Ulrike Orth

Mutations in MERTK , the human orthologue of the RCS rat retinal dystrophy gene, cause retinitis pigmentosa

Mutation of a receptor tyrosine kinase gene, Mertk, in the Royal College of Surgeons (RCS) rat results in defective phagocytosis of photoreceptor outer segments by the retinal pigment epithelium (RPE) and retinal degeneration. We screened the human orthologue, MERTK, located at 2q14.1 (ref. 10), in 328 DNA samples from individuals with various retinal dystrophies and found three mutations in three individuals with retinitis pigmentosa (RP). Our findings are the first conclusive evidence implicating the RPE phagocytosis pathway in human retinal disease.

Mutations in ARHGEF6, encoding a guanine nucleotide exchange factor for Rho GTPases, in patients with X-linked mental retardation.

X-linked forms of mental retardation (XLMR) include a variety of different disorders and may account for up to 25% of all inherited cases of mental retardation. So far, seven X-chromosomal genes mutated in nonspecific mental retardation (MRX) have been identified: FMR2, GDI1, RPS6KA3, IL1RAPL, TM4SF2, OPHN1 and PAK3 (refs 2–9). The products of the latter two have been implicated in regulation of neural plasticity by controlling the activity of small GTPases of the Rho family. Here we report the identification of a new MRX gene, ARHGEF6 (also known as αPIX or Cool-2), encoding a protein with homology to guanine nucleotide exchange factors for Rho GTPases (Rho GEF). Molecular analysis of a reciprocal X/21 translocation in a male with mental retardation showed that this gene in Xq26 was disrupted by the rearrangement. Mutation screening of 119 patients with nonspecific mental retardation revealed a mutation in the first intron of ARHGEF6 (IVS1-11T→C) in all affected males in a large Dutch family. The mutation resulted in preferential skipping of exon 2, predicting a protein lacking 28 amino acids. ARHGEF6 is the eighth MRX gene identified so far and the third such gene to encode a protein that interacts with Rho GTPases.

Human monoamine oxidase A and B genes map to xp11.23 and are deleted in a patient with norrie disease

Monoamine oxidase A and B (MAO A and B) are the central enzymes that catalyze oxidative deamination of biogenic amines throughout the body. The regional locations of genes encoding MAO A and B on the X chromosome were determined by using full-length cDNA clones for human MAO A and B, respectively. Using somatic cell hybrids, in situ hybridization, and field-inversion gel electrophoresis as well as deletion mapping in a patient with Norrie disease, we concluded that these two genes are close to each other and to the DXS7 locus (Xp 11.3).

Mutations of the Mitochondrial Holocytochrome c–Type Synthase in X-Linked Dominant Microphthalmia with Linear Skin Defects Syndrome

The microphthalmia with linear skin defects syndrome (MLS, or MIDAS) is an X-linked dominant male-lethal disorder almost invariably associated with segmental monosomy of the Xp22 region. In two female patients, from two families, with MLS and a normal karyotype, we identified heterozygous de novo point mutations--a missense mutation (p.R217C) and a nonsense mutation (p.R197X)--in the HCCS gene. HCCS encodes the mitochondrial holocytochrome c-type synthase that functions as heme lyase by covalently adding the prosthetic heme group to both apocytochrome c and c(1). We investigated a third family, displaying phenotypic variability, in which the mother and two of her daughters carry an 8.6-kb submicroscopic deletion encompassing part of the HCCS gene. Functional analysis demonstrates that both mutant proteins (R217C and Delta 197-268) were unable to complement a Saccharomyces cerevisiae mutant deficient for the HCCS orthologue Cyc3p, in contrast to wild-type HCCS. Moreover, ectopically expressed HCCS wild-type and the R217C mutant protein are targeted to mitochondria in CHO-K1 cells, whereas the C-terminal-truncated Delta 197-268 mutant failed to be sorted to mitochondria. Cytochrome c, the final product of holocytochrome c-type synthase activity, is implicated in both oxidative phosphorylation (OXPHOS) and apoptosis. We hypothesize that the inability of HCCS-deficient cells to undergo cytochrome c-mediated apoptosis may push cell death toward necrosis that gives rise to severe deterioration of the affected tissues. In summary, we suggest that disturbance of both OXPHOS and the balance between apoptosis and necrosis, as well as the X-inactivation pattern, may contribute to the variable phenotype observed in patients with MLS.

