Marijke Joosse

A mutation in SLC11A3 is associated with autosomal dominant hemochromatosis

Hereditary hemochromatosis (HH) is a very common disorder characterized by iron overload and multi-organ damage. Several genes involved in iron metabolism have been implicated in the pathology of HH (refs. 1–4). We report that a mutation in the gene encoding Solute Carrier family 11, member A3 (SLC11A3), also known as ferroportin, is associated with autosomal dominant hemochromatosis.

Disruption of a long-range cis-acting regulator for Shh causes preaxial polydactyly

Preaxial polydactyly (PPD) is a common limb malformation in human. A number of polydactylous mouse mutants indicate that misexpression of Shh is a common requirement for generating extra digits. Here we identify a translocation breakpoint in a PPD patient and a transgenic insertion site in the polydactylous mouse mutant sasquatch (Ssq). The genetic lesions in both lie within the same respective intron of the LMBR1/Lmbr1 gene, which resides ≈1 Mb away from Shh. Genetic analysis of Ssq reveals that the Lmbr1 gene is incidental to the phenotype and that the mutation directly interrupts a cis-acting regulator of Shh. This regulator is most likely the target for generating PPD mutations in human.

High prevalence of mutations in the microtubule-associated protein tau in a population study of frontotemporal dementia in the Netherlands

Mutations in microtubule-associated protein tau recently have been identified in familial cases of frontotemporal dementia (FTD). We report the frequency of tau mutations in a large population-based study of FTD carried out in the Netherlands from January 1994 to June 1998. Thirty-seven patients had >/=1 first-degree relative with dementia. A mutation in the tau gene was found in 17.8% of the group of patients with FTD and in 43% of patients with FTD who also had a positive family history of FTD. Three distinct missense mutations (G272V, P301L, R406W) accounted for 15.6% of the mutations. These three missense mutations, and a single amino acid deletion (DeltaK280) that was detected in one patient, strongly reduce the ability of tau to promote microtubule assembly. We also found an intronic mutation at position +33 after exon 9, which is likely to affect the alternative splicing of tau. Tau mutations are responsible for a large proportion of familial FTD cases; however, there are also families with FTD in which no mutations in tau have been found, which indicates locus and/or allelic heterogeneity. The different tau mutations may result in disturbances in the interactions of the protein tau with microtubules, resulting in hyperphosphorylation of tau protein, assembly into filaments, and subsequent cell death.

A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases.

Sialic acid storage diseases (SASD, MIM 269920) are autosomal recessive neurodegenerative disorders that may present as a severe infantile form (ISSD) or a slowly progressive adult form, which is prevalent in Finland (Salla disease). The main symptoms are hypotonia, cerebellar ataxia and mental retardation; visceromegaly and coarse features are also present in infantile cases. Progressive cerebellar atrophy and dysmyelination have been documented by magnetic resonance imaging (ref. 4). Enlarged lysosomes are seen on electron microscopic studies and patients excrete large amounts of free sialic acid in urine. A H+/anionic sugar symporter mechanism for sialic acid and glucuronic acid is impaired in lysosomal membranes from Salla and ISSD patients. The locus for Salla disease was assigned to a region of approximately 200 kb on chromosome 6q14–q15 in a linkage study using Finnish families. Salla disease and ISSD were further shown to be allelic disorders. A physical map with P1 and PAC clones was constructed to cover the 200-kb area flanked by the loci D6S280 and D6S1622, providing the basis for precise physical positioning of the gene. Here we describe a new gene, SLC17A5 (also known as AST), encoding a protein (sialin) with a predicted transport function that belongs to a family of anion/cation symporters (ACS). We found a homozygous SLC17A5 mutation (R39C) in five Finnish patients with Salla disease and six different SLC17A5 mutations in six ISSD patients of different ethnic origins. Our observations suggest that mutations in SLC17A5 are the primary cause of lysosomal sialic acid storage diseases.

Phenotypic Variation in Hereditary Frontotemporal Dementia with Tau Mutations

Several mutations in the tau gene have been found in families with hereditary frontotemporal dementia and parkinsonism linked to chromosome 17q21‐22 (FTDP‐17). This study is the first attempt to correlate genotype and phenotype in six families with FTDP‐17 with mutations in the tau gene (ΔK280, G272V, P301L, and R406W). We have investigated tau pathology in 1 P301L and 1 R406W patient. The R406W family showed a significantly higher age at onset (59.2 ± 5.5 years) and longer duration of illness (12.7 ± 1.5 years) than the families with the other mutations. The six families showed considerable variation in clinical presentation, but none of them had early parkinsonism. Mutism developed significantly later in the R406W family than in the other families. Frontotemporal atrophy on neuroimaging in the R406W family was less severe than in the P301L and ΔK280 families. The P301L brain contained many pretangles in the frontal and temporal cortex, and the dentate gyrus of hippocampus, showing three tau bands (64, 68, and 72 kd) of extracted tau from the frontal cortex. The presence of many neurofibrillary tangles, many diffuse and classic neuritic plaques in the temporal and parietal cortex, and the hippocampus of the same P301L brain correlated with the presence of four sarkosyl‐insoluble (60, 64, 68, and 72 kd) tau bands. The coexistence of characteristic P301L and Alzheimer pathology in the same brain needs further explanation. The R406W brain showed abundant neurofibrillary tangles in several brain regions, and four tau bands (60, 64, 68, and 72 kd) of extracted tau from these regions. The slower progression of the disease in the R406W family might be explained by the microtubule‐binding properties of the mutant protein.

