DRD2 and PPP1R1B (DARPP-32) polymorphisms independently confer increased risk for autism spectrum disorders and additively predict affected status in male-only affected sib-pair families.

BACKGROUND: The neurotransmitter dopamine (DA) modulates executive functions, learning, and emotional processing, all of which are impaired in individuals with autism spectrum disorders (ASDs). Our previous findings suggest a role for dopamine-related genes in families with only affected males. METHODS: We examined two additional genes which affect DA function, the DRD2 and PPP1R1B (DARPP-32) genes, in a cohort of 112 male-only affected sib-pair families. Selected polymorphisms spanning these genes were genotyped and both family-based and population-based tests were carried out for association analysis. General discriminant analysis was used to examine the gene-gene interactions in predicting autism susceptibility. RESULTS: There was a significantly increased frequency of the DRD2 rs1800498TT genotype (P = 0.007) in affected males compared to the comparison group, apparently due to over-transmission of the T allele (P = 0.0003). The frequency of the PPP1R1B rs1495099CC genotype in affected males was also higher than that in the comparison group (P = 0.002) due to preferential transmission of the C allele from parents to affected children (P = 0.0009). Alleles rs1800498T and rs1495099C were associated with more severe problems in social interaction (P = 0.0002 and P = 0.0016, respectively) and communication (P = 0.0004 and P = 0.0046), and increased stereotypic behaviours (P = 0.0021 and P = 0.00072). General discriminant analysis found that the DRD2 and PPP1R1B genes additively predicted ASDs (P = 0.00011; Canonical R = 0.26) and explain ~7% of the variance in our families. All findings remained significant following corrections for multiple testing. CONCLUSION: Our findings support a role for the DRD2 and PPP1R1B genes in conferring risk for autism in families with only affected males and show an additive effect of these genes towards prediction of affected status in our families.

A DRD1 haplotype is associated with risk for autism spectrum disorders in male-only affected sib-pair families.

Individuals with autism spectrum disorders (ASDs) have impairments in executive function and social cognition, with males generally being more severely affected in these areas than females. Because the dopamine D1 receptor (encoded by DRD1) is integral to the neural circuitry mediating these processes, we examined the DRD1 gene for its role in susceptibility to ASDs by performing single marker and haplotype case-control comparisons, family-based association tests, and genotype-phenotype assessments (quantitative transmission disequilibrium tests: QTDT) using three DRD1 polymorphisms, rs265981C/T, rs4532A/G, and rs686T/C. Our previous findings suggested that the dopaminergic system may be more integrally involved in families with affected males only than in other families. We therefore restricted our study to families with two or more affected males (N = 112). There was over-transmission of rs265981-C and rs4532-A in these families (P = 0.040, P = 0.038), with haplotype TDT analysis showing over-transmission of the C-A-T haplotype (P = 0.022) from mothers to affected sons (P = 0.013). In addition, haplotype case-control comparisons revealed an increase of this putative risk haplotype in affected individuals relative to a comparison group (P = 0.004). QTDT analyses showed associations of the rs265981-C, rs4532-A, rs686-T alleles, and the C-A-T haplotype with more severe problems in social interaction, greater difficulties with nonverbal communication and increased stereotypies compared to individuals with other haplotypes. Preferential haplotype transmission of markers at the DRD1 locus and an increased frequency of a specific haplotype support the DRD1 gene as a risk gene for core symptoms of ASD in families having only affected males.

SALL1 mutation analysis in Townes-Brocks syndrome: twelve novel mutations and expansion of the phenotype.

Townes-Brocks syndrome is an autosomal dominantly inherited disorder, which comprises multiple birth defects including renal, ear, anal, and limb malformations. TBS has been shown to result from mutations in SALL1, a human gene related to the developmental regulator SAL of Drosophila melanogaster. The SALL1 gene product is a zinc finger protein thought to act as a transcription factor. It contains four highly conserved, evenly distributed C2H2 double zinc finger domains. A single C2H2 motif is attached to the second domain, and at the amino terminus SALL1 contains a C2HC motif. Most mutations causing TBS are clustered in the N-terminal third of the SALL1 coding region and result in the production of truncated proteins containing only one or none of the C2H2 domains and the N-terminal transcriptional repressor domain of SALL1. Twenty-three SALL1 mutations were reported prior to this work, 22 of which are located in exon 2, 5' of the second double zinc finger-encoding region. Here we present 12 novel mutations in SALL1 associated with Townes-Brocks syndrome in 13 unrelated families. These include three nonsense mutations, three short insertions and six short deletions. Thus the number of SALL1 mutations increases to 35. Rare phenotypical features among mutation positive patients include hypothyroidism, vaginal aplasia with bifid uterus, cryptorchidism, bifid scrotum without hypospadia scrotalis, unilateral chorioretinal coloboma with loss of vision, dorsal hypoplasia of the corpus callosum, and umbilical hernia.

