Pontocerebellar Hypoplasia Type 9: A New Case with a Novel Mutation and Review of Literature.

Pontocerebellar hypoplasia type 9 (PCH-9) is a very rare autosomal recessive neurodegenerative disorder. Affected infants present early with severe developmental delay, spasticity, with the unique magnetic resonance imaging picture of thin corpus callosum, atrophied pons, and cerebellum. It is caused by loss of function mutations in the AMPD2 gene, encoding for the adenosine monophosphate deaminase enzyme-paralog 2. This gene is expressed in different somatic tissues with high level of expression in cerebellum and its encoded enzyme catalyzes a critical step in de novo biosynthesis of purines and its deficiency in the developing neurons severely affects neuronal differentiation and cell viability. We clinically evaluated an Emirati patient presented with severe developmental and growth delay, as well as corpus callosum agenesis and atrophy of brainstem and cerebellum. We performed exome sequencing, Sanger sequencing, and segregation analysis to identify the genetic cause of the phenotype, followed by in silico and in vitro analysis. We identified the novel variant (NM_004037.9:c.1471G > A) in AMPD2 gene leading to a single amino acid substitution (p.Gly491Arg) in adenosine monophosphate deaminase-2 enzyme. This variant is predicted to be pathogenic using several in silico tools, and resulted in a decrease in the enzyme function in the patient's polymorphonuclear cells by 82% (95% confidence interval: 73.3-91.7%, p = 0.029) compared with the control. This data establishes that the affected child is affected by PCH-9. Furthermore, we review all reported cases in literature to summarize the main clinical features of this rare disease.

Spectrum of genetic disorders and gene variants in the United Arab Emirates national population: insights from the CTGA database.

Like many other Arab countries, the United Arab Emirates (UAE) has a relatively high prevalence of genetic disorders. Here we present the first review and analysis of all genetic disorders and gene variants reported in Emirati nationals and hosted on the Catalogue for Transmission Genetics in Arabs (CTGA), an open-access database hosting bibliographic data on human gene variants associated with inherited or heritable phenotypes in Arabs. To date, CTGA hosts 665 distinct genetic conditions that have been described in Emiratis, 621 of which follow a clear Mendelian inheritance. Strikingly, over half of these are extremely rare according to global prevalence rates, predominantly with an autosomal recessive mode of inheritance. This is likely due to the relatively high consanguinity rates within the Emirati population. The 665 conditions include disorders that are unique to the Emirati population, as well as clearly monogenic disorders that have not yet been mapped to a causal genetic locus. We also describe 1,365 gene variants reported in Emiratis, most of which are substitutions and over half are classified as likely pathogenic or pathogenic. Of these, 235 had not been reported on the international databases dbSNP and Clinvar, as of December 2022. Further analysis of this Emirati variant dataset allows a comparison of clinical significance as reported by Clinvar and CTGA, where the latter is derived from the study cited. A total of 307 pathogenic/likely pathogenic variants from CTGA's Emirati dataset, were classified as benign, variants of uncertain significance, or were missing a clinical significance or had not been reported by Clinvar. In conclusion, we present here the spectrum of genetic disorders and gene variants reported in Emiratis. This review emphasizes the importance of ethnic databases such as CTGA in addressing the underrepresentation of Arab variant data in international databases and documenting population-specific discrepancies in variant interpretation, reiterating the value of such repositories for clinicians and researchers, especially when dealing with rare disorders.

Entwined African and Asian genetic roots of medieval peoples of the Swahili coast.

The urban peoples of the Swahili coast traded across eastern Africa and the Indian Ocean and were among the first practitioners of Islam among sub-Saharan people1,2. The extent to which these early interactions between Africans and non-Africans were accompanied by genetic exchange remains unknown. Here we report ancient DNA data for 80 individuals from 6 medieval and early modern (AD 1250-1800) coastal towns and an inland town after AD 1650. More than half of the DNA of many of the individuals from coastal towns originates from primarily female ancestors from Africa, with a large proportion-and occasionally more than half-of the DNA coming from Asian ancestors. The Asian ancestry includes components associated with Persia and India, with 80-90% of the Asian DNA originating from Persian men. Peoples of African and Asian origins began to mix by about AD 1000, coinciding with the large-scale adoption of Islam. Before about AD 1500, the Southwest Asian ancestry was mainly Persian-related, consistent with the narrative of the Kilwa Chronicle, the oldest history told by people of the Swahili coast3. After this time, the sources of DNA became increasingly Arabian, consistent with evidence of growing interactions with southern Arabia4. Subsequent interactions with Asian and African people further changed the ancestry of present-day people of the Swahili coast in relation to the medieval individuals whose DNA we sequenced.

