotype. We performed whole-genome sequencing and identified a novel homozygous nonsense mutation c.62T>G (NM_152564.4), p.(Leu21*) in the VPS13B gene. Our findings extended the previously reported phenotype of CS. We conclude that transient, prepubertal alacrima and anhidrosis are part of the phenotypic spectrum of CS associated with a novel homozygous nonsense mutation in the VPS13B gene.

Gastrointestinal dysfunction in autism displayed by altered motility and achalasia in Foxp1 +/- mice

Gastrointestinal dysfunctions in individuals with autism spectrum disorder are poorly understood, although they are common among this group of patients. FOXP1 haploinsufficiency is characterized by autistic behavior, language impairment, and intellectual disability, but feeding difficulties and gastrointestinal problems have also been reported. Whether these are primary impairments, the result of altered eating behavior, or side effects of psychotropic medication remains unclear. To address this question, we investigated Foxp1 ^+/- mice reflecting FOXP1 haploinsufficiency. These animals show decreased body weight and altered feeding behavior with reduced food and water intake. A pronounced muscular atrophy was detected in the esophagus and colon, caused by reduced muscle cell proliferation. Nitric oxide-induced relaxation of the lower esophageal sphincter was impaired and achalasia was confirmed in vivo by manometry. Foxp1 targets (Nexn, Rbms3, and Wls) identified in the brain were dysregulated in the adult Foxp1 ^+/- esophagus. Total gastrointestinal transit was significantly prolonged due to impaired colonic contractility. Our results have uncovered a previously unknown dysfunction (achalasia and impaired gut motility) that explains the gastrointestinal disturbances in patients with FOXP1 syndrome, with potential wider relevance for autism.

Refinements of LC-MS/MS Spectral Counting Statistics Improve Quantification of Low Abundance Proteins

Mass spectrometry-based spectral count has been a common choice of label-free proteome quantification due to the simplicity for the sample preparation and data generation. The discriminatory nature of spectral count in the MS data-dependent acquisition, however, inherently introduces the spectral count variation for low-abundance proteins in multiplicative LC-MS/MS analysis, which hampers sensitive proteome quantification. As many low-abundance proteins play important roles in cellular processes, deducing low-abundance proteins in a quantitatively reliable manner greatly expands the depth of biological insights. Here, we implemented the Moment Adjusted Imputation error model in the spectral count refinement as a post PLGEM-STN for improving sensitivity for quantitation of low-abundance proteins by reducing spectral count variability. The statistical framework, automated spectral count refinement by integrating the two statistical tools, was tested with LC-MS/MS datasets of MDA-MB468 breast cancer cells grown under normal and glucose deprivation conditions. We identified about 30% more quantifiable proteins that were found to be low-abundance proteins, which were initially filtered out by the PLGEM-STN analysis. This newly developed statistical framework provides a reliable abundance measurement of low-abundance proteins in the spectral count-based label-free proteome quantification and enabled us to detect low-abundance proteins that could be functionally important in cellular processes.

GDP-Mannose Pyrophosphorylase: A Biologically Validated Target for Drug Development Against Leishmaniasis

Leishmaniases are neglected tropical diseases that threaten about 350 million people in 98 countries around the world. In order to find new antileishmanial drugs, an original approach consists in reducing the pathogenic effect of the parasite by impairing the glycoconjugate biosynthesis, necessary for parasite recognition and internalization by the macrophage. Some proteins appear to be critical in this way, and one of them, the GDP-Mannose Pyrophosphorylase (GDP-MP), is an attractive target for the design of specific inhibitors as it is essential for Leishmania survival and it presents significant differences with the host counterpart. Two GDP-MP inhibitors, compounds A and B, have been identified in two distinct studies by high throughput screening and by a rational approach based on molecular modeling, respectively. Compound B was found to be the most promising as it exhibited specific competitive inhibition of leishmanial GDP-MP and antileishmanial activities at the micromolar range with interesting selectivity indexes, as opposed to compound A. Therefore, compound B can be used as a pharmacological tool for the development of new specific antileishmanial drugs.