Cntnap2 is disrupted in a family with gilles de la tourette syndrome and obsessive compulsive disorder

Gilles de la Tourette syndrome (GTS) is a sporadic or inherited complex neuropsychiatric disorder characterized by involuntary motor and vocal tics. There is comorbidity with disorders like obsessive compulsive disorder and attention deficit hyperactivity disorder. Until now linkage analysis has pointed to a number of chromosomal locations, but has failed to identify a clear candidate gene(s). We have investigated a GTS family with a complex chromosomal insertion/translocation involving chromosomes 2 and 7. The affected father [46,XY,inv(2) (p23q22),ins(7;2) (q35-q36;p21p23)] and two affected children [46,XX,der(7)ins(7;2)(q35-q36;p21p23) and 46,XY,der(7)ins(7;2)(q35-q36;p213p23)] share a chromosome 2p21-p23 insertion on chromosome 7q35-q36, thereby interrupting the contactin-associated protein 2 gene (CNTNAP2). This gene encodes a membrane protein located in a specific compartment at the nodes of Ranvier of axons. We hypothesize that disruption or decreased expression of CNTNAP2 could lead to a disturbed distribution of the K(+) channels in the nervous system, thereby influencing conduction and/or repolarization of action potentials, causing unwanted actions or movements in GTS.

FTDP-17: An early-onset phenotype with parkinsonism and epileptic seizures caused by a novel mutation

Recently, mutations in the tau gene on chromosome 17 were found causative for autosomal dominantly inherited frontotemporal dementia and parkinsonism (FTDP‐17). We describe a family carrying a missense mutation at nucleotide 1137 C → T, resulting in the amino acid substitution P301S. Methods of investigations include clinical, electrophysiological, and imaging techniques. This kindred presents with a novel phenotype characterized by an early onset of rapidly progressive frontotemporal dementia and parkinsonism in combination with epileptic seizures. We define the dopaminergic deficits as being predominantly presynaptic by the use of single‐photon emission computed tomography with a dopamine transporter ligand. The association of this early‐onset phenotype with P301S mutation is not entirely consistent with current criteria for the diagnosis of frontotemporal dementias and may encourage the search for tau mutations in diseases similar but not identical to FTDP‐17. Also, the change from proline to serine suggests that this mutation might contribute to tau hyperphosphorylation.

Hereditary Vascular Retinopathy, Cerebroretinal Vasculopathy, and Hereditary Endotheliopathy with Retinopathy, Nephropathy, and Stroke Map to a Single Locus on Chromosome 3p21.1-p21.3

We performed a genomewide search for linkage in an extended Dutch family with hereditary vascular retinopathy associated with migraine and Raynaud phenomenon. Patients with vascular retinopathy are characterized by microangiopathy of the retina, accompanied by microaneurysms and telangiectatic capillaries. The genome search, using a high throughput capillary sequencer, revealed significant evidence of linkage to chromosome 3p21.1-p21.3 (maximum pairwise LOD score 5.25, with D3S1578). Testing of two additional families that had a similar phenotype, cerebroretinal vasculopathy, and hereditary endotheliopathy with retinopathy, nephropathy, and stroke, revealed linkage to the same chromosomal region (combined maximum LOD score 6.30, with D3S1588). Haplotype analysis of all three families defined a 3-cM candidate region between D3S1578 and D3S3564. Our study shows that three autosomal dominant vasculopathy syndromes with prominent cerebroretinal manifestations map to the same 3-cM interval on 3p21, suggesting a common locus.

A novel acid α-glucosidase mutation identified in a Pakistani family with glycogen storage disease type II

A novel mutation, C118t, in exon 2 of the acid α-glucosidase gene has been found in an infant with glycogen storage disease type II. This mutation is predicted to result in protein truncation. The phenotype was that of the severe infantile form of the disorder with lack of motor development, but with eye regard, social smile and vocalization. The parents were heterozygous for C118T and belong to an Islamic community opposed to termination of pregnancy. As the C118T mutation results in the loss of one of two AvaI sites present in an informative PCR product, reliable premarriage carrier detection became possible and was acceptable to the members of this extended family.