X-linked dominant hypophosphatemia is closely linked to DNA markers DXS41 and DXS43 at Xp22

SummaryTwo families with X-linked dominant hypophosphatemia (McKusick No. *30780) were investigated for linkage of the disease locus with several marker genes defined by cloned, single-copy DNA sequences derived from defined regions of the X chromosome. Close linkage was found with DNA markers DXS41 (p99-6) and DXS43 (pD2) at Xp22, suggesting a location of the HPDR gene on the distal short arm of the X chromosome.

Gene of X-chromosomal congenital stationary night blindness is closely linked to DXS7 on Xp.

SummaryCongenital stationary night blindness is characterized by disturbed or absent night vision that is always present at or shortly after birth and nonprogressive. The X-linked form of the disease (CSNBX; McKusick catalog no. 31050) differs from the autosomal types in that the former is frequently associated with myopia. X-chromosome-specific polymorphic DNA markers were used to carry out linkage analysis in three European families segregating for CSNBX. Close linkage without recombination was found between the disease locus and the anonymous locus DXS7, mapped to Xp11.3, assigning the mutation to the proximal short arm of the X chromosome. Linkage data obtained with markers flanking DXS7 provided further support for this localization of the gene locus. Thus, in addition to retinitis pigmentosa and Norrie disease, CSNBX represents the third well-known hereditary eye disease the locus of which is mapped on the proximal Xp and closely linked to DXS7.

Ichthyosis follicularis, alopecia, and photophobia (IFAP) syndrome : Clinical and neuropathological observations in a 33-year-old man

The syndrome of ichthyosis follicularis, alopecia, and photophobia (IFAP) is an uncommon neuroichthyosis described in only 10 males so far. We report on a man with congenital ichthyosis and alopecia with apparently normal development in early infancy. Photophobia and generalized myoclonicastatic seizures began during or after the first year of age and were associated with progressive impairment of motor skills and mental abilities. He died at 33 years of age. Neuropathological findings showed an unusual deformation of the temporal lobes and olivocerebellar atrophy. Cytogenetic and molecular studies did not uncover deletions in either Xp22.2 to 3 or in Xq27.3 to qter.

Molecular analysis of the L1CAM gene in patients with X-linked hydrocephalus demonstrates eight novel mutations and suggests non-allelic heterogeneity of the trait

Eight novel mutations were identified in the gene encoding L1CAM, a neural cell adhesion protein, in patients/families with X-linked hydrocephalus (XHC) providing additional evidence for extreme allelic heterogeneity of the trait. The two nonsense mutations (Gln440Ter and Gln1042Ter) result most likely in functional null-alleles and complete absence of L1CAM at the cell surface. The four missense mutations (Leu482Pro, Ser542Pro, Met741Thr, and Val752Met) as well as delSer526 may considerably alter the structure of L1CAM. Interestingly, a missense mutation in an XHC family predicting the Val768Ile change in the second fibronectin type III domain of L1CAM was found not only in the two affected cousins and their obligate carrier mothers but also in two unaffected male relatives of the patients. Several possible explanations of this finding are discussed; the most likely being that Val768Ile is a rare non-pathogenic variant. If this were indeed the case, our data suggest that the XHC in this family is not due to a mutation of the L1CAM gene, i.e., that, in addition to the extreme allelic heterogeneity of XHC, a non-allelic form of genetic heterogeneity may also exist in this trait.

Five novel mutations in the L1CAM gene in families with X linked hydrocephalus.

Five novel mutations have been identified in the gene encoding L1CAM, a neural cell adhesion protein, in families with X linked hydrocephalus (XHC). Interestingly, all five mutations are in the evolutionarily highly conserved Ig-like domains of the protein. The two frameshift mutations (52insC and 955delG) and the nonsense mutation (Trp276Ter) most probably result in functional null alleles and complete absence of L1CAM at the cell surface. The two missense mutations (Tyr194Cys and Pro240Leu) may considerably alter the structure of the L1CAM protein. These data provide convincing evidence that XHC is genetically extremely heterogeneous.

A Y/5 translocation in a 45,X male with cri du chat syndrome

SummaryIn a patient described as a 45,X male with cri du chat syndrome, combined cytogenetic and molecular methods revealed Y euchromatic material to be translocated onto the short arm of one chromosome 5, resulting in a chromosome der(5)(5qter→5p14::Yp11.31→Ypter). The translocated Y euchromatin comprised only the distal short arm including the pseudoautosomal region and the so-called deletion intervals 1 and 2. A review of 45,X males from the literature showed that; most of them carry a paternally transmitted Y/autosome translocations; resulting in various autosomal deletions. Depending on the segment concerned, the deletion led to congenital malformations.