Clinical features and molecular basis of pseudohypoaldosteronism type 1.

Pseudohypoaldosteronism (PHA) type 1 is a disease showing mineralocorticoid resistance in the kidney and/or other mineralocorticoid target tissues. Patients with PHA1 present very high plasma aldosterone and renin levels, but they develop excessive salt wasting. There are three types of PHA1. The systemic form of PHA1 is inherited in an autosomal recessive manner and causes severe life-long salt loss in multiple target tissues, such as sweat glands, salivary glands, the colonic epithelium, and the lung. In the systemic form of PHA1, life-long salt supplementation is necessary. The second type is the renal form, where aldosterone resistance is shown only in the kidney, and its inheritance is autosomal dominant. In the renal form of PHA1, salt supplementation generally becomes unnecessary by 1-3 yr of age. The third type is the secondary PHA1, which is strongly associated with urinary tract infections and/or urinary tract malformations. This review summarizes the clinical features and molecular basis of PHA1. Understanding of its pathogenesis can be helpful for the early diagnosis and clinical care of affected children with PHA1.

A Japanese Family with Central Hypothyroidism Caused by a Novel IGSF1 Mutation.

BACKGROUND: Hemizygous mutations in the immunoglobulin superfamily member 1 (IGSF1) gene have been demonstrated to cause congenital central hypothyroidism in males. This study reports a family with a novel mutation in the IGSF1 gene located on the long arm of the X chromosome. PATIENT FINDINGS: A two-month-old boy was diagnosed with central hypothyroidism because of prolonged jaundice. A thyrotropin-releasing hormone (TRH) stimulation test indicated dysfunction in both the hypothalamus and the pituitary gland, and prompted the IGSF1 gene to be analyzed. The patient had a novel nonsense variant, c.2713C>T (p.Q905X), in exon 14 of the IGSF1 gene. Studies of the family revealed that the patient's sister and mother were heterozygous carriers of the IGSF1 mutation. The patient's maternal uncle carried the same mutation as the proband but had no overt symptoms. The mother and uncle started levothyroxine supplementation because of subclinical hypothyroidism. SUMMARY: A novel mutation (c.2713C>T, p.Q905X) of the IGSF1 gene was identified that causes congenital central hypothyroidism in a Japanese family. The findings further expand the clinical heterogeneity of this entity.

Clinical and molecular analysis of six Japanese patients with a renal form of pseudohypoaldosteronism type 1.

Pseudohypoaldosteronism type 1 (PHA1) is a rare condition characterized by neonatal salt loss with elevated plasma aldosterone and renin levels. Two types of PHA1 have been described: an autosomal recessive systemic form and an autosomal dominant renal form, in which the target organ defect is confined to the renal tubules. The dominant renal form of PHA1 is caused by heterozygous mutations in the NR3C2 gene, which encodes the mineralocorticoid receptor (MR). We determined clinical and biochemical parameters in two familial and four sporadic Japanese patient and analyzed the status of the NR3C2 gene. Failure to thrive was noted in five of the six patients. In one of the familial cases, the mother had an episode of failure to thrive when she was a toddler, but received no medical treatment. NaCl supplementation was discontinued in four of the six patients after they reached one year of age and they have grown normally thereafter. However, in one patient, 9 g/day of salt has been required to maintain serum Na concentration after 1 year of age. Analysis of NR3C2 identified three novel mutations [c. C1951T (p.R651X), c.304_305delGC (p.A102fsX103), c.del 603A (p.T201fsX34)] and one previously reported mutation [c.A2839G (p.947X)]. p.R651X was identified in one familial case and one unrelated sporadic patient. The patient who has been supplemented with large amount of salt was heterozygous for c.del 603A in exon 2. In conclusion, our study expands the spectrum of phenotypes, and characterized mutations of NR3C2 in the renal form of PHA1.

Increased Na reabsorption via the Na-Cl cotransporter in autosomal recessive pseudohypoaldosteronism.