Curation and bioinformatic analysis of strabismus genes supports functional heterogeneity and proposes candidate genes with connections to RASopathies

Strabismus refers to the misalignment of the eyes and occurs in 2-4% of individuals. The low-resolution label "strabismus" covers a range of heterogeneous defects, which makes it challenging to unravel this condition. Consequently a coherent understanding of the causes is lacking. Here, we attempt to gain a better understanding of the underlying genetics by combining gene curation, diverse bioinformatic analyses (including gene ontology, pathway mapping, expression and network-based methods) and literature review. Through a phenotype-based curation process, we identify high-confidence and permissive sets of 54 and 233 genes potentially involved in strabismus. These genes can be grouped into 10 modules that together span a heterogeneous set of biological and molecular functions, and can be linked to clinical sub-phenotypes. Multiple lines of evidence associate retina and cerebellum biology with the strabismus genes. We further highlight a potential role of the Ras-MAPK pathway. Independently, sets of 11 genes and 15 loci tied to strabismus with definitive genetic basis have been compiled from the literature. We identify strabismus candidate genes for 5 of the 15 reported loci (CHD7; SLC9A6; COL18A1, COL6A2; FRY, BRCA2, SPG20; PARK2). Finally, we synthesize a Strabismus Candidate Gene Collection, which together with our curated gene sets will serve as a resource for future research. The results of this informatics study support the heterogeneity and complexity of strabismus and point to specific biological pathways and brain regions for future focus.

Triple-A syndrome: a wide spectrum of adrenal dysfunction

OBJECTIVE

Triple-A or Allgrove syndrome is an autosomal recessive disorder due to mutations in the AAAS gene, which encodes a nucleoporin named ALADIN. It is characterized by a classical clinical triad: alacrima, achalasia and adrenal insufficiency, the canonic symptoms that are associated with progressive peripheral neuropathy. Only a few cohorts have been reported. The objective of the present study was to characterize the various spectra of adrenal function in Triple-A patients.

METHODS

A retrospective clinical and biological monitoring of 14 patients (10 families) was done in a single multidisciplinary French center. All had AAAS gene sequenced and adrenal function evaluation.

RESULTS

Nine different AAAS mutations were found, including one new mutation: c.755G>C, p.(Trp252Ser). Regarding adrenal function, defects of the zona fasciculata and reticularis were demonstrated by increased basal ACTH levels and low DHEAS levels in all cases regardless of the degree of glucocorticoid deficiency. In contrast, mineralocorticoid function was always conserved: i.e., normal plasma renin level associated with normal aldosterone level. The main prognostic feature was exacerbation of neuropathy and cognitive disorders.

CONCLUSIONS

These data suggest that, in Triple-A patients, adrenal function can be deficient, insufficient or compensated. In our cohort after the first decade of life, there does not appear to be any degradation of adrenal function over time. However, patients with compensated adrenal function should be informed and educated to manage a glucocorticoid replacement therapy in case of stressful conditions, with no need for systematic long-term treatment.

A Novel GMPPA Mutation in Two Adult Sisters with Achalasia, Alacrima, Short Stature, Dysmorphism, and Intellectual Disability

The alacrima, achalasia, and mental retardation syndrome (AAMR) is a newly described autosomal recessive disorder characterized by the onset of these 3 main features at birth or in early infancy. At present, only 16 cases have been reported. Recently, it was shown that AAMR is due to mutations in the guanosine diphosphate (GDP)-mannose pyrophosphorylase A (GMPPA) gene. These mutations induce a significant GDP-mannose overload, which may affect protein glycosylation. Herein, for the first time, we describe 2 adult sisters with AAMR with a previously not reported deleterious homozygous missense mutation c.1118G>C (p.Arg373Pro) in the GMPPA gene, born to healthy consanguineous heterozygous parents from an ancient endogamous population. The main symptoms in both sisters started soon after birth with achalasia and feeding difficulties, requiring surgical treatment. Both sisters showed alacrima identified during the first months of life, delayed psychomotor development, speech delay, facial dysmorphism, limb defects, short stature, and moderate intellectual disability. Alacrima and feeding difficulties due to achalasia during the neonatal period or first months of life, in the absence of adrenal cortical insufficiency, should spur to investigate AAMR by sequencing the GMPPA gene.

