The essential role of centrosomal NDE1 in human cerebral cortex neurogenesis.

We investigated three families whose offspring had extreme microcephaly at birth and profound mental retardation. Brain scans and postmortem data showed that affected individuals had brains less than 10% of expected size (≤10 standard deviation) and that in addition to a massive reduction in neuron production they displayed partially deficient cortical lamination (microlissencephaly). Other body systems were apparently unaffected and overall growth was normal. We found two distinct homozygous mutations of NDE1, c.83+1G>T (p.Ala29GlnfsX114) in a Turkish family and c.684_685del (p.Pro229TrpfsX85) in two families of Pakistani origin. Using patient cells, we found that c.83+1G>T led to the use of a novel splice site and to a frameshift after NDE1 exon 2. Transfection of tagged NDE1 constructs showed that the c.684_685del mutation resulted in a NDE1 that was unable to localize to the centrosome. By staining a patient-derived cell line that carried the c.83+1G>T mutation, we found that this endogeneously expressed mutated protein equally failed to localize to the centrosome. By examining human and mouse embryonic brains, we determined that NDE1 is highly expressed in neuroepithelial cells of the developing cerebral cortex, particularly at the centrosome. We show that NDE1 accumulates on the mitotic spindle of apical neural precursors in early neurogenesis. Thus, NDE1 deficiency causes both a severe failure of neurogenesis and a deficiency in cortical lamination. Our data further highlight the importance of the centrosome in multiple aspects of neurodevelopment.

Prognosis for splicing factor PRPF8 retinitis pigmentosa, novel mutations and correlation between human and yeast phenotypes.

PRPF8-retinitis pigmentosa is said to be severe but there has been no overview of phenotype across different mutations. We screened RP patients for PRPF8 mutations and identified three new missense mutations, including the first documented mutation outside exon 42 and the first de novo mutation. This brings the known RP-causing mutations in PRPF8 to nineteen. We then collated clinical data from new and published cases to determine an accurate prognosis for PRPF8-RP. Clinical data for 75 PRPF8-RP patients were compared, revealing that while the effect on peripheral retinal function is severe, patients generally retain good visual acuity in at least one eye until the fifth or sixth decade. We also noted that prognosis for PRPF8-RP differs with different mutations, with p.H2309P or p.H2309R having a worse prognosis than p.R2310K. This correlates with the observed difference in growth defect severity in yeast lines carrying the equivalent mutations, though such correlation remains tentative given the limited number of mutations for which information is available. The yeast phenotype is caused by lack of mature spliceosomes in the nucleus, leading to reduced RNA splicing function. Correlation between yeast and human phenotypes suggests that splicing factor RP may also result from an underlying splicing deficit.

Genotype-phenotype correlation for leber congenital amaurosis in Northern Pakistan.

OBJECTIVES: To report the genetic basis of Leber congenital amaurosis (LCA) in northern Pakistan and to describe the phenotype. METHODS: DNA from 14 families was analyzed using single-nucleotide polymorphism and microsatellite genotyping and direct sequencing to determine the genes and mutations involved. The history and examination findings from 64 affected individuals were analyzed to show genotype-phenotype correlation and phenotypic progression. RESULTS: Homozygous mutations were found in RPGRIP1 (4 families), AIPL1 and LCA5 (3 families each), and RPE65, CRB1, and TULP1 (1 family each). Six of the mutations are novel. An additional family demonstrated linkage to the LCA9 locus. Visual acuity, severe keratoconus, cataract, and macular atrophy were the most helpful features in predicting the genotype. Many of the phenotypic variables became more prevalent with increasing age. CONCLUSIONS: Leber congenital amaurosis in northern Pakistan is genetically heterogeneous. Mutations in RPGRIP1, AIPL1, and LCA5 accounted for disease in 10 of the 14 families. This study illustrates the differences in phenotype, for both the anterior and posterior segments, seen between patients with identical or different mutations in the LCA genes and also suggests that at least some of the phenotypic variation is age dependent. CLINICAL RELEVANCE: The LCA phenotype, especially one including different generations in the same family, may be used to refine a molecular diagnostic strategy.