Familial pericentric inversion of chromosome 18: behavioral abnormalities in patients heterozygous for either the dup(18p)/del(18q) or dup(18q)/del(18p) recombinant chromosome.

We describe a family in which the largest hitherto reported pericentric inversion of chromosome 18, inv(18)(p11.22q23), segregates. Individuals heterozygous for the nonrecombinant inversion were unaffected. However, those heterozygous for either the dup(18p)/del(18q) or dup(18q) /del(18p) recombinant exhibited mild learning difficulty, personality disorders and deficient social behavior in the absence of mental retardation. Of the three family members tested, the behavioral abnormalities were more prominent in the two individuals with the dup(18p)/del(18q) recombinant than in the one with the dup(18q)/del(18p) recombinant. Genetic counseling issues for this family, in particular for the affected, include the enhanced probability of reduced fertility as well as the recurrence risk of the parental inversion equaling 1/2 in surviving offspring. This observation kindles the interest in determining the frequency of subtelomeric rearrangements in individuals with learning difficulty and deficiency in social interaction, phenotypic features often considered to be of multifactorial causation.

A mixed epigenetic/genetic model for oligogenic inheritance of autism with a limited role for UBE3A.

The genetic contribution to autism is often attributed to the combined effects of many loci (ten or more). This conclusion is based in part on the much lower concordance for dizygotic (DZ) than for monozygotic (MZ) twins, and is consistent with the failure to find strong evidence for linkage in genome-wide studies. We propose that the twin data are compatible with oligogenic inheritance combined with a major, genetic or epigenetic, de novo component to the etiology. Based on evidence that maternal but not paternal duplications of chromosome 15q cause autism, we attempted to test the hypothesis that autism involves oligogenic inheritance (two or more loci) and that the Angelman gene (UBE3A), which encodes the E6-AP ubiquitin ligase, is one of the contributing genes. A search for epigenetic abnormalities led to the discovery of a tissue-specific differentially methylated region (DMR) downstream of the UBE3A coding exons, but the region was not abnormal in autism lymphoblasts or brain samples. Based on evidence for allele sharing in 15q among sib-pairs, abnormal DNA methylation at the 5'-CpG island of UBE3A in one of 17 autism brains, and decreased E6-AP protein in some autism brains, we propose a mixed epigenetic and genetic model for autism with both de novo and inherited contributions. The role of UBE3A may be quantitatively modest, but interacting proteins such as those ubiquitinated by UBE3A may be candidates for a larger role in an oligogenic model. A mixed epigenetic and genetic and mixed de novo and inherited (MEGDI) model could be relevant to other "complex disease traits".

The HOXA1 A218G polymorphism and autism: lack of association in white and black patients from the South Carolina Autism Project.

A recent study has suggested that the A218G polymorphism in the homeobox A1 (HOXA1) gene may influence susceptibility to autism. We have determined the frequencies of the A and G alleles of the HOXA1 A218G polymorphism in both white and black patients from the South Carolina Autism Project (SCAP) and controls. Marked differences were found in allele frequencies between the races, but no deviations from Hardy-Weinberg equilibrium were seen in either white or black SCAP family members. More direct tests, comparing genotype frequencies between probands and controls and tracking transmission of the A versus G alleles to affected offspring, did not support the contention that allele status for the HOXA1 A218G polymorphism influences one's susceptibility to autism.

Absence of MeCP2 mutations in patients from the South Carolina autism project.

