Early onset LGMDR19 with unusual features related to GMPPB gene in South Indian siblings with variable phenotype

Guanosine diphosphate mannose (GDP-mannose) pyrophosphorylase B (GMPPB) is a cytoplasmic enzyme that catalyzes the synthesis of GDP-mannose, a crucial substrate for several glycosylation pathways. Reports of pathogenic variants in the GMPPB gene are infrequent. As of April 2023, 109 cases with pathogenic variants in the GMPPB gene have been reported worldwide. Here, we present two siblings born of consanguineous parentage from Southern India. This is a retrospective study on genetically confirmed siblings with GMPPB pathogenic variants. The siblings, a 15-year-old girl, and a 13-year-old boy, presented with progressive limb-girdle weakness and cataracts from early childhood. The girl had exertion-induced breathlessness and mental subnormality. Creatine kinase levels were 5750 and 4320 IU/L. An echocardiogram of the heart revealed global hypokinesia, moderate left ventricular dysfunction, and mild mitral regurgitation with dilation of the left atrium and left ventricle in the girl child. Exome sequencing showed a homozygous pathogenic variant (NM₀21971. 4 (GMPPB): c. 358A>G (p. Met120Val) in Exon 4 of GMPPB gene in both the siblings. Thus, we report an unusual early onset LGMD phenotype of GMPPB-related disorder with cardiac involvement and cataracts from a single family.