"Crying without tears" as an early diagnostic sign-post of triple A (Allgrove) syndrome: two case reports

BACKGROUND

Triple A syndrome (or Allgrove syndrome) is a rare autosomal recessive disorder characterized by alacrima, achalasia, adrenal insufficiency and autonomic/neurological abnormalities. The majority of cases are caused by mutations in the AAAS gene located on chromosome 12q13. However, the clinical picture as well as genetic testing may be complex since symptomatology is variable and mutations cannot be identified in all clinically diagnosed patients. We present two unrelated patients with triple-A syndrome illustrating the importance of alacrima as an early clinical sign.

CASE PRESENTATION

A 3.5 year old girl presented with repeated hypoglycaemic myoclonic events. Adrenal insufficiency was diagnosed. In addition, alacrima, obvious since early infancy, was incidentally reported by the mother and finally lead to the clinical diagnosis of triple A syndrome. This was confirmed by positive mutation analysis of the AAAS gene. The second patient, an 8 months old boy was presented because of anisocoria and unilateral optic atrophy. MRI revealed cerebellar vermis hypotrophy. Psychomotor retardation, failure to thrive, and frequent vomiting lead to further diagnostic work-up. Achalasia was diagnosed radiologically. In addition, the mother mentioned absence of tears since birth leading to the clinical diagnosis of triple A syndrome. In contrast to the first cases genetic testing was negative.

CONCLUSION

These two patients illustrate the heterogeneity of triple A syndrome in both terms, clinical expression and genetic testing. We particularly aim to stress the importance of alacrima, which should be considered as a red flag symptom. Further differential diagnosis is required in every child affected by alacrima.

Congenital disorders of glycosylation (CDG): Quo vadis?

The survey summarizes in its first part the current status of knowledge on the Congenital Disorders of Glycosylation (CDG) with regard to their phenotypic spectrum, diagnostic and therapeutic strategies, and pathophysiology. It documents the clinical and basic research activities, and efforts to involve patients and their families. In the second part, it tries to look into the future of CDG. More specific biomarkers are needed for fast CDG diagnosis and treatment monitoring. Whole genome sequencing will play an increasingly important role in the molecular diagnosis of unsolved CDG. Epigenetic defects are expected to join the rapidly expanding genetic and allelic heterogeneity of the CDG family. Novel treatments are urgently needed particularly for PMM2-CDG, the most prevalent CDG. Patient services such as apps should be developed e.g. to document the natural history and monitor treatment. Networking (EURO-CDG, the European Reference Networks (MetabERN)) is an efficient tool to disseminate knowledge and boost collaboration at all levels. The final goal is of course to improve the quality of life of the patients and their families.

Isolated Hearing Impairment Caused by SPATA5 Mutations in a Family with Variable Phenotypic Expression

Biallelic mutations in the SPATA5 gene, encoding ATPase family protein, are an important cause of newly recognized epileptic encephalopathy classified as epilepsy, hearing loss, and mental retardation syndrome (EHLMRS, OMIM: 616577). Herein we describe a family in which two SPATA5 mutations with established pathogenicity (p.Thr330del and c.1714+1G>A) were found in the proband and her younger sister. The proband had a similar clinical picture to the previous descriptions of EHLMRS. In the sister, the only manifestation was an isolated sensorineural hearing loss. Our findings extend the phenotypic spectrum of SPATA5-associated diseases and indicate that SPATA5 defects may account for a fraction of isolated sensorineural hearing impairment cases.

Muscle Pathology as a Diagnostic Clue to Allgrove Syndrome

Allgrove or triple A syndrome is a rare autosomal recessive disorder that can present with a variable range of multi-system manifestations, including optic atrophy, cerebellar ataxia, upper and lower motoneuron signs and various neuropathic abnormalities. These cases are a diagnostic challenge, particularly when the eponymous combination of achalasia, Addisonianism and alacrima is incomplete. Therefore, it is in the differential diagnosis for multisystem conditions and should be known to pathologists who diagnose disorders of skeletal muscle. Here, we describe new findings in skeletal muscle histology from the case of a boy of consanguineous Turkish origin whose achalasia provided the only specific clinical clue to the diagnosis. These include myocyte nuclear abnormalities with partially abnormal anti-lamin A/C immunohistochemistry and altered nuclear ultrastructure but without overt abnormalities of nuclear pore morphology. In this case, the condition was associated with a hitherto unreported c.762delC mutation in the nucleoporin gene AAAS.

