De novo HNF1 homeobox B mutation as a cause for chronic, treatment-resistant hypomagnesaemia

Summary 29-year-old female presenting with an 8-year history of unexplained hypomagnesaemia, which was severe enough to warrant intermittent inpatient admission for intravenous magnesium. Urinary magnesium was inappropriately normal in the context of hypomagnesaemia indicating magnesium wasting. Ultrasound imaging demonstrated unilateral renal cysts and computed tomography of kidneys, ureters and bladder showed a bicornuate uterus. Referral to genetic services and subsequent testing revealed a de novo HNF1B deletion. Learning points: HNF1B loss-of-function mutations are one of the most common monogenic causes of congenital anomalies of the kidney and urinary tract. Those with HNF1B mutations may have some of a constellation of features (renal and hepatic cysts, deranged liver function tests, maturity onset diabetes of the young type 5 (MODY5), bicornuate uterus, hyperparathyroidism, hyperuricaemic gout, but presenting features are highly heterogeneous amongst patients and no genotype/phenotype correlation exists. HNF1B mutations are inherited in an autosomal dominant pattern but up to 50% of cases are de novo. HNF1B mutations can be part of the Chr17q12 deletion syndrome, a contiguous gene deletion syndrome. Inorganic oral magnesium replacements are generally poorly tolerated with side effects of diarrhoea. Organic magnesium compounds, such as magnesium aspartate, are better absorbed oral replacement therapies.


Summary
29-year-old female presenting with an 8-year history of unexplained hypomagnesaemia, which was severe enough to warrant intermittent inpatient admission for intravenous magnesium. Urinary magnesium was inappropriately normal in the context of hypomagnesaemia indicating magnesium wasting. Ultrasound imaging demonstrated unilateral renal cysts and computed tomography of kidneys, ureters and bladder showed a bicornuate uterus. Referral to genetic services and subsequent testing revealed a de novo HNF1B deletion.

Background
This case presentation explores the history and treatment of a patient with an HNF1B mutation. This patient presented with hypomagnesaemia in her late teens -a less usual presentation of this condition; cases are more commonly picked up prenatally due to abnormal kidney echogenicity or cysts. We provide a review of the genetic basis for the constellation of features found in association with HNF1B mutations, discuss the steps taken to make a diagnosis and provide some guidance on oral magnesium replacement therapies and their relative merits.

Case presentation
A 29-year-old female presented with an eight-year history of hypomagnesaemia. This had been noted at the Correspondence should be addressed to M Korbonits Email m.korbonits@qmul.ac.uk

Learning points:
• HNF1B loss-of-function mutations are one of the most common monogenic causes of congenital anomalies of the kidney and urinary tract.
• Those with HNF1B mutations may have some of a constellation of features (renal and hepatic cysts, deranged liver function tests, maturity onset diabetes of the young type 5 (MODY5), bicornuate uterus, hyperparathyroidism, hyperuricaemic gout, but presenting features are highly heterogeneous amongst patients and no genotype/ phenotype correlation exists.
• HNF1B mutations are inherited in an autosomal dominant pattern but up to 50% of cases are de novo.
• HNF1B mutations can be part of the Chr17q12 deletion syndrome, a contiguous gene deletion syndrome.
• Inorganic oral magnesium replacements are generally poorly tolerated with side effects of diarrhoea. Organic magnesium compounds, such as magnesium aspartate, are better absorbed oral replacement therapies.
age of 21 years whilst being treated for mumps-related pancreatitis. The hypomagnesaemia caused symptoms of headaches and lethargy and replacement with magnesium glycerophosphate 4 mg three times daily had been instituted. It was suspected that her compliance with the medication was poor as the patient still required occasional inpatient admission for symptomatic hypomagnesaemia and received intravenous magnesium infusions.

Investigation
Serum magnesium was 0.51 mmol/L at presentation to our department, despite the oral replacement therapy. 24-h urinary magnesium (3.7 mmol/day, normal range: 3-5 mmol/day) was noted to be inappropriately normal in the context of low serum magnesium (0.46 mmol/L, normal range: 0.7-1 mmol/L) with hypocalciuria (24-h urinary calcium 0.8 mmol/day, normal range: 2.5-7.5 mmol/day). Serum parathormone was 4.5 pmol/L. Diabetes mellitus was excluded by a normal HbA1c (33 mmol/mol, non-diabetic <42 mmol/mol) and fasting glucose measurement (5 mmol/L, non-diabetic <6.1 mmol/L). Subsequent HbA1c checks have all been within normal range. Estimated glomerular filtration rate was 83 mL/min. CT scanning of the abdomen to exclude renal tract calcification revealed the presence of several hyperdense rounded lesions in the left kidney. The right kidney was normal. A bicornuate uterus was seen ( Fig. 1). A subsequent ultrasound (Fig. 2) demonstrated 3 cysts in the left kidney (upper pole 3.1 cm, mid kidney 1.7 cm and lower pole 1.4 cm). The liver, spleen, pancreas and bladder were normal.

Treatment
Initially, the patient was commenced on oral magnesium glycerol phosphate 4 mg three times daily. However, she remained periodically symptomatic and was admitted periodically for IV magnesium. Serum magnesium ran in the range of 0.46-0.54 mmol/L (normal range 0.7-1 mmol/L). Later, this dose was doubled for a trial period, with no resulting increase in serum magnesium (0.46 mmol/L). The patient was changed to oral magnesium aspartate 10 mmol twice daily with an appreciable increase in serum magnesium levels (0.57-0.61 mmol/L) and reduction of her symptoms.

