Clinical, radiological and computational studies on two novel GNPTG variants causing mucolipidosis III gamma phenotypes with varying severity.

Mucolipidosis III gamma (ML III γ) is a slowly progressive disorder that affects multiple parts of the body such as the skeleton, joints, and connective tissue structures. It is caused by pathogenic variants in the GNPTG gene that provides instructions for producing the γ subunit of GlcNAc-1-phosphotransferase. In this study we aim to characterize clinical findings and biological insights on two novel GNPTG variants causing ML III γ phenotypes with varying severity. We report on two siblings with ML III γ bearing the previously undescribed c.477C > G (p.Y159*) nonsense variant in a homozygous state as well as a patient with ML III γ bearing the novel c.110 + 19_111-17del variant in a homozygous state. These variants were revealed by whole-exome sequencing and Sanger sequencing, respectively. Their parents, who are heterozygotes for the same mutation, are healthy. The clinical and radiographic presentation of ML III γ in our patients who had c.477C > G (p.Y159*) variant is consistent with a relatively severe form of the disease, which is further supported by a working three-dimensional model of the GlcNAc-1-phosphotransferase γ subunit. On the other hand, it is seen that our patient who carries the c.110 + 19_111-17del variant has a milder phenotype. Our findings help broaden the spectrum of GNPTG variants causing ML III γ and offer structural and mechanistic insights into loss of GlcNAc-1-phosphotransferase γ subunit function.