BACKGROUND: The autosomal recessive form of pseudohypoaldosteronism type 1 (AR-PHA1) is caused by loss-of-function mutations in the epithelial sodium channel subunit genes and is characterized by a multisystemic and lifelong severe salt-wasting tendency. However, we observed a male AR-PHA1 patient who exhibited less frequent salt wasting with advancing age, despite the cessation of daily salt supplementation. OBJECTIVE: To elucidate the mechanism for the above phenomenon. METHODS: We evaluated the sodium-reabsorption ability of his distal nephrons (from the distal convoluted tubules to the collecting ducts) and compared it to that of a patient with the dominant form of PHA1 (AD-PHA1) carrying a heterozygous NR3C2 (mineralocorticoid receptor) gene mutation. In addition, immunoblotting of the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) protein was conducted using urine samples from the AR- and AD-PHA1 patients. RESULTS: The levels of sodium reabsorption that occurred via the distal nephrons were almost identical in the two PHA1 patients, despite their different molecular pathogeneses. Immunoblotting showed an increased urinary NCC protein level in the AR-PHA1 patient. CONCLUSION: Taken together, increased sodium reabsorption via the upregulation of the expression of NCC might have been responsible, at least in part, for the clinical improvement seen in an AR-PHA1 patient.

Gitelman's syndrome with mental retardation.

A 56-year-old mentally retarded Japanese woman (intelligence quotient: 49) was admitted to our hospital with the chief complaints of headache, dizziness, vomiting, and lower limb paralysis. Laboratory tests showed severe hypokalemia, metabolic alkalosis, hypomagnesemia, and hypocalciuria. These findings suggested a diagnosis of Gitelman's syndrome (GS). We examined the thiazide-sensitive Na-Cl cotransporter (TSC) gene for the mutations that can be responsible for Gitelman's syndrome, and confirmed the diagnosis. After potassium and magnesium supplementation, her paralysis improved dramatically. The marriage of her parents was consanguineous. She had nine siblings (all with mental retardation), among whom five had died of unknown causes during childhood. Familial mental retardation has never been detected before in Gitelman's syndrome. Here we report a rare case of Gitelman's syndrome with familial mental retardation.

Novel TRPM6 mutations in 21 families with primary hypomagnesemia and secondary hypocalcemia.

Primary hypomagnesemia with secondary hypocalcemia is a rare autosomal recessive disorder characterized by profound hypomagnesemia associated with hypocalcemia. Pathophysiology is related to impaired intestinal absorption of magnesium accompanied by renal magnesium wasting as a result of a reabsorption defect in the distal convoluted tubule. Recently, mutations in the TRPM6 gene coding for TRPM6, a member of the transient receptor potential (TRP) family of cation channels, were identified as the underlying genetic defect. Here, the results of a TRPM6 mutational analysis of 21 families with 28 affected individuals are presented. In this large patient cohort, a retrospective clinical evaluation based on a standardized questionnaire was also performed. Genotype analysis revealed TRPM6 mutations in 37 of 42 expected mutant alleles. Sixteen new TRPM6 mutations were identified, including stop mutations, frame-shift mutations, splice-site mutations, and deletions of exons. Electrophysiologic analysis of mutated ion channels after heterologous expression in Xenopus oocytes proved complete loss of function of TRPM6. Clinical evaluation revealed a homogeneous clinical picture at manifestation with onset in early infancy with generalized cerebral convulsions. Initial laboratory evaluation yielded extremely low serum magnesium levels, low serum calcium levels, and inadequately low parathyroid hormone levels. Treatment usually consisted of acute intravenous magnesium supplementation leading to relief of clinical symptoms and normocalcemia, followed by lifelong oral magnesium supplementation. Serum magnesium levels remained in the subnormal range despite adequate therapy. This is best explained by a disturbed magnesium conservation in the distal convoluted tubule, which emerged in all patients upon magnesium supplementation. Delay of diagnosis resulted in permanent neurologic damage in three patients.

Two heterozygous mutations of CLDN16 in a Japanese patient with FHHNC.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC, MIN 248250) is a rare autosomal recessive tubular disorder that eventually progresses to renal failure. However, the progression to end-stage renal failure can vary from patient to patient. A primary defect is related to impaired tubular resorption of magnesium and calcium in the thick ascending limb of Henle's loop. Recently, paracellin-1 was identified as a renal tight junction protein predominantly expressed in TAL. Mutations of its gene (CLDN16) have been shown to cause FHHNC. We describe a sporadic Japanese case of FHHNC. The male patient showed hematuria, hypercalciuria, and nephrocalcinosis at 5 years of age. Hypomagnesemia was also noticed at this time. As renal function gradually deteriorated, further evaluation was performed at 14 years of age and a diagnosis of FHHNC was made. Despite several medications (magnesium supplementation, citrate, and hydrochlorothiazide), he eventually progressed to renal insufficiency at 19 years of age. Analysis of the CLDN16 gene demonstrated two heterozygous mutations (R149Q and R216C). Mutations of the same amino acids have already been described in FHHNC and thus these mutations might be the cause of the disease in our patient. Hence, we confirm the genetic impairment of the CLDN16 gene in a Japanese patient with FHHNC.