A homozygous MED11 C-terminal variant causes a lethal neurodegenerative disease.

PURPOSE: The mediator (MED) multisubunit-complex modulates the activity of the transcriptional machinery, and genetic defects in different MED subunits (17, 20, 27) have been implicated in neurologic diseases. In this study, we identified a recurrent homozygous variant in MED11 (c.325C>T; p.Arg109Ter) in 7 affected individuals from 5 unrelated families. METHODS: To investigate the genetic cause of the disease, exome or genome sequencing were performed in 5 unrelated families identified via different research networks and Matchmaker Exchange. Deep clinical and brain imaging evaluations were performed by clinical pediatric neurologists and neuroradiologists. The functional effect of the candidate variant on both MED11 RNA and protein was assessed using reverse transcriptase polymerase chain reaction and western blotting using fibroblast cell lines derived from 1 affected individual and controls and through computational approaches. Knockouts in zebrafish were generated using clustered regularly interspaced short palindromic repeats/Cas9. RESULTS: The disease was characterized by microcephaly, profound neurodevelopmental impairment, exaggerated startle response, myoclonic seizures, progressive widespread neurodegeneration, and premature death. Functional studies on patient-derived fibroblasts did not show a loss of protein function but rather disruption of the C-terminal of MED11, likely impairing binding to other MED subunits. A zebrafish knockout model recapitulates key clinical phenotypes. CONCLUSION: Loss of the C-terminal of MED subunit 11 may affect its binding efficiency to other MED subunits, thus implicating the MED-complex stability in brain development and neurodegeneration.

PDZD8 Disruption Causes Cognitive Impairment in Humans, Mice, and Fruit Flies.

BACKGROUND: The discovery of coding variants in genes that confer risk of intellectual disability (ID) is an important step toward understanding the pathophysiology of this common developmental disability. METHODS: Homozygosity mapping, whole-exome sequencing, and cosegregation analyses were used to identify gene variants responsible for syndromic ID with autistic features in two independent consanguineous families from the Arabian Peninsula. For in vivo functional studies of the implicated gene's function in cognition, Drosophila melanogaster and mice with targeted interference of the orthologous gene were used. Behavioral, electrophysiological, and structural magnetic resonance imaging analyses were conducted for phenotypic testing. RESULTS: Homozygous premature termination codons in PDZD8, encoding an endoplasmic reticulum-anchored lipid transfer protein, showed cosegregation with syndromic ID in both families. Drosophila melanogaster with knockdown of the PDZD8 ortholog exhibited impaired long-term courtship-based memory. Mice homozygous for a premature termination codon in Pdzd8 exhibited brain structural, hippocampal spatial memory, and synaptic plasticity deficits. CONCLUSIONS: These data demonstrate the involvement of homozygous loss-of-function mutations in PDZD8 in a neurodevelopmental cognitive disorder. Model organisms with manipulation of the orthologous gene replicate aspects of the human phenotype and suggest plausible pathophysiological mechanisms centered on disrupted brain development and synaptic function. These findings are thus consistent with accruing evidence that synaptic defects are a common denominator of ID and other neurodevelopmental conditions.

Bi-allelic null variant in matrix metalloproteinase-15, causes congenital cardiac defect, cholestasis jaundice, and failure to thrive.

Here, we delineate the phenotype of two siblings with a bi-allelic frameshift variant in MMP15 gene with congenital cardiac defects, cholestasis, and dysmorphism. Genome sequencing analysis revealed a recently reported homozygous frameshift variant (c.1058delC, p.Pro353Glnfs*102) in MMP15 gene that co-segregates with the phenotype in the family in a recessive mode of inheritance. Relative quantification of MMP15 mRNA showed evidence of degradation of the mutated transcript, presumably by nonsense mediated decay. Likewise, MMP15: p.Gly231Arg, a concurrently reported homozygous missense variant in another patient exhibiting a similar phenotype, was predicted to disrupt zinc ion binding to the MMP-15 enzyme catalytic domain, which is essential for substrate proteolysis, by structural modeling. Previous animal models and cellular findings suggested that MMP15 plays a crucial role in the formation of endocardial cushions. These findings confirm that MMP15 is an important gene in human development, particularly cardiac, and that its loss of function is likely to cause a severe disorder phenotype.

