Bi-Allelic Loss-of-Function Variant in MAN1B1 Cause Rafiq Syndrome and Developmental Delay

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 <i>MAN1B1</i> gene mutation. To date, 24 pathogenic <i>MAN1B1</i> 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 <i>MAN1B1</i> mutation (c.772_775del) that co-segregated with RAFQS in this family. Analysis of public single-cell transcriptomic data revealed that <i>MAN1B1</i> is predominantly expressed in dorsal progenitors and intermediate excitatory neurons during human brain development. Knockdown of <i>Man1b1</i> 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 <i>Man1b1</i> 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 <i>MAN1B1</i> in the etiology of RAFQS and demonstrate that loss-of-function mutation in <i>MAN1B1</i> disrupt neuro-developmental processes, providing mechanistic insights into the pathogenesis of this disorder.