Hemimelic extra toes and Hammer toe are distinct mutations that show a genetic interaction

The mutations underlying the mouse Hemimelic extra toes(Hx) phenotype, characterized by preaxial polydactyly and shortening of the tibia, and theHammer toe(Hm) phenotype, characterized by syndactyly, have been localized very close to each other on mouse Chromosome (Chr) 5p (Green 1989; Mouse Genome Database 1999). Only a single recombination event between the Hm andHx mutations was detected in 3.664 offspring (Sweet 1982), and thus the two mutations might affect the same gene. TheHx mutant is characterized by preaxial polydactyly on all four feet, with the hind limbs more severely affected than the fore limbs. TheHx mutation arose in the B10.D2/nSn strain, but variable expression of the phenotype has been reported, possibly caused by the outbred genetic background used (Knudsen and Kochhar 1981). The heterozygous phenotype typically includes shortening of the radius, tibia, and talus with extra preaxial metacarpals, metatarsals, and digits. The fibula and ulna are normal in size but often bent. The humerus, femur, and limb girdles are normal, and no other skeletal defects are present (Green 1989). It has been assumed that the homozygous Hx condition is embryolethal at an early stage of development (Knudsen and Kochhar 1981). The Hm mutant (C3HeB/FeJLe-a/a-Ca J Sl Hm) displays a semi-dominant phenotype of soft-tissue syndactyly between digits 2, 3, 4, and 5 on all four feet, while digits 1 and 2 are separated normally. The hind feet are always more severely affected than the fore feet. Heterozygous mutants show partial syndactyly that extends to the base of the most distal phalanges. Syndactyly in the homozygous mutant is complete and extends to the top of the most distal phalanges. Both heterozygous and homozygous mutants show incomplete syndactyly of the digits on the fore feet that is gradually decreasing towards the preaxial side (Green 1989). Two human congenital hand malformations, preaxial polydactyly (PPD) and complex polysyndactyly (CPS), have been localized to human Chr 7q36 (Heutink et al. 1994; Tsukurov et al. 1994). This region is syntenic to the region of mouse Chr 5 where the Hx andHm phenotypes have been localized (Mouse Genome Database 1999). PPD is characterized by a triphalangeal thumb and index finger duplication. Tibial and radial abnormalities have been observed (Zguricas et al. 1999). CPS is characterized by preand postaxial limb anomalies combined with syndactyly of the fingers. The striking similarity between human and mouse phenotypes and the syntenic chromosomal location suggest that the mouseHmandHx mutations are equivalent to the human CPS and PPD mutations. To see whether the variable phenotype of the heterozygous Hx mutant stabilizes in an inbred genetic background, and to test whether theHx andHm phenotypes are variant expressions of the same genetic defect, we crossed the Hx mutation into the C3HeB/ FeJLe-a/a-Ca J background of theHmmouse. After ten generations the phenotype was stable, with all heterozygous Hx mutant mice showing preaxial polydactyly and normal tibia. This finding indicates that the variation in the phenotype of the original Hx mutant is most likely caused by the presence of genetic modifiers elsewhere on the genome that influence the expression of the Hx mutant phenotype. Because the Hx phenotype is still present in the Hm background, theHm andHx phenotypes are not the result of variant expression of a single mutation caused by differences in genetic background, but represent two distinct mutations. We then tested whether homozygous Hx mice are viable in this C3HeB/FeJLe-a/a-Ca J genetic background. Polymorphic marker D5mit387 lies in the proximity of theHx/Hm locus (Mouse Genome Database 1999) and was used to distinguish the mutant Hx from wildtype (+) alleles. In aHx mutant backcross we observed three recombinations in 161 offspring, correlating with a genetic distance betweenD5mit387 and Hx of 1.8 cM. Offspring from heterozygousHx crosses were tested for the presence of homozygosity for theD5mit387allele associated with the Hx mutation. In 93 offspring we found 28 mice homozygous for this allele (see Table 1). This finding is in agreement with Mendelian inheritance, assuming that the homozygous Hx condition is fully viable ( P < 0.05, chi square test). The entire offspring from a backcross of homozygousHx mutants withwt mates showed theHx mutant phenotype, confirming homozygosity of the parent Hx mice. HomozygousHx mutant mice could phenotypically be distinguished from heterozygousHx mutant mice; all four feet showed preaxial polydactyly with six to eight digits, but unlike the heterozygous Hx mutant mice, the bones of the extra preaxial digits tended to be fused. All homozygousHx mutant mice showed shortening of the tibia, which was not observed in the heterozygous Hx mutant, and in a few cases shortening of the radius was observed. Bone staining of neonates and adult animals revealed no other skeletal abnormalities. Intercrossing homozygous Hx mice yielded fully viable and fertile offspring that were born with normal embryo resorption rates and litter sizes (data not shown). We then used the homozygous Hx C3HeB/FeJLe-a/a-Ca J mutants (the same genetic background as the Hm mutant), to test whether theHx and Hm mutant phenotypes are caused by two independent mutations or whether they show genetic interaction. We therefore crossed homozygous Hm mutant mice with homo-