Spectrum of neuro-genetic disorders in the United Arab Emirates national population.

Clinical and molecular characterization of neuro-genetic disorders among UAE national patients seen in the Genetic Clinic at Tawam hospital over a period of 3 years. A retrospective chart review of all Emirati patients assessed by clinical geneticists due to neuro-genetic disorders including global developmental delay, ASD, ID, ADHD, and epilepsy in combination with abnormalities of other organ systems. Each patient had proper assessment including detailed history, three-generation family history, developmental history and detailed physical examination looking for other system involvement. Hearing test and ophthalmological examination were performed when needed. Magnetic resonance imaging (MRI) of the brain, echocardiogram, and renal ultrasound were pursued as indicated. Detailed psychological evaluation and psychometric assessment were done when indicated. The review was done for a period between January 2018 and December 2020. Genetic investigations included chromosome karyotype, FISH study, metabolic/biochemical tests, chromosome microarray, gene sequencing, targeted mutation testing, trio whole exome and trio genome sequencing. A total of 644 patients with developmental delay, ID, learning difficulty, ASD, ADHD, or NNDs, were seen in genetic clinic from January 2018 to December 2020. A total of 506 patients were included in this review, all completed the genetic evaluations during the study period. There were 398 (61.8%) males and 246 (38.2%) females, with a ratio of 1.6:1. Positive family history of NDD was documented in 132 families, while 115 families had negative history and family history was unknown/unclear in the remaining. Fifty seven (11.26% [57/506]) patients had positive microarray results. Hundred ninety seven (38.9% [197/506]) patients had positive molecular testing. Genetic disorders were found in 133 (67.5% [133/197]) and inborn errors of metabolism were found in 42 (21.3% [42/197]). Consanguinity was documented in 139 patients with positive molecular diagnoses (139/197, 70.5%). Sixty nine (35% [69/197]) patients had autosomal dominant disorders, majority were De Novo (84%). Ninety-five (48% [95/197]) patients had autosomal recessive diseases, 40 mutations involved inborn errors of metabolism and 50 mutations involved genetic disorders. Pathogenic variants causing both autosomal dominant and recessive disorders were found in 98 patients (49.7% [98/197]), likely pathogenic variants causing both autosomal dominant and recessive disorders were found in 66 patients (33.5% [66/197]). X-linked related disorders were found in 10 patients (5% [10/197]). Mitochondrial mutation was found in one patient. Novel mutations were found in 76 patients (76/197 i.e., 38.56%). Twenty two patients had variants of unknown significant. The remaining 252 studied patients (252/506 i.e., 49.8%), remained undiagnosed. This study shows that neuro-genetic disorders in the UAE are very heterogeneous at clinical and molecular levels. Using microarray, WES and WGS a diagnosis was reached in 50% of the patients while no diagnosis was reached in other half of the studied patients. It is possible that some mutations were missed by WGS and WES. However, it is also possible that many of disorders in UAE population are novel and the causative mutation is not yet discovered. More researches need to be done in this population to uncover the molecular basis of these disorders.

Combining exome/genome sequencing with data repository analysis reveals novel gene-disease associations for a wide range of genetic disorders.

PURPOSE: Within this study, we aimed to discover novel gene-disease associations in patients with no genetic diagnosis after exome/genome sequencing (ES/GS). METHODS: We followed two approaches: (1) a patient-centered approach, which after routine diagnostic analysis systematically interrogates variants in genes not yet associated to human diseases; and (2) a gene variant centered approach. For the latter, we focused on de novo variants in patients that presented with neurodevelopmental delay (NDD) and/or intellectual disability (ID), which are the most common reasons for genetic testing referrals. Gene-disease association was assessed using our data repository that combines ES/GS data and Human Phenotype Ontology terms from over 33,000 patients. RESULTS: We propose six novel gene-disease associations based on 38 patients with variants in the BLOC1S1, IPO8, MMP15, PLK1, RAP1GDS1, and ZNF699 genes. Furthermore, our results support causality of 31 additional candidate genes that had little published evidence and no registered OMIM phenotype (56 patients). The phenotypes included syndromic/nonsyndromic NDD/ID, oral-facial-digital syndrome, cardiomyopathies, malformation syndrome, short stature, skeletal dysplasia, and ciliary dyskinesia. CONCLUSION: Our results demonstrate the value of data repositories which combine clinical and genetic data for discovering and confirming gene-disease associations. Genetic laboratories should be encouraged to pursue such analyses for the benefit of undiagnosed patients and their families.