Outcome and follow-up
Follow-up has been over a period of 9 years. Referral was made to a genetic testing service, where the patient was tested for an HNF1B mutation. A heterozygous whole gene deletion was identified in HNF1B. Subsequent array-based comparative genomic hybridization analysis demonstrated a 1.5 Mb deletion within chromosome 17q12 (34,822,460-36,375,192, GRCh37/hg19) (Fig. 3). Neither parent shared the deletion indicating that it was a de novo event in our patient. More recently, the patient has sought advice on conception and has been referred for pre-implantation screening to eliminate the risk of transmission of the HNF1B mutation.
A typical facial phenotype is observed in some patients with 17q12 deletions -high rounded forehead, arched eyebrows, small chin/set back lower jaw and downward slanting eyes. In retrospect, our patient has the latter two characteristics. Less commonly, they may be of short stature with height in the lower 3% of population and have spinal curvature (9). Our patient was shorter than average (152 cm) but was not in the bottom 3rd centile for height and she had exceeded mid-parental height prediction (father 159.5 cm, mother 150 cm, predicted height 148 cm).

Hypomagnesaemia
The occurrence of hypomagnesaemia is described with various types of HNF1B mutations (7,10,11). It is believed to occur through magnesium wasting in the renal distal convoluted tubule. The co-existence of hypermagnesuria and hypo/normocalciuria helps to localise its site of the dysfunction (7). Magnesium is predominately reabsorbed in the thick ascending limb of the loop of Henle and the distal convoluted tubule. Magnesium and calcium reabsorption in the thick ascending limb both rely on the adequate function of the intercellular tight junction proteins claudin 16 and 19. Failure of these prevents paracellular passage of both calcium and magnesium and results in the syndrome of familial hypercalciuric hypomagnesemia with nephrocalcinosis (12). As hypercalciuria is not accompanying hypomagnesaemia in HNF1B mutation patients, the site of abnormal magnesium handling is unlikely to be in the thick ascending limb .
In the distal convoluted tubule, HNF1B binds to the promoter of the FXYD domain containing ion transport regulator 2 (FXYD2) gene (7), which encodes the γ-subunit of a basal membrane Na + /K + -ATPase in the kidney. This γ-subunit helps to stabilise the α-subunit within the Na + / K + -ATPase. Loss of the γ-subunits affect the Na + -, K + -and ATP-binding affinities of the Na + /K + -ATPase (13). Although the exact method of extrusion of magnesium from the cells of the distal convoluted tubule into the circulation is not yet known, one possibility is that reduced activity in this Na + /K + -ATPase pump and a consequent reduction in Na + removal from the cell, results in a higher intracellular Na + concentration. This could then prevent its exchange for intracellular Mg 2+ through the soluble carrier family 41 member A1 (SLC41A1) (14) or A3 (SLC41A3) (15,16). Mechanisms believed to be involved in the re-absorption of magnesium in the distal convoluted tubule and its extrusion into the capillaries are shown in Fig. 4. We believe, our patient loses magnesium via the distal convoluted tubule.

Oral magnesium replacement
Our patient was not well controlled on magnesium glycerophosphate while she had significantly better control on magnesium aspartate. In a study using rats, magnesium was better absorbed from organic magnesium salt replacements (magnesium aspartate and particularly magnesium gluconate) than from inorganic salt replacements (e.g. magnesium sulphate and magnesium carbonate) (17). That some magnesium salts are better absorbed than others is reflected in our patient in respect of higher serum magnesium levels upon switching replacement regime from magnesium glycerophosphate to magnesium aspartate. The estimated average requirement for magnesium for a 19-to 30-year-old female is 255 mg (10.6 mmol)/day. This increases to 290 mg (12.7 mmol)/ day in pregnancy and 255 mg (11.3 mmol/day) in lactation (18). Our patient may therefore benefit from a small amount of additional magnesium therapy during these periods if she achieves conception.

MODY5
Although not seen in our patient, HNF1B mutations can also cause maturity onset diabetes of the young type 5 (MODY5). This is again thought to be mediated by the formation of cysts -this time in the pancreas, where mouse models have shown a reduction in expression of transcription factors downstream of Hnf1b, which are involved in the formation of pancreatic ducts and the development of endocrine cells. A reduction in the number of multipotent progenitor cells has also been noted, which resulted in pancreatic hypoplasia (19). In one study, it was noted that MODY5 generally occurred before the age of 30 years (20); therefore, our patient's chances of developing diabetes mellitus are now probably lower. We will follow her glycaemic state with annual HbA1c and fasting glucose levels.

Uterine anomalies and reproductive considerations
LHX1, also located in the deleted area, is a transcription factor, which is essential for the normal development and elongation of the female reproductive tract (21). Interestingly, HNF1B can activate LHX1 (22). Patients with HNF1B point mutations, therefore without 17q12 deletion and normal LHX1, can show genital abnormalities (23). In our patient, both mechanismsloss of LHX1 or loss of HNF1B-induced stimulation of the LHX1 promoter -could play a role in the development of the uterine abnormality. Bicornuate uterus is one of the most common uterine malformations (24), and it is associated with a spontaneous abortion rate of 36%, a pre-term birth-rate of 23% and a live birth-rate of 55.2% (24,25).
In summary, the heterozygous large deletion of 17q12 area explains the complex phenotype of this patient.

Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Figure 4
Mechanism of magnesium reabsorption into DCT cells and proposed mechanism for extrusion into the capillaries. Green channel, co-transporter; red channel, ion channel; orange channel, sodium potassium ATPase pump and purple channel, proposed mechanism for magnesium extrusion. EGFR, epidermal growth factor receptor.

Funding
This work was supported by a Wellcome Trust Clinical Training fellowship to CES (grant number 097970/Z/11/Z).