Rivka Carmi

Mutation of TRPM6 causes familial hypomagnesemia with secondary hypocalcemia

Familial hypomagnesemia with secondary hypocalcemia (OMIM 602014) is an autosomal recessive disease that results in electrolyte abnormalities shortly after birth. Affected individuals show severe hypomagnesemia and hypocalcemia, which lead to seizures and tetany. The disorder has been thought to be caused by a defect in the intestinal absorption of magnesium, rather than by abnormal renal loss of magnesium. Restoring the concentrations of serum magnesium to normal values by high-dose magnesium supplementation can overcome the apparent defect in magnesium absorption and in serum concentrations of calcium. Life-long magnesium supplementation is required to overcome the defect in magnesium handling by these individuals. We previously mapped the gene locus to chromosome 9q in three large inbred kindreds from Israel. Here we report that mutation of TRPM6 causes hypomagnesemia with secondary hypocalcemia and show that individuals carrying mutations in this gene have abnormal renal magnesium excretion.

Mutation of a nuclear receptor gene, NR2E3 , causes enhanced S cone syndrome, a disorder of retinal cell fate

Hereditary human retinal degenerative diseases usually affect the mature photoreceptor topography by reducing the number of cells through apoptosis, resulting in loss of visual function. Only one inherited retinal disease, the enhanced S-cone syndrome (ESCS), manifests a gain in function of photoreceptors. ESCS is an autosomal recessive retinopathy in which patients have an increased sensitivity to blue light; perception of blue light is mediated by what is normally the least populous cone photoreceptor subtype, the S (short wavelength, blue) cones. People with ESCS also suffer visual loss, with night blindness occurring from early in life, varying degrees of L (long, red)- and M (middle, green)-cone vision, and retinal degeneration. The altered ratio of S- to L/M-cone photoreceptor sensitivity in ESCS may be due to abnormal cone cell fate determination during retinal development. In 94% of a cohort of ESCS probands we found mutations in NR2E3 (also known as PNR), which encodes a retinal nuclear receptor recently discovered to be a ligand-dependent transcription factor. Expression of NR2E3 was limited to the outer nuclear layer of the human retina. Our results suggest that NR2E3 has a role in determining photoreceptor phenotype during human retinogenesis.

Identification of the gene (BBS1) most commonly involved in Bardet-Biedl syndrome, a complex human obesity syndrome.

Bardet-Biedl syndrome (BBS, OMIM 209900) is a genetic disorder with the primary features of obesity, pigmentary retinopathy, polydactyly, renal malformations, mental retardation and hypogenitalism. Individuals with BBS are also at increased risk for diabetes mellitus, hypertension and congenital heart disease. What was once thought to be a homogeneous autosomal recessive disorder is now known to map to at least six loci: 11q13 (BBS1), 16q21 (BBS2), 3p13–p12 (BBS3), 15q22.3–q23 (BBS4), 2q31 (BBS5) and 20p12 (BBS6). There has been considerable interest in identifying the genes that underlie BBS, because some components of the phenotype are common. Cases of BBS mapping ro BBS6 are caused by mutations in MKKS; mutations in this gene also cause McKusick-Kaufman syndrome (hydrometrocolpos, post-axial polydactyly and congenital heart defects). In addition, we recently used positional cloning to identify the genes underlying BBS2 (ref. 16) and BBS4 (ref. 17). The BBS6 protein has similarity to a Thermoplasma acidophilum chaperonin, whereas BBS2 and BBS4 have no significant similarity to chaperonins. It has recently been suggested that three mutated alleles (two at one locus, and a third at a second locus) may be required for manifestation of BBS (triallelic inheritance). Here we report the identification of the gene BBS1 and show that a missense mutation of this gene is a frequent cause of BBS. In addition, we provide data showing that this common mutation is not involved in triallelic inheritance.