Biochemical analysis of leishmanial and human GDP-Mannose Pyrophosphorylases and selection of inhibitors as new leads

Leishmaniases are an ensemble of diseases caused by the protozoan parasite of the genus Leishmania. Current antileishmanial treatments are limited and present main issues of toxicity and drug resistance emergence. Therefore, the generation of new inhibitors specifically directed against a leishmanial target is an attractive strategy to expand the chemotherapeutic arsenal. GDP-Mannose Pyrophosphorylase (GDP-MP) is a prominent therapeutic target involved in host-parasite recognition which has been described to be essential for parasite survival. In this work, we produced and purified GDP-MPs from L. mexicana (LmGDP-MP), L. donovani (LdGDP-MP), and human (hGDP-MP), and compared their enzymatic properties. From a rationale design of 100 potential inhibitors, four compounds were identified having a promising and specific inhibitory effect on parasite GDP-MP and antileishmanial activities, one of them exhibits a competitive inhibition on LdGDP-MP and belongs to the 2-substituted quinoline series.

Mutations in CRLF1 cause familial achalasia

We here report a family from Libya with three siblings suffering from early onset achalasia born to healthy parents. We analyzed roughly 5000 disease-associated genes by a next-generation sequencing (NGS) approach. In the analyzed sibling we identified two heterozygous variants in CRLF1 (cytokine receptor-like factor 1). Mutations in CRLF1 have been associated with autosomal recessive Crisponi or cold-induced sweating syndrome type 1 (CS/CISS1), which among other symptoms also manifests with early onset feeding difficulties. Segregation analysis revealed compound heterozygosity for all affected siblings, while the unaffected mother carried the c.713dupC (p.Pro239Alafs*91) and the unaffected father carried the c.178T>A (p.Cys60Ser) variant. The c.713dupC variant has already been reported in affected CS/CISS1 patients, the pathogenicity of the c.178T>A variant was unclear. As reported previously for pathogenic CRLF1 variants, cytokine receptor-like factor 1 protein secretion from cells transfected with the c.178T>A variant was severely impaired. From these results we conclude that one should consider a CRLF1-related disorder in early onset achalasia even if other CS/CISS1 related symptoms are missing.

Clinical and Laboratory Diagnosis of Peroxisomal Disorders

The peroxisomal disorders (PDs) are a heterogeneous group of genetic diseases in man caused by an impairment in peroxisome biogenesis or one of the metabolic functions of peroxisomes. Thanks to the revolutionary technical developments in gene sequencing methods and their increased use in patient diagnosis, the field of genetic diseases in general and peroxisomal disorders in particular has dramatically changed in the last few years. Indeed, several novel peroxisomal disorders have been identified recently and in addition it has been realized that the phenotypic spectrum of patients affected by a PD keeps widening, which makes clinical recognition of peroxisomal patients increasingly difficult. Here, we describe these new developments and provide guidelines for the clinical and laboratory diagnosis of peroxisomal patients.

A novel TRAPPC11 mutation in two Turkish families associated with cerebral atrophy, global retardation, scoliosis, achalasia and alacrima

BACKGROUND

Triple A syndrome (MIM #231550) is associated with mutations in the AAAS gene. However, about 30% of patients with triple A syndrome symptoms but an unresolved diagnosis do not harbour mutations in AAAS.

OBJECTIVE

Search for novel genetic defects in families with a triple A-like phenotype in whom AAAS mutations are not detected.

METHODS

Genome-wide linkage analysis, whole-exome sequencing and functional analyses were used to discover and verify a novel genetic defect in two families with achalasia, alacrima, myopathy and further symptoms. Effect and pathogenicity of the mutation were verified by cell biological studies.

RESULTS

We identified a homozygous splice mutation in TRAPPC11 (c.1893+3A>G, [NM_021942.5], g.4:184,607,904A>G [hg19]) in four patients from two unrelated families leading to incomplete exon skipping and reduction in full-length mRNA levels. TRAPPC11 encodes for trafficking protein particle complex subunit 11 (TRAPPC11), a protein of the transport protein particle (TRAPP) complex. Western blot analysis revealed a dramatic decrease in full-length TRAPPC11 protein levels and hypoglycosylation of LAMP1. Trafficking experiments in patient fibroblasts revealed a delayed arrival of marker proteins in the Golgi and a delay in their release from the Golgi to the plasma membrane. Mutations in TRAPPC11 have previously been described to cause limb-girdle muscular dystrophy type 2S (MIM #615356). Indeed, muscle histology of our patients also revealed mild dystrophic changes. Immunohistochemically, β-sarcoglycan was absent from focal patches.