A recurrent, homozygous EMC10 frameshift variant is associated with a syndrome of developmental delay with variable seizures and dysmorphic features.

PURPOSE: The endoplasmic reticulum membrane complex (EMC) is a highly conserved, multifunctional 10-protein complex related to membrane protein biology. In seven families, we identified 13 individuals with highly overlapping phenotypes who harbor a single identical homozygous frameshift variant in EMC10. METHODS: Using exome, genome, and Sanger sequencing, a recurrent frameshift EMC10 variant was identified in affected individuals in an international cohort of consanguineous families. Multiple families were independently identified and connected via Matchmaker Exchange and internal databases. We assessed the effect of the frameshift variant on EMC10 RNA and protein expression and evaluated EMC10 expression in normal human brain tissue using immunohistochemistry. RESULTS: A homozygous variant EMC10 c.287delG (Refseq NM_206538.3, p.Gly96Alafs*9) segregated with affected individuals in each family, who exhibited a phenotypic spectrum of intellectual disability (ID) and global developmental delay (GDD), variable seizures and variable dysmorphic features (elongated face, curly hair, cubitus valgus, and arachnodactyly). The variant arose on two founder haplotypes and results in significantly reduced EMC10 RNA expression and an unstable truncated EMC10 protein. CONCLUSION: We propose that a homozygous loss-of-function variant in EMC10 causes a novel syndromic neurodevelopmental phenotype. Remarkably, the recurrent variant is likely the result of a hypermutable site and arose on distinct founder haplotypes.

Life-threatening arrhythmias with autosomal recessive TECRL variants.

AIMS: Sudden death and aborted sudden death have been observed in patients with biallelic variants in TECRL. However, phenotypes have only begun to be described and no data are available on medical therapy after long-term follow-up. METHODS AND RESULTS: An international, multi-centre retrospective review was conducted. We report new cases associated with TECRL variants and long-term follow-up from previously published cases. We present 10 cases and 37 asymptomatic heterozygous carriers. Median age at onset of cardiac symptoms was 8 years (range 1-22 years) and cases were followed for an average of 10.3 years (standard deviation 8.3), right censored by death in three cases. All patients on metoprolol, bisoprolol, or atenolol were transitioned to nadolol or propranolol due to failure of therapy. Phenotypes typical of both long QT syndrome and catecholaminergic polymorphic ventricular tachycardia (CPVT) were observed. We also observed divergent phenotypes in some cases despite identical homozygous variants. None of 37 heterozygous family members had a cardiac phenotype. CONCLUSION: Patients with biallelic pathogenic TECRL variants present with variable cardiac arrhythmia phenotypes, including those typical of long QT syndrome and CPVT. Nadolol and propranolol may be superior beta-blockers in this setting. No cardiac disease or sudden death was present in patients with a heterozygous genotype.

Phosphoglucomutase-1 deficiency: Early presentation, metabolic management and detection in neonatal blood spots.

Phosphoglucomutase 1 deficiency is a congenital disorder of glycosylation (CDG) with multiorgan involvement affecting carbohydrate metabolism, N-glycosylation and energy production. The metabolic management consists of dietary D-galactose supplementation that ameliorates hypoglycemia, hepatic dysfunction, endocrine anomalies and growth delay. Previous studies suggest that D-galactose administration in juvenile patients leads to more significant and long-lasting effects, stressing the urge of neonatal diagnosis (0-6 months of age). Here, we detail the early clinical presentation of PGM1-CDG in eleven infantile patients, and applied the modified Beutler test for screening of PGM1-CDG in neonatal dried blood spots (DBSs). All eleven infants presented episodic hypoglycemia and elevated transaminases, along with cleft palate and growth delay (10/11), muscle involvement (8/11), neurologic involvement (5/11), cardiac defects (2/11). Standard dietary measures for suspected lactose intolerance in four patients prior to diagnosis led to worsening of hypoglycemia, hepatic failure and recurrent diarrhea, which resolved upon D-galactose supplementation. To investigate possible differences in early vs. late clinical presentation, we performed the first systematic literature review for PGM1-CDG, which highlighted respiratory and gastrointestinal symptoms as significantly more diagnosed in neonatal age. The modified Butler-test successfully identified PGM1-CDG in DBSs from seven patients, including for the first time Guthrie cards from newborn screening, confirming the possibility of future inclusion of PGM1-CDG in neonatal screening programs. In conclusion, severe infantile morbidity of PGM1-CDG due to delayed diagnosis could be prevented by raising awareness on its early presentation and by inclusion in newborn screening programs, enabling early treatments and galactose-based metabolic management.