IDENTIFICATION OF MUTATIONS IN THE CONNEXIN 26 GENE THAT CAUSE AUTOSOMAL RECESSIVE NONSYNDROMIC HEARING LOSS

Mutations in the Cx26 gene have been shown to cause autosomal recessive nonsyndromic hearing loss (ARNSHL) at the DFNB1 locus on chromosome 13q12. Using direct sequencing, we screened the Cx26 coding region of affected and nonaffected members from seven ARNSHL families either linked to the DFNB1 locus or in which the ARNSHL phenotype cosegregated with markers from chromosome 13q12. Cx26 mutations were found in six of the seven families and included two previously described mutations (W24X and W77X) and two novel Cx26 mutations: a single base pair deletion of nucleotide 35 resulting in a frameshift and a C‐to‐T substitution at nucleotide 370 resulting in a premature stop codon (Q124X). We have developed and optimized allele‐specific PCR primers for each of the four mutations to rapidly determine carrier and noncarrier status within families. We also have developed a single stranded conformational polymorphism (SSCP) assay which covers the entire Cx26 coding region. This assay can be used to screen individuals with nonsyndromic hearing loss for mutations in the CX26 gene. Hum Mutat 11:387–394, 1998.

Identification of the gene that, when mutated, causes the human obesity syndrome BBS4.

Bardet–Biedl syndrome (BBS, MIM 209900) is a heterogeneous autosomal recessive disorder characterized by obesity, pigmentary retinopathy, polydactyly, renal malformations, mental retardation, and hypogenitalism. The disorder is also associated with diabetes mellitus, hypertension, and congenital heart disease. Six distinct BBS loci map to 11q13 (BBS1), 16q21 (BBS2), 3p13–p12 (BBS3), 15q22.3–q23 (BBS4), 2q31 (BBS5), and 20p12 (BBS6). Although BBS is rare in the general population (<1/100,000), there is considerable interest in identifying the genes causing BBS because components of the phenotype, such as obesity and diabetes, are common. We and others have demonstrated that BBS6 is caused by mutations in the gene MKKS (refs. 12,13), mutation of which also causes McKusick–Kaufman syndrome (hydrometrocolpos, post-axial polydactyly, and congenital heart defects). MKKS has sequence homology to the alpha subunit of a prokaryotic chaperonin in the thermosome Thermoplasma acidophilum. We recently identified a novel gene that causes BBS2. The BBS2 protein has no significant similarity to other chaperonins or known proteins. Here we report the positional cloning and identification of mutations in BBS patients in a novel gene designated BBS4.

Comparative Genomic Analysis Identifies an ADP-Ribosylation Factor–like Gene as the Cause of Bardet-Biedl Syndrome (BBS3)

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous, pleiotropic human disorder characterized by obesity, retinopathy, polydactyly, renal and cardiac malformations, learning disabilities, and hypogenitalism. Eight BBS loci have been mapped, and seven genes have been identified. BBS3 was previously mapped to chromosome 3 by linkage analysis in a large Israeli Bedouin kindred. The rarity of other families mapping to the BBS3 locus has made it difficult to narrow the disease interval sufficiently to identify the gene by positional cloning. We hypothesized that the genomes of model organisms that contained the orthologues to known BBS genes would also likely contain a BBS3 orthologue. Therefore, comparative genomic analysis was performed to prioritize BBS candidate genes for mutation screening. Known BBS proteins were compared with the translated genomes of model organisms to identify a subset of organisms in which these proteins were conserved. By including multiple organisms that have relatively small genome sizes in the analysis, the number of candidate genes was reduced, and a few genes mapping to the BBS3 interval emerged as the best candidates for this disorder. One of these genes, ADP-ribosylation factor-like 6 (ARL6), contains a homozygous stop mutation that segregates completely with the disease in the Bedouin kindred originally used to map the BBS3 locus, identifying this gene as the BBS3 gene. These data illustrate the power of comparative genomic analysis for the study of human disease and identifies a novel BBS gene.