CONCLUSIONS

The identified novel TRAPPC11 mutation represents an expansion of the myopathy phenotype described before and is characterised particularly by achalasia, alacrima, neurological and muscular phenotypes.

Transgenic Rescue of the LARGEmyd Mouse: A LARGE Therapeutic Window?

LARGE is a glycosyltransferase involved in glycosylation of α-dystroglycan (α-DG). Absence of this protein in the LARGE_myd mouse results in α-DG hypoglycosylation, and is associated with central nervous system abnormalities and progressive muscular dystrophy. Up-regulation of LARGE has previously been proposed as a therapy for the secondary dystroglycanopathies: overexpression in cells compensates for defects in multiple dystroglycanopathy genes. Counterintuitively, LARGE overexpression in an FKRP-deficient mouse exacerbates pathology, suggesting that modulation of α-DG glycosylation requires further investigation. Here we demonstrate that transgenic expression of human LARGE (LARGE-LV5) in the LARGE_myd mouse restores α-DG glycosylation (with marked hyperglycosylation in muscle) and that this corrects both the muscle pathology and brain architecture. By quantitative analyses of LARGE transcripts we also here show that levels of transgenic and endogenous LARGE in the brains of transgenic animals are comparable, but that the transgene is markedly overexpressed in heart and particularly skeletal muscle (20–100 fold over endogenous). Our data suggest LARGE overexpression may only be deleterious under a forced regenerative context, such as that resulting from a reduction in FKRP: in the absence of such a defect we show that systemic expression of LARGE can indeed act therapeutically, and that even dramatic LARGE overexpression is well-tolerated in heart and skeletal muscle. Moreover, correction of LARGE_myd brain pathology with only moderate, near-physiological LARGE expression suggests a generous therapeutic window.

Genetics of enteric neuropathies

Abnormal development or disturbed functioning of the enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, is associated with the development of neuropathic gastrointestinal motility disorders. Here, we review the underlying molecular basis of these disorders and hypothesize that many of them have a common defective biological mechanism. Genetic burden and environmental components affecting this common mechanism are ultimately responsible for disease severity and symptom heterogeneity. We believe that they act together as the fulcrum in a seesaw balanced with harmful and protective factors, and are responsible for a continuum of symptoms ranging from neuronal hyperplasia to absence of neurons.

Prospective phenotyping of NGLY1-CDDG, the first congenital disorder of deglycosylation

PURPOSE

The cytosolic enzyme N-glycanase 1, encoded by NGLY1, catalyzes cleavage of the β-aspartyl glycosylamine bond of N-linked glycoproteins, releasing intact N-glycans from proteins bound for degradation. In this study, we describe the clinical spectrum of NGLY1 deficiency (NGLY1-CDDG).

METHODS

Prospective natural history protocol.

RESULTS

In 12 individuals ages 2 to 21 years with confirmed, biallelic, pathogenic NGLY1 mutations, we identified previously unreported clinical features, including optic atrophy and retinal pigmentary changes/cone dystrophy, delayed bone age, joint hypermobility, and lower than predicted resting energy expenditure. Novel laboratory findings include low cerebral spinal fluid (CSF) total protein and albumin and unusually high antibody titers toward rubella and/or rubeola following vaccination. We also confirmed and further quantified previously reported findings noting that decreased tear production, transient transaminitis, small feet, a complex hyperkinetic movement disorder, and varying degrees of global developmental delay with relatively preserved socialization are the most consistent features.

CONCLUSION

Our prospective phenotyping expands the clinical spectrum of NGLY1-CDDG, offers prognostic information, and provides baseline data for evaluating therapeutic interventions.Genet Med 19 2, 160-168.

