Digenic Inheritance of Hepatocyte Nuclear Factor-1α and -1β With Maturity-Onset Diabetes of the Young, Polycystic Thyroid, and Urogenital Malformations

After an occasional blood glucose reading of 13.4 mmo/l, the diagnosis of diabetes was established in a 13-year-old girl. She was asymptomatic except for mild chronic lower abdominal discomfort. Her fasting glucose was 9.3 mmol/l, A1C was 8.5% (normal 4.2–6.1), and urine ketones were negative. After brief insulin treatment, she received 0.5 mg/day glimepiride, with a current A1C of 5.6%.

Her father (Fig. 1), diagnosed with diabetes at the age of 27 years, has received insulin since the age of 38 years. The paternal grandmother had diabetes treated with glibenclamide until she died 20 years ago, aged 62 years. Two asymptomatic sisters of the index patient were diagnosed with diabetes at the age of 14 years and 19 years. Their fasting serum glucose was 7.3 and 6.8 mmol/l, respectively, and A1C was 6.0 and 6.1%. Two hours after a 75-g oral glucose challenge, their glucose levels were 10.1 and 13.1 mmol/l, respectively.

Fasting C-peptide was 299.7 pmol/l in the index patient, and 599.4 and 566.1 pmol/l in her sisters; all had normal insulin sensitivity (homeostasis model assessment 2.0, 1.7, and 1.8, respectively) and normal BMI (<25 kg/m²). Liver, kidney, and thyroid function tests including serum thyrotropin, free T₄ and T₃, creatinine, and other standard laboratory parameters were normal in all sisters, and serum islet cell autoantibodies and GAD65 antibodies were negative.

β-Cell function was assessed after intravenous challenge with glucose (0.5 g/kg bolus) and l-arginine (0.7g/min for 30 min). In the index patient and her sisters, insulin secretion was significantly impaired after intravenous glucose (maximum serum insulin 223.8, 124.1, and 145.6 pmol/l, respectively). In contrast, there was a sustained insulin release after arginine infusion (maximum serum insulin 543.1, 363.0, and 655.0 pmol/l, respectively).

Abdominal ultrasound in the index patient showed agenesis of the right kidney and a didelphic uterus with hypoplastic right uterine horn and hemiatresia of the cervix, which was confirmed by hysteroscopy. In both of her sisters, no abnormalities of the urogenital system were identified by ultrasound. In the index patient and the younger sister, polycystic changes in both thyroid lobes with >50 cysts up to 4 mm in size were detected by high-resolution (12 MHz) ultrasound, and single thyroid cysts were identified in the father and the older sister. No cystic or other lesions in the liver were identified in any individual.

HNF-1β sequence analysis in the index patient revealed a heterozygous genomic missense variant (c.1006C>G) in exon 4, resulting in the substitution of a highly conserved residue (p.His336Asp) in the protein's transactivation domain. This novel variant was detected in 1 of 400 chromosomes in healthy individuals by denaturing high-performance liquid chromatography. Sequencing identified HNF-1β c.1006C>G in the younger sister and the father but not in the older diabetic sister. Thus, this mutation could not exclusively account for the diabetes phenotype in this family.

We next analyzed HNF-1α and identified a heterozygous genomic mutation (c.526 + 1delGTAA) in the canonic splice site of intron 2 in all diabetic individuals but not in unaffected family members (Fig. 1). This novel mutation is predicted to result in aberrant HNF-1α splicing (3), with the introduction of a premature termination codon at amino acid position 194 and deletion of the POU A and transactivation domains.

To rule out further modifying gene effects, additional candidate genes putatively involved in the HNF transcriptional network were sequenced. In the promoter of HNF-6, a sequential heterozygous single nucleotide polymorphism (c.1-400A>C, c.1-390C>A, and c.1-385G>A) was identified in all diabetic patients but also in the unaffected mother. No sequence variation was detected in the index patient's genomic DNA in the coding regions of HNF-4α, insulin promoter factor 1/pancreatic duodenal homeobox-1, NeuroD (causing MODY 1, 4, and 6, respectively), nor in HNF-6, HNF-3β, and chicken ovalbumin upstream promoter transcription factors I and II.

We have identified a novel heterozygous HNF-1α splice site mutation that segregates with diabetes and impaired glucose-dependent insulin secretion, typical of MODY3 (4). In addition, polycystic thyroid, renal, and genital abnormalities were found to extend the clinical phenotype of MODY in this kindred. In a recent report, renal agenesis has been described in two families with HNF-1α mutations, including a patient with a different HNF-1α splice site variant and a bicornute uterus (5). The association of HNF-1α mutants with a polycystic thyroid phenotype, however, has not been observed thus far. Considering that HNF-1α mutations are the most common cause of MODY, extrapancreatic manifestations seem overall rare in HNF-1α mutation carriers.

In several, but not all, diabetic patients in these kindred, a second mutation was identified in HNF-1β, leading to a nonconservative amino acid substitution in the highly conserved region of the transactivation domain. This molecular feature and the low allelic frequency suggest that c.1006C>G is indeed a pathogenic HNF-1β variant. Strikingly, urogenital and polycystic thyroid changes but not metabolic characteristics were associated with the mutant HNF-1β allele, while MODY segregated with the HNF-1α variant.

In polarized epithelial cells, HNF-1α and -1β cooperate in a network of transcriptional regulators including HNF-4α and -3β, and both HNF-1α and -β homo- and heterodimerize for DNA binding via their NH₂-terminal dimerization domains (6). It is conceivable that the extended MODY phenotype observed in these kindred may result from the digenic inactivation of HNF-1α and -1β. In the kidney, inactivation of HNF-1β inhibits the expression of the polycystic kidney disease gene Pkhd1 (7), and distinct functional characteristics of HNF-1β mutants lead to a spectrum of kidney malformations including polycystic phenotypes (8). HNF-1α and -1β interact with HNF-3β, a forkhead transcription factor expressed in early thyroid organogenesis and in the adult thyroid (9). The putative role of HNF-1 transcription factors in thyroid disease involving differential transactivation of HNF-3β or other target genes, however, must await experimental confirmation.