The methyl-CpG binding protein 2 (MeCP2) gene has recently been identified as the gene responsible for Rett syndrome (RS), a pervasive developmental disorder considered by many to be one of the autism spectrum disorders. Most female patients with MeCP2 mutations exhibit the classic features of RS, including autistic behaviors. Most male patients with MeCP2 mutations exhibit moderate to severe developmental delay/mental retardation. Ninety nine patients from the South Carolina autism project (SCAP) were screened for MeCP2 mutations, including all 41 female patients from whom DNA samples were available plus the 58 male patients with the lowest scores on standard IQ tests and/or the Vineland Adaptive Behavior Scale. No pathogenic mutations were observed in these patients. One patient had the C582T variant, previously reported in the unaffected father of an RS patient. Two other patients had single nucleotide polymorphisms in the 3' UTR of the gene, G1470A and C1516G. These variants were seen in 12/82 and 1/178 phenotypically normal male controls, respectively. The findings from this and other studies suggest that mutations in the coding sequence of the MeCP2 gene are not a significant etiological factor in autism.

Molecular cytogenetic characterization of a de novo unbalanced translocation leading to trisomy 17q25-->qter and monosomy 18p11.3-->pter in a girl with dysmorphic features.

An 18-year-old, gravida 1 underwent percutaneous umbilical blood sampling (PUBS) because of positive triple screen, oligohydramnios and markedly short fetal bones. Chromosome analysis showed an abnormal chromosome 18 with unidentified chromatin at the end of the p-arm. Parental karyotypes were normal. FISH analyses with wcp18 showed additional material of unknown origin on the derivative chromosome 18. Further FISH analysis with subtelomeric probes showed normal signals for the long arm of chromosome 18 (18q23) while no signals were observed for the short arm (18p11.32). These findings were confirmed using a YAC probe from the short arm of 18. The infant was delivered at 30 weeks of gestation. At age 3 months, she was developmentally delayed and has multiple dysmorphic features. Further molecular cytogenetic studies including M-FISH and subtelomere probes showed that the additional material on chromosome 18 consisted of the distal 17q25-->qter region. Based on these studies the karyotype has been interpreted as 46,XX,der(18)t(17;18)(q25;p11.32). To the best of our knowledge, this is the first report of partial monosomy 18p and partial trisomy 17q in a patient with no major CNS malformations. This case shows the importance of molecular cytogenetic techniques in detailed characterization of de novo chromosome rearrangements.

Hydrocephalus and intestinal aganglionosis: is L1CAM a modifier gene in Hirschsprung disease?

Congenital hydrocephalus associated with aqueductal stenosis and/or agenesis of the corpus callosum has been described in newborn males with mutations in L1CAM, a gene that encodes a neural cell adhesion molecule. These males usually have severe mental retardation and may have spastic paraplegia and adducted thumbs. In contrast, Hirschsprung disease, or absence of ganglion cells in the distal gut, has rarely been described in such individuals. We report a male infant who had severe hydrocephalus identified in the prenatal period with evidence of aqueductal stenosis and adducted thumbs at birth. He developed chronic constipation, and rectal biopsy confirmed the diagnosis of Hirschsprung disease. Molecular testing of the L1CAM gene revealed a G2254A mutation, resulting in a V752M amino acid substitution. A common polymorphism in RET, but no mutation, was identified. Our patient represents the third example of coincident hydrocephalus and Hirschsprung disease in an individual with an identified L1CAM mutation. We hypothesize that L1CAM-mediated cell adhesion may be important for the ability of ganglion cell precursors to populate the gut, and that L1CAM may modify the effects of a Hirschsprung disease-associated gene to cause intestinal aganglionosis.

Prenatal diagnosis of L1 cell adhesion molecule mutations. Capabilities and limitations.

OBJECTIVE: Discuss the capability for and limitations of prenatal detection of L1 cell adhesion molecule (L1CAM) mutations. METHODS: Haplotype analysis by PCR and PAGE. Mutation detection by SSCP, followed by dideoxy sequencing. Confirmation of sequencing results with PCR and NcoI digestion. RESULTS: A 1-bp deletion was found in exon 2 of L1CAM in all affected males and obligate carriers in the pedigree. Prenatal detection is now possible for subsequent pregnancies. CONCLUSION: In a large gene with widespread mutations such as L1CAM, a mutation must be detected in another family member before direct prenatal mutation testing can be done within the required timeframe. If the proper family members are available, haplotyping offers a fast but indirect test with several limitations.