Complement Factor H Gene Mutation Associated with Autosomal Recessive Atypical Hemolytic Uremic Syndrome

Atypical hemolytic uremic syndrome (HUS) presents with the clinical features of hypertension, microangiopathic hemolytic anemia, and acute renal failure. Both dominant and recessive modes of inheritance have been reported. This study describes the genetic and functional analysis of a large Bedouin kindred with autosomal recessive HUS. The kindred consists of several related nuclear families in which all parent unions of affected children are consanguineous. A previous report demonstrated that a dominant form of HUS maps to chromosome 1q and that complement factor H (CFH), a regulatory component of the complement system, lies within the region and is involved in the dominant disorder. Early-onset and persistent hypocomplementemia in this Bedouin kindred prompted us to evaluate the CFH gene. Linkage analysis was performed, demonstrating linkage between the disorder and the markers near the CFH gene. Mutation analysis of the CFH coding region revealed a single missense mutation. Functional analyses demonstrate that the mutant CFH is properly expressed and synthesized but that it is not transported normally from the cell. This is the first study reporting that a recessive, atypical, early-onset, and relapsing HUS is associated with the CFH protein and that a CFH mutation affects intracellular trafficking and secretion.

Use of isolated inbred human populations for identification of disease genes.

The genetic mapping of disease loci involves the use of patient phenotype and genotype data in the search for genetic markers that segregate, or are associated with, a trait or disorder. Genetically isolated populations offer many advantages for such studies. The high degree of inbreeding and/or founder effects in some small population isolates result in an increased incidence of recessive disorders. Monogenic disorders are less likely to show non-allelic heterogeneity in isolated populations than in more diverse populations. The use of isolated populations also reduces the complexity of polygenic disorders by reducing the number of loci probably involved in the disorder. Finally, a variety of strategies can be used with particular efficacy for the mapping of disease genes in isolated populations.

Evaluation of Complex Inheritance Involving the Most Common Bardet-Biedl Syndrome Locus (BBS1)

Bardet-Biedl syndrome (BBS) is a genetic disorder with the primary features of obesity, pigmentary retinopathy, polydactyly, renal malformations, mental retardation, and hypogenitalism. Patients with BBS are also at increased risk for diabetes mellitus, hypertension, and congenital heart disease. BBS is known to map to at least six loci: 11q13 (BBS1), 16q21 (BBS2), 3p13-p12 (BBS3), 15q22.3-q23 (BBS4), 2q31 (BBS5), and 20p12 (BBS6). Although these loci were all mapped on the basis of an autosomal recessive mode of inheritance, it has recently been suggested-on the basis of mutation analysis of the identified BBS2, BBS4, and BBS6 genes-that BBS displays a complex mode of inheritance in which, in some families, three mutations at two loci are necessary to manifest the disease phenotype. We recently identified BBS1, the gene most commonly involved in Bardet-Biedl syndrome. The identification of this gene allows for further evaluation of complex inheritance. In the present study we evaluate the involvement of the BBS1 gene in a cohort of 129 probands with BBS and report 10 novel BBS1 mutations. We demonstrate that a common BBS1 missense mutation accounts for approximately 80% of all BBS1 mutations and is found on a similar genetic background across populations. We show that the BBS1 gene is highly conserved between mice and humans. Finally, we demonstrate that BBS1 is inherited in an autosomal recessive manner and is rarely, if ever, involved in complex inheritance.

Clinical presentation and outcome in primary familial hypomagnesaemia

The clinical presentation and long term outcome (mean follow up eight years, range 0.25 to 21) of 15 patients with autosomal recessive primary familial hypomagnesaemia is described. The most common (67%) presenting events were generalised hypocalcaemic-hypomagnesaemic seizures at a mean (SD) age of 4.9 (2.5) weeks. Thirteen infants, treated soon after diagnosis with high dose enteral magnesium developed normally. Their serum calcium returned to normal concentrations but serum magnesium could not be maintained at normal concentrations (0.53 (0.12 SD) mmol/l; normal >0.62). Delay in establishing a diagnosis led to a convulsive disorder with permanent neurological impairment in two infants. Reported complications of prolonged hypomagnesaemia such as renal stones, hypertension, arrhythmias, sudden death, or dyslipidaemia were not observed.