The pharmacological chaperone N-n-butyl-deoxygalactonojirimycin enhances ß-galactosidase processing and activity in fibroblasts of a patient with infantile GM1-gangliosidosis.

GM1-gangliosidosis, a lysosomal storage disorder, is associated with ~ 161 missense variants in the GLB1 gene. Affected patients present with ß-galactosidase (ß-Gal) deficiency in lysosomes. Loss of function in ER-retained misfolded enzymes with missense variants is often due to subcellular mislocalization. Deoxygalactonojirimycin (DGJ) and its derivatives are pharmaceutical chaperones that directly bind to mutated ß-Gal in the ER promoting its folding and trafficking to lysosomes and thus enhancing its activity. An Emirati child has been diagnosed with infantile GM1-gangliosidosis carrying the reported p.D151Y variant. We show that p.D151Y ß-Gal in patient's fibroblasts retained < 1% residual activity due to impaired processing and trafficking. The amino acid substitution significantly affected the enzyme conformation; however, p.D151Y ß-Gal was amenable for partial rescue in the presence of glycerol or at reduced temperature where activity was enhanced with ~ 2.3 and 7 folds, respectively. The butyl (NB-DGJ) and nonyl (NN-DGJ) derivatives of DGJ chaperoning function were evaluated by measuring their IC50s and ability to stabilize the wild-type ß-Gal against thermal degradation. Although NN-DGJ showed higher affinity to ß-Gal, it did not show a significant enhancement in p.D151Y ß-Gal activity. However, NB-DGJ promoted p.D151Y ß-Gal maturation and enhanced its activity up to ~ 4.5% of control activity within 24 h which was significantly increased to ~ 10% within 6 days. NB-DGJ enhancement effect was sustained over 3 days after washing it out from culture media. We therefore conclude that NB-DGJ might be a promising therapeutic chemical chaperone in infantile GM1 amenable variants and therefore warrants further analysis for its clinical applications.

Correction: A restricted spectrum of missense KMT2D variants cause a multiple malformations disorder distinct from Kabuki syndrome.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Regulation of human cerebral cortical development by EXOC7 and EXOC8, components of the exocyst complex, and roles in neural progenitor cell proliferation and survival.

PURPOSE: The exocyst complex is a conserved protein complex that mediates fusion of intracellular vesicles to the plasma membrane and is implicated in processes including cell polarity, cell migration, ciliogenesis, cytokinesis, autophagy, and fusion of secretory vesicles. The essential role of these genes in human genetic disorders, however, is unknown. METHODS: We performed homozygosity mapping and exome sequencing of consanguineous families with recessively inherited brain development disorders. We modeled an EXOC7 splice variant in vitro and examined EXOC7 messenger RNA (mRNA) expression in developing mouse and human cortex. We modeled exoc7 loss-of-function in a zebrafish knockout. RESULTS: We report variants in exocyst complex members, EXOC7 and EXOC8, in a novel disorder of cerebral cortex development. In EXOC7, we identified four independent partial loss-of-function (LOF) variants in a recessively inherited disorder characterized by brain atrophy, seizures, and developmental delay, and in severe cases, microcephaly and infantile death. In EXOC8, we found a homozygous truncating variant in a family with a similar clinical disorder. We modeled exoc7 deficiency in zebrafish and found the absence of exoc7 causes microcephaly. CONCLUSION: Our results highlight the essential role of the exocyst pathway in normal cortical development and how its perturbation causes complex brain disorders.

A restricted spectrum of missense KMT2D variants cause a multiple malformations disorder distinct from Kabuki syndrome.

