A novel intermediate mucolipidosis II/IIIαβ caused by GNPTAB mutation in the cytosolic N-terminal domain

Abstract

Mucolipidosis (ML) II and ML IIIα/β are allelic autosomal recessive metabolic disorders due to mutations in GNPTAB. The gene encodes the enzyme UDP-GlcNAc-1-phosphotransferase (GNPT), which is critical to proper trafficking of lysosomal acid hydrolases. The ML phenotypic spectrum is dichotomous. Criteria set for defining ML II and ML IIIα/β are inclusive for all but the few patients with phenotypes that span the archetypes. Clinical and biochemical findings of the 'intermediate' ML in eight patients with the c.10A>C missense mutation in GNPTAB are presented to define this intermediate ML and provide a broader insight into ML pathogenesis. Extensive clinical information, including radiographic examinations at various ages, was obtained from a detailed study of all patients. GNPTAB was sequenced in probands and parents. GNPT activity was measured and cathepsin D sorting assays were performed in fibroblasts. Intermediate ML patients who share the c.10A>C/p.K4Q mutation in GNPTAB demonstrate a distinct, consistent phenotype similar to ML II in physical and radiographic features and to ML IIIα/β in psychomotor development and life expectancy. GNPT activity is reduced to 7-12% but the majority of newly synthesized cathepsin D remains intracellular. The GNPTAB c.10A>C/p.K4Q missense allele results in an intermediate ML II/III with distinct clinical and biochemical characteristics. This delineation strengthens the utility of the discontinuous genotype-phenotype correlation in ML II and ML IIIα/β and prompts additional studies on the tissue-specific pathogenesis in GNPT-deficient ML.

Figure 1

GNPTAB c.10A>C/p.K4Q ML clinical features in three unrelated patients: short stature, normocephaly, coarse facies, limited mobility in all joints, short and broad hands with fingers postured in ulnar deviation. A₁ and A₂: Patient 1 at 8 and 12 years; B: Patient 2 at 6 years; C₁ and C₂: Patient 3 at 8 and 20 years.

Figure 2

GNPTAB c.10A>C/p.K4Q ML clinical features in five siblings: Similar findings as in Figure 1 are shown early and late in disorder's natural course. D: Patient 4 at 21.5 years; E: Patient 5 at 20 years; F: Patient 6 at 17.5 years; G: Patient 7 at 7.5 years; H: Patient 8 at 6 years.

Figure 3

Lateral radiographs of spinal columns in eight patients with GNPTAB c.10A>C/p.K4Q ML compared with one ML II and two ML III α/β patients. Thoracolumbar vertebral bodies remain significantly foreshortened with concave frontal and convex upper/lower margins; delayed and deficient ossification. Similar findings are seen in ML II. In ML IIIα/β vertebral bodies are oblong, and show mild platyspondyly and irregular margins higher anteriorly than posteriorly. In all types of ML ossification is most deficient in either T12 or L1 or in both, often at the top of thoracolumbar kyphoscoliosis. A. Patient 1 at 8 years; B: Patient 2 at 6 years; C: Patient 3 at 9 years; D: Patient 4 at 19 years; E: Patient 5 at 19 years; F: Patient 6 at 18 years; G: Patient 7 at 7 years; H: Patient 8 at 4 years; ML II: female ML II patient at 4 years; ML III-1: male with ML III α/β at 18 years; ML-III-2: female with ML III α/β at 13 years.

Figure 4

Hand radiographs in GNPTAB c.10A>C/p.K4Q ML patients: comparison with ML II and ML IIIα/β. DM in the hands is similar to that in ML II and distinctly more severe than in ML IIIα/β. Figures show shortening and diaphyseal widening in all tubular hand bones; severely delayed and deficient ossification of epiphyses, carpal bones and the distal ulnar and radial epiphyses; and apparent regression of mid-diaphyseal widening in older patients concomitant with progressive generalized osteopenia. A: Patient 1 at 8 years; B: Patient 2 at 4 years; C: Patient 3 at 9 years; D: Patient 4 at 9 years; E: Patient 5 at 19 years; H: Patient 8 at 6 years; ML II: female with ML II at 4 years; ML III-2: female with ML III α/β at 13 years.

Figure 5

Sorting of newly synthesized cathepsin D in control and ML fibroblasts GM01586 (ML II), patient 2 (K4Q compound heterozygote), and GM00113 (K4Q homozygote). Autoradiographs of ³⁵S-labeled immunoprecipitates of cell and secreted cathepsin D are shown. The percentage of intracellular enzyme is determined by dividing the amount of cell-associated cathepsin D by the total cathepsin D (cell-associated and secreted).