Mutation of the variant alpha-tubulin TUBA8 results in polymicrogyria with optic nerve hypoplasia.

The critical importance of cytoskeletal function for correct neuronal migration during development of the cerebral cortex has been underscored by the identities of germline mutations underlying a number of human neurodevelopmental disorders. The proteins affected include TUBA1A, a major alpha-tubulin isoform, and microtubule-associated components such as doublecortin, and LIS1. Mutations in these genes are associated with the anatomical abnormality lissencephaly, which is believed to reflect failure of neuronal migration. An important recent observation has been the dependence of cortical neuronal migration upon acetylation of alpha-tubulin at lysine 40 by the histone acetyltransferase Elongator complex. Here, we describe a recognizable autosomal recessive syndrome, characterized by generalized polymicrogyria in association with optic nerve hypoplasia (PMGOH). By autozygosity mapping, we show that the molecular basis for this condition is mutation of the TUBA8 gene, encoding a variant alpha-tubulin of unknown function that is not susceptible to the lysine 40 acetylation that regulates microtubule function during cortical neuron migration. Together with the unique expression pattern of TUBA8 within the developing cerebral cortex, these observations suggest a role for this atypical microtubule component in regulating mammalian brain development.

Loss of the metalloprotease ADAM9 leads to cone-rod dystrophy in humans and retinal degeneration in mice.

Cone-rod dystrophy (CRD) is an inherited progressive retinal dystrophy affecting the function of cone and rod photoreceptors. By autozygosity mapping, we identified null mutations in the ADAM metallopeptidase domain 9 (ADAM9) gene in four consanguineous families with recessively inherited early-onset CRD. We also found reduced photoreceptor responses in Adam9 knockout mice, previously reported to be asymptomatic. In 12-month-old knockout mice, photoreceptors appear normal, but the apical processes of the retinal pigment epithelium (RPE) cells are disorganized and contact between photoreceptor outer segments (POSs) and the RPE apical surface is compromised. In 20-month-old mice, there is clear evidence of progressive retinal degeneration with disorganized POS and thinning of the outer nuclear layer (ONL) in addition to the anomaly at the POS-RPE junction. RPE basal deposits and macrophages were also apparent in older mice. These findings therefore not only identify ADAM9 as a CRD gene but also identify a form of pathology wherein retinal disease first manifests at the POS-RPE junction.

Macular dystrophy associated with the Arg172Trp substitution in peripherin/RDS: genotype-phenotype correlation.

PURPOSE: To document the evolution of geographic atrophy in the peripherin/RDS Arg172Trp substitution, provide age-related estimates of visual acuity, and compare with other missense mutations with a similar phenotype (Arg142Trp, Arg172Gln, and Arg195Leu). METHODS: Total area of geographic atrophy in 18 affected individuals with the peripherin/RDS Arg172Trp substitution was measured from retinal photographs and plotted as a function of age. Visual acuity data from these individuals were collated with previously published cases of Arg172Trp substitution to obtain age-related estimates of visual acuity. These were compared with published series with the Arg142Trp, Arg172Gln, and Arg195Leu substitutions, using linear regression. RESULTS: In patients with the Arg172Trp substitution, the increase in total area of chorioretinal atrophy and decline in visual acuity showed significant association with age (R2 = 0.619, P < 0.001; R2 = 0.761, P < 0.001). A trend was observed toward earlier age at onset and worse visual acuity with the Arg172Trp substitution as compared with the Arg142Trp and Arg172Gln substitutions. Linear regression analysis showed that until the age of 60 years, at any given age, visual acuity with the Arg172Trp substitution was significantly worse than the Arg142Trp (P < 0.001) and the Arg172Gln substitutions (P = 0.04). Patients above the age of 60 years were excluded as a floor effect on visual acuity was observed with visual acuity being worse than 6/60 for most patients. CONCLUSION: This paper demonstrates that visual prognosis in macular dystrophies associated with peripherin/RDS may be mutation specific and, for the Arg172Trp substitution, worse than the Arg142Trp and Arg172Gln substitutions.