KONJAC1 and 2 Are Key Factors for GDP-Mannose Generation and Affect l-Ascorbic Acid and Glucomannan Biosynthesis in Arabidopsis

Humans are unable to synthesize l-ascorbic acid (AsA), yet it is required as a cofactor in many critical biochemical reactions. The majority of human dietary AsA is obtained from plants. In Arabidopsis thaliana, a GDP-mannose pyrophosphorylase (GMPP), VITAMIN C DEFECTIVE1 (VTC1), catalyzes a rate-limiting step in AsA synthesis: the formation of GDP-Man. In this study, we identified two nucleotide sugar pyrophosphorylase-like proteins, KONJAC1 (KJC1) and KJC2, which stimulate the activity of VTC1. The kjc1kjc2 double mutant exhibited severe dwarfism, indicating that KJC proteins are important for growth and development. The kjc1 mutation reduced GMPP activity to 10% of wild-type levels, leading to a 60% reduction in AsA levels. On the contrary, overexpression of KJC1 significantly increased GMPP activity. The kjc1 and kjc1kjc2 mutants also exhibited significantly reduced levels of glucomannan, which is also synthesized from GDP-Man. Recombinant KJC1 and KJC2 enhanced the GMPP activity of recombinant VTC1 in vitro, while KJCs did not show GMPP activity. Yeast two-hybrid assays suggested that the stimulation of GMPP activity occurs via interaction of KJCs with VTC1. These results suggest that KJCs are key factors for the generation of GDP-Man and affect AsA level and glucomannan accumulation through the stimulation of VTC1 GMPP activity.

Homozygous missense mutation in the LMAN2L gene segregates with intellectual disability in a large consanguineous Pakistani family

BACKGROUND

Intellectual disability (ID) is a neurodevelopmental disorder affecting 1%-3% of the population worldwide. It is characterised by high phenotypic and genetic heterogeneity and in most cases the underlying cause of the disorder is unknown. In our study we investigated a large consanguineous family from Baluchistan, Pakistan, comprising seven affected individuals with a severe form of autosomal recessive ID (ARID) and epilepsy, to elucidate a putative genetic cause.

METHODS AND RESULTS

Whole exome sequencing (WES) of a trio, including a child with ID and epilepsy and its healthy parents that were part of this large family, revealed a homozygous missense variant p.R53Q in the lectin mannose-binding 2-like (LMAN2L) gene. This homozygous variant was co-segregating in the family with the phenotype of severe ID and infantile epilepsy; unaffected family members were heterozygous variant carriers. The variant was predicted to be pathogenic by five different in silico programmes and further three-dimensional structure modelling of the protein suggests that variant p.R53Q may impair protein-protein interaction. LMAN2L (OMIM: 609552) encodes for the lectin, mannose-binding 2-like protein which is a cargo receptor in the endoplasmic reticulum important for glycoprotein transport. Genome-wide association studies have identified an association of LMAN2L to different neuropsychiatric disorders.

CONCLUSION

This is the first report linking LMAN2L to a phenotype of severe ARID and seizures, indicating that the deleterious homozygous p.R53Q variant very likely causes the disorder.

Enzyme complexity in intermediary metabolism

A good appraisal of the function of enzymes is essential for the understanding of inborn errors of metabolism. However, it is clear now that the 'one gene, one enzyme, one catalytic function' rule oversimplifies the actual situation. Genes often encode several related proteins, which may differ in their subcellular localisation, regulation or function. Furthermore, enzymes often show several catalytic activities. In some cases, this is because they are multifunctional, possessing two or more different active sites that catalyse different, physiologically related reactions. In enzymes with broad specificity or in multispecificity enzymes, a single type of catalytic site performs the same reaction on different physiological substrates at similar rates. Enzymes that act physiologically in only one reaction often show nonetheless substrate promiscuity: they act at low rates on compounds that resemble their physiological substrate(s), thus forming non-classical metabolites, which are in some cases eliminated by metabolite repair. In addition to their catalytic role, enzymes may have moonlighting functions, i.e. non-catalytic functions that are most often not related with their catalytic activity. Deficiency in such functions may participate in the phenotype of inborn errors of metabolism. Evolution has also made that some enzymes have lost their catalytic activity to become allosteric proteins.

Mannose metabolism: more than meets the eye

Mannose is a simple sugar with a complex life. It is a welcome therapy for genetic and acquired human diseases, but it kills honeybees and blinds baby mice. It could cause diabetic complications. Mannose chemistry, metabolism, and metabolomics in cells, tissues and mammals can help explain these multiple systemic effects. Mannose has good, bad or ugly outcomes depending on its steady state levels and metabolic flux. This review describes the role of mannose at cellular level and its impact on organisms.