Cystinosis as a lysosomal storage disease with multiple mutant alleles: Phenotypic-genotypic correlations

Cystinosis is an autosomal recessive lysosomal storage disease with an unclear enzymatic defect causing lysosomal cystine accumulation with no corresponding elevation of plasma cystine levels leading to multisystemic dysfunction. The systemic manifestations include a proximal renal tubular defect (Fanconi-like), endocrinal disturbances, eye involvements, with corneal, conjunctival and retinal depositions, and neurological manifestations in the form of brain and muscle dysfunction. Most of the long-term ill effects of cystinosis are observed particularly in patients with long survival as a result of a renal transplant. Its responsible CTNS gene that encodes the lysosomal cystine carrier protein (cystinosin) has been mapped on the short arm of chromosome 17 (Ch17 p13). There are three clinical forms based on the onset of main symptoms: nephropathic infantile form, nephropathic juvenile form and non-nephropathic adult form with predominant ocular manifestations. Avoidance of eye damage from sun exposure, use of cystine chelators (cysteamine) and finally renal transplantation are the main treatment lines. Pre-implantation genetic diagnosis for carrier parents is pivotal in the prevention of recurrence.

Sporadic Fibrodysplasia Ossificans Progressiva in an Egyptian Infant with c.617G > A Mutation in ACVR1 Gene: A Case Report and Review of Literature

Fibrodysplasia ossificans progressiva (FOP) is an autosomal dominant severe musculoskeletal disease characterized by extensive new bone formation within soft connective tissues and unique skeletal malformations of the big toes which represent a birth hallmark for the disease. Most of the isolated classic cases of FOP showed heterozygous mutation in the ACVR1 gene on chromosome 2q23 that encodes a bone morphogenetic protein BMP (ALK2). The most common mutation is (c.617G > A) leading to the amino acid substitution of arginine by histidine (p.Arg206His). We currently report on an Egyptian infant with a sporadic classic FOP in whom c.617G > A mutation had been documented. The patient presented with the unique congenital malformation of big toe and radiological evidence of heterotopic ossification in the back muscles. The triggering trauma was related to the infant's head, however; neither neck region nor sites of routine intramuscular vaccination given during the first year showed any ossifications. Characterization of the big toe malformation is detailed to serve as an early diagnostic marker for this rare disabling disease.

A novel homozygous p.Arg527Leu LMNA mutation in two unrelated Egyptian families causes overlapping mandibuloacral dysplasia and progeria syndrome

Mandibuloacral dysplasia (MAD) is a rare disease resulting from a mutation of LMNA gene encoding lamins A and C. The most common mutation associated with this disease is a homozygous arginine 527 replacement by histidine. Three female patients originating from two unrelated families from Northeast Egypt were examined. Their growth was retarded; they had microcephaly, widened cranial sutures, prominent eyes and cheeks, micrognathia, dental crowding, hypoplastic mandible, acro-osteolysis of distal phalanges, and joint contractures. In addition, they presented some progeroid features, such as pinched nose, premature loss of teeth, loss of hair, scleroderma-like skin atrophy, spine rigidity, and waddling gait. The clinical presentation of the disease varied between the patient originating from Family 1 and patients from Family 2, suggesting that unknown, possibly epigenetic factors, modify the course of the disease. The first symptoms of the disease appeared at the age of 2.5 (a girl from Family 1), 5, and 3 years (girls from Family 2). All patients had the same, novel homozygous c.1580G>T LMNA mutation, resulting in the replacement of arginine 527 by leucine. Computational predictions of such substitution effects suggested that it might alter protein stability and increase the tendency for protein aggregation, and as a result, might influence its interaction with other proteins. In addition, restriction fragment-length polymorphism analysis performed in 178 unrelated individuals showed that up to 1.12% of inhabitants of Northeast Egypt might be heterozygous carriers of this mutation, suggesting the presence of a founder effect in this area.

Mutation analysis of the GLUT2 gene in three unrelated Egyptian families with Fanconi-Bickel syndrome: revisited gene atlas for renumbering

BACKGROUND

Fanconi-Bickel syndrome (FBS) is an autosomal recessive disorder caused by defects in the facilitative glucose transporter 2 (GLUT2 or SLC2A2) gene which codes for the glucose transporter protein 2 expressed in hepatocytes and renal tubular cells causing a defect in carbohydrate metabolism, hepatomegaly, severe hypophosphatemic rickets and failure to thrive.

SUBJECTS AND METHODS

Among 17 unrelated Egyptian families with heritable renal tubular acidosis, three families clinically suspected as FBS were enrolled for this study after providing written informed consent. The three families had positive consanguinity and index cases with characteristic clinical features of FBS (hepatorenal glycogen accumulation, glucose and galactose intolerance, fasting hypoglycemia, a characteristic tubular nephropathy). Laboratory work-up included urinalysis, renal and liver function tests, fasting and postprandial blood sugar, serum calcium, phosphorus, alkaline phosphatase, sodium and potassium, lipid profile and arterial blood gas analysis. Imaging studies included bone survey and abdominal ultrasound. Liver biopsy was performed to confirm pathological diagnosis of the liver enlargement. Molecular analysis was performed for all family members-polymerase chain reaction followed by direct sequencing of the coding segments as well as the flanking introns.

RESULTS

Three different mutations were detected, one specific for each family, including two new mutations. In the first family, exon 3, two bases (GA) were deleted (c.253_254delGA causing a frameshift mutation (p. Glu85fs); the patient presented with early symptoms but unfortunately died despite adequate treatment. In the second family, a mutation was found in exon 6, in the splicing acceptor site with intron 5 (c.776-1G>C or IVS5-1G>A). The third family showed a missense mutation C-to-T substitution at c.1250 (c.1250C>T) causing change of codon 417 (CCG) for proline to CTG for leucine (p. P417L); this is a well-known mutation in the Arab population previously localized in exon 9; however, it is currently renumbered to exon 10.

CONCLUSION

Neither the new mutations nor the reported one were particularly more frequent; however, the third mutation (c.1250C>T) needs more attention in survey studies especially if performed in Arab patients as it has been renumbered because of the 'change' of gene structure since the initial reports.

p.R672C mutation of MYH3 gene in an Egyptian infant presented with Freeman-Sheldon syndrome

OBJECTIVE

To define the mutation type in a clinically suspected Egyptian child with Freeman-Sheldon syndrome (FSS); it involves certain skeletal malformations with some facial characteristics; skeletal malformations include camptodactyly with ulnar deviation, talipes equinovarus, while the facial characteristics include deep-sunken eyes with hypertelorism, long philtrum, small pinched nose and pursed mouth.

METHODS

Amplification of exon 17 of the embryonic myosin heavy chain (MYH3) gene was done using one forward and two different reverse primers, and then the cleaned PCR product was sequenced.

RESULT

A de novo missense mutation (c.2014C>T with replacement C > Y) in MYH3 gene leading to change of arginine at position 672 by cytosine in protein sequence.

CONCLUSION

Mutation analysis remains to be the standard way for definitive diagnosis in FSS. The authors currently report, for the first time in an Egyptian infant aged 16 months who presented with FSS, a c.2014C>T missense mutation of MYH3 gene, with no family history or consanguinity.

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.

CONCLUSION

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.