Mutations in LCA5, encoding the ciliary protein lebercilin, cause Leber congenital amaurosis.

Leber congenital amaurosis (LCA) causes blindness or severe visual impairment at or within a few months of birth. Here we show, using homozygosity mapping, that the LCA5 gene on chromosome 6q14, which encodes the previously unknown ciliary protein lebercilin, is associated with this disease. We detected homozygous nonsense and frameshift mutations in LCA5 in five families affected with LCA. In a sixth family, the LCA5 transcript was completely absent. LCA5 is expressed widely throughout development, although the phenotype in affected individuals is limited to the eye. Lebercilin localizes to the connecting cilia of photoreceptors and to the microtubules, centrioles and primary cilia of cultured mammalian cells. Using tandem affinity purification, we identified 24 proteins that link lebercilin to centrosomal and ciliary functions. Members of this interactome represent candidate genes for LCA and other ciliopathies. Our findings emphasize the emerging role of disrupted ciliary processes in the molecular pathogenesis of LCA.

An SCN9A channelopathy causes congenital inability to experience pain.

The complete inability to sense pain in an otherwise healthy individual is a very rare phenotype. In three consanguineous families from northern Pakistan, we mapped the condition as an autosomal-recessive trait to chromosome 2q24.3. This region contains the gene SCN9A, encoding the alpha-subunit of the voltage-gated sodium channel, Na(v)1.7, which is strongly expressed in nociceptive neurons. Sequence analysis of SCN9A in affected individuals revealed three distinct homozygous nonsense mutations (S459X, I767X and W897X). We show that these mutations cause loss of function of Na(v)1.7 by co-expression of wild-type or mutant human Na(v)1.7 with sodium channel beta(1) and beta(2) subunits in HEK293 cells. In cells expressing mutant Na(v)1.7, the currents were no greater than background. Our data suggest that SCN9A is an essential and non-redundant requirement for nociception in humans. These findings should stimulate the search for novel analgesics that selectively target this sodium channel subunit.

Quantification of homozygosity in consanguineous individuals with autosomal recessive disease.

Individuals born of consanguineous union have segments of their genomes that are homozygous as a result of inheriting identical ancestral genomic segments through both parents. One consequence of this is an increased incidence of recessive disease within these sibships. Theoretical calculations predict that 6% (1/16) of the genome of a child of first cousins will be homozygous and that the average homozygous segment will be 20 cM in size. We assessed whether these predictions held true in populations that have preferred consanguineous marriage for many generations. We found that in individuals with a recessive disease whose parents were first cousins, on average, 11% of their genomes were homozygous (n = 38; range 5%-20%), with each individual bearing 20 homozygous segments exceeding 3 cM (n = 38; range of number of homozygous segments 7-32), and that the size of the homozygous segment associated with recessive disease was 26 cM (n = 100; range 5-70 cM). These data imply that prolonged parental inbreeding has led to a background level of homozygosity increased approximately 5% over and above that predicted by simple models of consanguinity. This has important clinical and research implications.

Exonic microdeletions in the X-linked PQBP1 gene in mentally retarded patients: a pathogenic mutation and in-frame deletions of uncertain effect.

