Polycystic Kidney Disease with Hyperinsulinemic Hypoglycemia Caused by a Promoter Mutation in Phosphomannomutase 2

Abstract

Hyperinsulinemic hypoglycemia (HI) and congenital polycystic kidney disease (PKD) are rare, genetically heterogeneous disorders. The co-occurrence of these disorders (HIPKD) in 17 children from 11 unrelated families suggested an unrecognized genetic disorder. Whole-genome linkage analysis in five informative families identified a single significant locus on chromosome 16p13.2 (logarithm of odds score 6.5). Sequencing of the coding regions of all linked genes failed to identify biallelic mutations. Instead, we found in all patients a promoter mutation (c.-167G>T) in the phosphomannomutase 2 gene (PMM2), either homozygous or in trans with PMM2 coding mutations. PMM2 encodes a key enzyme in N-glycosylation. Abnormal glycosylation has been associated with PKD, and we found that deglycosylation in cultured pancreatic β cells altered insulin secretion. Recessive coding mutations in PMM2 cause congenital disorder of glycosylation type 1a (CDG1A), a devastating multisystem disorder with prominent neurologic involvement. Yet our patients did not exhibit the typical clinical or diagnostic features of CDG1A. In vitro, the PMM2 promoter mutation associated with decreased transcriptional activity in patient kidney cells and impaired binding of the transcription factor ZNF143. In silico analysis suggested an important role of ZNF143 for the formation of a chromatin loop including PMM2 We propose that the PMM2 promoter mutation alters tissue-specific chromatin loop formation, with consequent organ-specific deficiency of PMM2 leading to the restricted phenotype of HIPKD. Our findings extend the spectrum of genetic causes for both HI and PKD and provide insights into gene regulation and PMM2 pleiotropy.

Keywords: PMM2; ZNF143; glycosylation; hyperinsulinemic hypoglycemia; polycystic kidney disease; promoter.

Figure 1.

Genetic studies identify a shared mutation. (A–E) Pedigrees of informative families used for linkage analysis. (F) The combined parametric multipoint linkage analysis reveals a single significant peak with a maximum LOD score of 6.5 on chromosome 16. (G) Sequencing of the linked region reveals the presence of the mutation c.-167G>T in the promoter of PMM2 in all patients. Shown are electropherogram traces of the region c.-173_–c.161 from a patient homozygous for the mutation, a control (wild-type) subject, and a heterozygote parent.

Figure 2.

Renal imaging and histology in children with HIPKD. (A) Ultrasound images of the kidney from patient 2.1, left kidney, age 11 years; (B) patient 2.2, right kidney, age 4 years; and (C) patient 6.1, right kidney, age 2 years. Note the presence of cysts of various sizes. Kidney length was >95th percentile for age in all patients. (D) Axial MRI image of the kidney from patient 2.1, age 11 years; (E and F) axial and coronal MRI images (without contrast) from patient 6.1, age 2 years. Note the massively enlarged kidneys with cysts of various sizes. (G) Macroscopic appearance of the nephrectomy specimen from patient 6.1, age 2 years, removed at the time of transplant. Note the large kidney size (20 cm longitudinal, normal <7.7 cm) and numerous macroscopic cysts. (H and I) Histology of the same kidney demonstrating multiple cysts lined by simple attenuated epithelium, with glomeruli noted in some (glomerulocystic disease; hematoxylin and eosin; original magnifications, ×20 and ×100, respectively). Dist, distance; Lt, left; Rt, right.

Figure 3.

Liver involvement in HIPKD. (A) Shown is an axial T2-weighted (STIR sequence) MRI image of patient 2.1 showing numerous small liver cysts. (B) Ultrasound image of the liver of patient 2.2 showing a cyst. (C and D) Photomicrographs of the liver biopsy specimen from patient 8.1, demonstrating abnormal and expanded portal tracts consistent with ductal plate malformation (hematoxylin and eosin; original magnifications, ×40 and ×100, respectively).

Figure 4.

The c.-167G>T mutation impairs promoter function. Luciferase activity is significantly reduced with mutant compared with wild-type promoter in (A) human pancreatic β cell line 1.1B4 and (B) human kidney cell line RPTEC/TERT1. Light gray bars show luciferase activity with wild-type promoter, black bars with the c.-167G>T mutation, and the crosshatched bars show activity with deletion of the ZNF143 binding site c.-163_-180del. Asterisk indicates significance (wild type versus respective mutation; P<0.01). (C) In kidney cells derived from patient 6.1, expression of the allele with the mutant promoter (c.-167G>T) is significantly reduced compared with the other allele. Asterisk indicates significance (P<0.01). (D) EMSA demonstrating reduced affinity of mutant promoter to ZNF143. Note the impaired binding of ZNF143 (indicated by arrow) to the mutant (lane 3) compared with wild-type (lane 1) PMM2 promoter. Binding is specific, because addition of excess unlabeled wild-type (lane 2) or mutant promoter probe (lane 4) abolishes binding to the labeled probes. Moreover, no specific binding is seen in the negative control (no ZNF143) to either wild-type (lane 5) or mutant (lane 6) promoter probe. (E) Densitometry confirms significantly reduced affinity of ZNF143 to mutant promoter (P<0.01). Bands from lane 1 and 3 indicated by arrow in (D) were measured by densitometry and results pooled from five independent experiments. (F) Deglycosylation alters insulin secretion in a murine pancreatic β cell line (MIN6). There is significant increase in insulin secretion after stimulation with high glucose (P<0.01), as expected. Note the significant (P<0.01) increase in insulin secretion with deglycosylation after further stimulation with phorbol-12 myristate-13 acetate (PMA).