Familial hypomagnesemia with hypercalciuria and nephrocalcinosis: unusual clinical associations and novel claudin16 mutation in an Egyptian family.
BACKGROUND
Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive tubular disorder that eventually progresses to renal failure, depending upon the extent of nephrocalcinosis. Its basic pathogenesis is impaired tubular resorption of magnesium and calcium in the thick ascending limb of the loop of Henle (TAL) due to a genetic defect in paracellin-1 (a tight junction protein expressed in TAL). Mutations of the claudin16 gene (CLDN16), formerly called paracellin-1 gene (PCLN-1), have been linked to FHHNC.


METHODS
An extended Egyptian family with more than one member affection by nephrocalcionsis was included and thoroughly investigated in the current study after giving informed consent. Thorough history was taken for polyuria, polydipsia and hypocalcemia symptoms, as well as clinical examination with stress on anthropometric measurements and radiological evaluation for kidneys and bones. Laboratory workup for the differential diagnosis of nephrocalcinosis was done: complete urinalysis, including urinary calcium excretion, urine pH and electrolytes, arterial blood gas (ABG), serum electrolytes (sodium, potassium, calcium, magnesium and phosphorous), renal function tests as well as parathyroid and gonadotropin-sex hormone assay. DNA extraction from peripheral blood leukocytes was done followed by amplification using primers previously described, purification and finally sequencing to analyze each exon of the CLDN16 gene.


RESULTS
Two sibs for a consanguineous couple were affected by nephrocalcinosis and showed persistent hypocalcemia, hypercalciuria, nephrocalcinosis with persistently alkaline urine and ocular manifestations in the form of congenital cataracts, high myopia and retinal abnormalities. The elder sib showed genitourinary abnormalities in the form of hypospadias and cryptorchidism. These two sibs had a homozygous two-base deletion in exon 1 of the CLDN16 gene (C. 233_234 del GG; Ins C), causing a frame shift mutation (Arg55 fs); however, their parents were heterozygote carriers for that mutation.


CONCLUSIONS
The above-mentioned clinical data in the two affected sibs together with the family history of end-stage renal disease associated with nephrocalcinosis and high myopia suggested a diagnosis of FHHNC, which was confirmed for the first time in an Egyptian family by a novel mutation in exon 1 of the CLDN16 gene. Genitourinary associations with FHHNC have not yet been reported in the literature. Here, we will try to highlight the principles of mutation detection based on sequencing with the use of the online NCBI databases, statistics and other search tools.
Al-Haggar
Mohammad
M
Genetics Unit, Mansoura University Children's Hospital, Mansoura, Egypt. m.alhaggar@yahoo.co.uk.
Bakr
Ashraf
A
Nephrology Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
Tajima
Toshihiro
T
Department of Pediatrics, Hokkaido University School of Medicine, Hokkaido, Japan.
Fujieda
Kenji
K
Department of Pediatrics, Asahikawa Medical College, Asahikawa, Japan.
Hammad
Ayman
A
Nephrology Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
Soliman
Othman
O
Genetics Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
Darwish
Ahmad
A
Nephrology Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
Al-Said
Afaf
A
Genetics Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
Yahia
Sohier
S
Genetics Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
Abdel-Hady
Dina
D
Genetics Unit, Mansoura University Children's Hospital, Mansoura, Egypt.
D001483:Base Sequence; D057167:Claudins; D003241:Consanguinity; D004252:DNA Mutational Analysis; D004534:Egypt; D005091:Exons; D016368:Frameshift Mutation; D020022:Genetic Predisposition to Disease; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D053565:Hypercalciuria / Q000175:diagnosis / Q000235:genetics* / Q000628:therapy; D033162:Incidental Findings; D007676:Kidney Failure, Chronic / Q000235:genetics*; D008275:Magnesium Deficiency / Q000175:diagnosis / Q000235:genetics* / Q000628:therapy; D008565:Membrane Proteins / Q000235:genetics*; D008969:Molecular Sequence Data; D009397:Nephrocalcinosis / Q000175:diagnosis / Q000235:genetics* / Q000628:therapy; D010375:Pedigree; D010641:Phenotype; D015499:Renal Tubular Transport, Inborn Errors / Q000175:diagnosis / Q000235:genetics* / Q000628:therapy
D057167:Claudins; D008565:Membrane Proteins; C502010:claudin 16