Mutations in PQBP1 were recently identified in families with syndromic and non-syndromic X-linked mental retardation (XLMR). Clinical features frequently associated with MR were microcephaly and/or short stature. The predominant mutations detected so far affect a stretch of six AG dinucleotides in the polar-amino-acid-rich domain (PRD), causing frameshifts in the fourth coding exon. We searched for PQBP1 exon 4 frameshifts in 57 mentally retarded males in whom initial referral description indicated at least one of the following criteria: microcephaly, short stature, spastic paraplegia or family history compatible with XLMR, and in 772 mentally retarded males not selected for specific clinical features or family history. We identified a novel frameshift mutation (23 bp deletion) in two half-brothers with specific clinical features, and performed prenatal diagnosis in this family. We also found two different 21 bp in-frame deletions (c.334-354del(21 bp) and c.393-413del(21 bp)) in four unrelated probands from various ethnic origins, each deleting one of five copies of an imperfect seven amino-acid repeat. Although such deletions have not been detected in 1180 X chromosomes from European controls, the c. 334-354del(21 bp) was subsequently found in two of 477 Xs from Indian controls. We conclude that pathogenic frameshift mutations in PQBP1 are rare in mentally retarded patients lacking specific associated signs and that the 21 bp in-frame deletions may be non-pathogenic, or alternatively could act subtly on PQBP1 function. This touches upon a common dilemma in XLMR, that is, how to distinguish between mutations and variants that may be non-pathogenic or represent risk factors for cognitive impairment.

MORM syndrome (mental retardation, truncal obesity, retinal dystrophy and micropenis), a new autosomal recessive disorder, links to 9q34.

A consanguineous pedigree is described where 14 individuals are affected with a novel autosomal recessive disorder, which causes static moderate mental retardation, truncal obesity, a congenital nonprogressive retinal dystrophy and micropenis in males. We have tentatively named this condition MORM syndrome. It shows similarities to Bardet-Biedl syndrome and Cohen syndrome, but can be distinguished by clinical features; the age of onset and nonprogressive nature of the visual impairment, the lack of characteristic facies, skin or gingival infection, microcephaly, 'mottled retina', polydactyly and small penis without testicular anomalies. Furthermore, linkage to the known Bardet-Biedl (BBS1-8) and Cohen syndrome loci was excluded. Autozygosity mapping identified a single homozygous subtelomeric region shared by all affecteds on chromosome 9q34.3, with a maximum LOD score of 5.64. We believe this to be the first example of the identification of a subtelomeric recessive locus by autozygosity mapping.

A centrosomal mechanism involving CDK5RAP2 and CENPJ controls brain size.

Autosomal recessive primary microcephaly is a potential model in which to research genes involved in human brain growth. We show that two forms of the disorder result from homozygous mutations in the genes CDK5RAP2 and CENPJ. We found neuroepithelial expression of the genes during prenatal neurogenesis and protein localization to the spindle poles of mitotic cells, suggesting that a centrosomal mechanism controls neuron number in the developing mammalian brain.

Protein-truncating mutations in ASPM cause variable reduction in brain size.

Mutations in the ASPM gene at the MCPH5 locus are expected to be the most common cause of human autosomal recessive primary microcephaly (MCPH), a condition in which there is a failure of normal fetal brain development, resulting in congenital microcephaly and mental retardation. We have performed the first comprehensive mutation screen of the 10.4-kb ASPM gene, identifying all 19 mutations in a cohort of 23 consanguineous families. Mutations occurred throughout the ASPM gene and were all predicted to be protein truncating. Phenotypic variation in the 51 affected individuals occurred in the degree of microcephaly (5-11 SDs below normal) and of mental retardation (mild to severe) but appeared independent of mutation position.

ASPM is a major determinant of cerebral cortical size.

One of the most notable trends in mammalian evolution is the massive increase in size of the cerebral cortex, especially in primates. Humans with autosomal recessive primary microcephaly (MCPH) show a small but otherwise grossly normal cerebral cortex associated with mild to moderate mental retardation. Genes linked to this condition offer potential insights into the development and evolution of the cerebral cortex. Here we show that the most common cause of MCPH is homozygous mutation of ASPM, the human ortholog of the Drosophila melanogaster abnormal spindle gene (asp), which is essential for normal mitotic spindle function in embryonic neuroblasts. The mouse gene Aspm is expressed specifically in the primary sites of prenatal cerebral cortical neurogenesis. Notably, the predicted ASPM proteins encode systematically larger numbers of repeated 'IQ' domains between flies, mice and humans, with the predominant difference between Aspm and ASPM being a single large insertion coding for IQ domains. Our results and evolutionary considerations suggest that brain size is controlled in part through modulation of mitotic spindle activity in neuronal progenitor cells.