Congenital disorders of glycosylation (CDG) are rare inherited disorders resulting from defects in cellular glycosylation machinery. Albumin has recently been shown to be N-glycosylated at two non-canonical glycosylation sites. We applied multiplexed mass spectrometry-based glycoproteomics to identify site-specific N-glycosylation alterations in albumin from patients with PMM2-CDG, MPI-CDG, SRD5A3-CDG, MAN1B1-CDG and PGM1-CDG. Our findings demonstrate that the glycosylation of albumin is indeed affected in CDG and indicate a potential role for albumin-derived glycopeptides as diagnostic biomarkers.

Rafiq syndrome (RAFQS) is a rare autosomal recessive disorder that is classified as a type II congenital disorder of glycosylation (CDG-II), and caused by MAN1B1 gene mutation. To date, 24 pathogenic MAN1B1 mutations have been reported in association with MAN1B1-CDG. However, the underlying pathogenic mechanisms remain poorly understood. In this study, we recruited a consanguineous family from Pakistan with multiple affected individuals exhibiting mild facial dysmorphism, developmental delay, and intellectual disability. Utilizing exome sequencing and homozygosity mapping, we identified a novel MAN1B1 mutation (c.772_775del) that co-segregated with RAFQS in this family. Analysis of public single-cell transcriptomic data revealed that MAN1B1 is predominantly expressed in dorsal progenitors and intermediate excitatory neurons during human brain development. Knockdown of Man1b1 in primarily cultured mouse excitatory neurons disrupted axon growth, dendrite formation, and spine maturation, and could not be rescued by truncated variants identified in the family. Furthermore, in utero, electroporation experiments revealed that Man1b1 knockdown in the murine cortex impaired neural stem cells' proliferation and differentiation, as well as cortical neuron migration. Collectively, these findings elucidate a critical role for MAN1B1 in the etiology of RAFQS and demonstrate that loss-of-function mutation in MAN1B1 disrupt neuro-developmental processes, providing mechanistic insights into the pathogenesis of this disorder.

Rafiq syndrome, MAN1B1-CDG, was described in 2010 and associated with genetic mutation in MAN1B1 gene in 2011. The disorder follows an autosomal recessive pattern of inheritance and typically presents with specific facial dysmorphism, intellectual disability, developmental delay, obesity, and hypotonia. The syndrome belongs to a group of metabolic disorders called Congenital Glycosylation Disorders (CGD). In this study, we discuss a 5-year-old male from Palestine who presented with developmental delay, hypotonia, characteristic facial dysmorphisms, impulsive behaviors, inability to speak, cryptorchidism, and other manifestations. This constellation of manifestations raised suspicion of a genetic disorder, prompting whole exome sequencing (WES), which revealed the presence of a homozygous likely pathogenic variant in the MAN1B1 gene (c.1976T>G)(p.Phe659Cys). We also reviewed all previously documented cases and compared the clinical features among them. After reviewing the family pedigree and its suspected cases, we found that the 2 most frequent features among them are intellectual disability and facial dysmorphism, whereas the least frequent one is truncal obesity. We discussed the importance of providing genetic counseling to parents of children with this and other rare, autosomal recessive disorders to prevent new cases from appearing.

Congenital disorders of glycosylation (CDG), inherited metabolic diseases caused by defects in glycosylation, are characterized by a high frequency of intellectual disability (ID) and various clinical manifestations. Two siblings with ID, dysmorphic features, and epilepsy were examined using mass spectrometry of serum transferrin, which revealed a CDG type 2 pattern. Whole-exome sequencing showed that both patients were homozygous for a novel pathogenic variant of MAN1B1 (NM_016219.4:c.1837del) inherited from their healthy parents. We conducted a HPLC analysis of sialylated N-linked glycans released from total plasma proteins and characterized the α1,2-mannosidase I activity of the lymphocyte microsome fraction. The accumulation of monosialoglycans was observed in MAN1B1-deficient patients, indicating N-glycan-processing defects. The enzymatic activity of MAN1B1 was compromised in patient-derived lymphocytes. The present patients exhibited unique manifestations including early-onset epileptic encephalopathy and cerebral infarction. They also showed coagulation abnormalities and hypertransaminasemia. Neither sibling had truncal obesity, which is one of the characteristic features of MAN1B1-CDG.

MAN1B1-CDG is a multisystem disorder caused by mutations in MAN1B1, encoding the endoplasmic reticulum mannosyl-oligosaccharide alpha-1,2-mannnosidase. A defect leads to dysfunction within the degradation of misfolded glycoproteins. We present two additional patients with MAN1B1-CDG and a resulting defect in endoplasmic reticulum-associated protein degradation. One patient (P2) is carrying the previously undescribed p.E663K mutation. A therapeutic trial in patient 1 (P1) using disulfiram with the rationale to generate an attenuation of translation and thus a balanced, restored ER glycoprotein synthesis failed. No improvement of the transferrin glycosylation profile was seen.