PURPOSE: To investigate if specific exon 38 or 39 KMT2D missense variants (MVs) cause a condition distinct from Kabuki syndrome type 1 (KS1). METHODS: Multiple individuals, with MVs in exons 38 or 39 of KMT2D that encode a highly conserved region of 54 amino acids flanked by Val3527 and Lys3583, were identified and phenotyped. Functional tests were performed to study their pathogenicity and understand the disease mechanism. RESULTS: The consistent clinical features of the affected individuals, from seven unrelated families, included choanal atresia, athelia or hypoplastic nipples, branchial sinus abnormalities, neck pits, lacrimal duct anomalies, hearing loss, external ear malformations, and thyroid abnormalities. None of the individuals had intellectual disability. The frequency of clinical features, objective software-based facial analysis metrics, and genome-wide peripheral blood DNA methylation patterns in these patients were significantly different from that of KS1. Circular dichroism spectroscopy indicated that these MVs perturb KMT2D secondary structure through an increased disordered to ɑ-helical transition. CONCLUSION: KMT2D MVs located in a specific region spanning exons 38 and 39 and affecting highly conserved residues cause a novel multiple malformations syndrome distinct from KS1. Unlike KMT2D haploinsufficiency in KS1, these MVs likely result in disease through a dominant negative mechanism.

Evaluating the Role of MAST1 as an Intellectual Disability Disease Gene: Identification of a Novel De Novo Variant in a Patient with Developmental Disabilities.

Intellectual disability (ID) is one of the most common developmental disorders characterized by a congenital limitation in intellectual functioning and adaptive behavior. More than 800 genes have been implicated so far in the pathogenesis of syndromic and non-syndromic ID conditions with the actual number is expected to be over two thousand. The advent of next-generation sequencing resulted in the identification of many novel ID genes with new genes are being reported on weekly basis. The level of evidence on ID genes varies with some of them being preliminary. MAST1 have been hinted at as being causative of ID but the evidence has been very sketchy. Extensive search of the literature identified three heterozygous de novo missense variants in MAST1 as possible causes of syndromic ID in three individuals where intellectual disability has been a major feature. Using exome sequencing, we identified a novel missense variant c.3539T>G, p.(Leu1180Arg) in MAST1 in an Emirati patient with intellectual disability, microcephaly, and dysmorphic features. In silico pathogenicity prediction analyses predict that all the four missense variants reported in this study are likely to be damaging. Immunostaining of cells expressing human MAST1 showed that majority large proportion of the expressed protein is colocalized the microtubule filaments in the cytoplasm. However, the identified variant c.3539T>G, p.(Leu1180Arg) as well as the other three variants seem to localize in a similar pattern to wild-type indicating a disease mechanism not involving mis-targeting. We, therefore, suggest that mutations in MAST1 should be considered as strong candidates for intellectual disability in humans.

Correction: DYRK1A-related intellectual disability: a syndrome associated with congenital anomalies of the kidney and urinary tract.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A Novel Homozygous Missense Variant in the NAGA Gene with Extreme Intrafamilial Phenotypic Heterogeneity.

Schindler disease is a rare autosomal recessive lysosomal storage disorder caused by a deficiency in alpha-N-acetylgalactosaminidase (α-NAGA) activity due to defects in the NAGA gene. Accumulation of the enzyme's substrates results in clinically heterogeneous symptoms ranging from asymptomatic individuals to individuals with severe neurological manifestations. Here, a 5-year-old Emirati male born to consanguineous parents presented with congenital microcephaly and severe neurological manifestations. Whole genome sequencing revealed a homozygous missense variant (c.838C>A; p.L280I) in the NAGA gene. The allele is a reported SNP in the ExAC database with a 0.0007497 allele frequency. The proband's asymptomatic sister and cousin carry the same genotype in a homozygous state as revealed from the family screening. Due to the extreme intrafamilial heterogeneity of the disease as seen in previously reported cases, we performed further analyses to establish the pathogenicity of this variant. Both the proband and his sister showed abnormal urine oligosaccharide patterns, which is consistent with the diagnosis of Schindler disease. The α-NAGA activity was significantly reduced in the proband and his sister with 5.9% and 12.1% of the mean normal activity, respectively. Despite the activity loss, p.L280I α-NAGA processing and trafficking were not affected. However, protein molecular dynamic simulation analysis revealed that this amino acid substitution is likely to affect the enzyme's natural dynamics and hinders its ability to bind to the active site. Functional analysis confirmed the pathogenicity of the identified missense variant and the diagnosis of Schindler disease. Extreme intrafamilial clinical heterogeneity of the disease necessitates further studies for proper genetic counseling and management.