Molecular analysis of holoprosencephaly in South America.

Holoprosencephaly (HPE) is a spectrum of brain and facial malformations primarily reflecting genetic factors, such as chromosomal abnormalities and gene mutations. Here, we present a clinical and molecular analysis of 195 probands with HPE or microforms; approximately 72% of the patients were derived from the Latin American Collaborative Study of Congenital Malformations (ECLAMC), and 82% of the patients were newborns. Alobar HPE was the predominant brain defect in almost all facial defect categories, except for patients without oral cleft and median or lateral oral clefts. Ethmocephaly, cebocephaly, and premaxillary agenesis were primarily observed among female patients. Premaxillary agenesis occurred in six of the nine diabetic mothers. Recurrence of HPE or microform was approximately 19%. The frequency of microdeletions, detected using Multiplex Ligation-dependant Probe Amplification (MLPA) was 17% in patients with a normal karyotype. Cytogenetics or QF-PCR analyses revealed chromosomal anomalies in 27% of the probands. Mutational analyses in genes SHH, ZIC2, SIX3 and TGIF were performed in 119 patients, revealing eight mutations in SHH, two mutations in SIX3 and two mutations in ZIC2. Thus, a detailed clinical description of new HPE cases with identified genetic anomalies might establish genotypic and phenotypic correlations and contribute to the development of additional strategies for the analysis of new cases.

Association of methylenetetrahydrofolate reductase gene 677C > T polymorphism and Down syndrome.

The association between Down syndrome (DS) and maternal polymorphisms in genes encoding folic acid metabolizing enzymes remains a controversial issue. A meta-analysis was performed to evaluate the association of maternal MTHFR 677C > T polymorphism and the risk of having a child with DS. Case-control studies were screened from major literature databases. Twenty articles from 13 countries worldwide, with a total of 2,101 DS and 2,702 control mothers, attended the inclusion criteria. We found a 50 % increase for the association of maternal homozygous TT genotype and DS in both fixed (OR = 1.51; 95 % CI 1.22-1.87) and random effects models (OR 1.54; 95 % 1.15-2.05). Similarly, a significant pooled OR was found for the heterozygote CT, with an OR 1.26; 95 % CI 1.10-1.43 (fixed effects model) and OR 1.28; 95 % 1.08-1.51 (random effects model). As ultra-violet B solar radiation highly depends on latitude, and can promote, in less pigmented skin, intravascular folate photolysis, we stratified the analysis by latitude region, defining as Tropical (between 23.5(°) S and 23.5(°) N), Sub-Tropical (between 23.5(°) and 40(°) N and S), and Northern (≥ 40(o) N). Significant association was only found for Sub-Tropical area, both using fixed and random effect models. In conclusion, MTHFR 677C > T polymorphism is a moderate risk factor for DS for some populations, and populations located in Sub-Tropical region seem to be at greater risk. Latitude, ethnicity, skin pigmentation, and red blood cell folate are important variables to be considered in future studies.

Dihydrofolate Reductase (DHFR) del19bp Polymorphism and Down Syndrome Offspring.

Down syndrome (DS) is the most common form of mental disability of genetic etiology. Nondisjunction of chromosome 21 is the leading cause of the syndrome. In general, free trisomy 21 cases originate from missegregation in maternal meiosis. Several reports have suggested an association between genetic variants in genes encoding folate metabolizing enzymes and the predisposition to chromosome missegregation. We have conducted a case-control study of 109 DS case mothers (MDS) and 248 control mothers (CM) to assess the association between DHFR del19bp polymorphism and an increased risk of bearing a DS child. Genomic DNA was extracted from buccal cells, and molecular analysis of DHFR del19pb polymorphism was performed by polymerase chain reaction (PCR). Both MDS and CM allelic and genotypic distributions were in Hardy-Weinberg equilibrium. The frequency of DHFR del19pb-mutated allele was 0.54 in MDS and 0.46 in CM. Overall analysis showed that the mutant allele was borderline associated with DS risk (OR 1.38; 95% CI 1.00-1.89; P = 0.05) and a weak positive association for del/del and/or wt/del genotypes of DHFR del19pb polymorphism compared to homozygous wt/wt genotype was identified (OR = 1.75; 95% CI 1.01-3.03; P = 0.05). When we have analyzed data stratified by age, there is an increased risk of bearing a DS child associated with the polymorphic allele (OR = 1.49; 95% CI 1.03-2.16; P = 0.03), suggesting that DHFR del 19-bp polymorphism could be an independent risk factor for DS in women aged < 40 years old.

DNA (cytosine-5)-methyltransferase 3B (DNMT 3B) polymorphism and risk of Down syndrome offspring.

Down syndrome (DS) is the most common form of human genetic mental retardation. Several polymorphisms in genes coding folic acid cycle enzymes have been associated to the risk of bearing a DS child; however, the results are controversial. S-adenosyl-l-methionine (SAM) is an important intermediate of folic acid pathway and acts as methyl donor and substrate for DNA (cytosine-5)-methyltransferase 3B (DNMT3B - EC 2.1.1.37) de novo methylation processes during embryogenesis. Recent studies suggest that a functional polymorphism of DNMT 3B in maternal genotype may be associated with a decreased risk of having a DS child. We herein investigate the association of this polymorphism with the occurrence of DS in a Brazilian population. We have genotyped 111 mothers of DS infants (MDS) and 212 control mothers (CM) through PCR-RFLP. The observed genotypic frequencies were CC = 0.22; CT = 0.49 and TT = 0.29 in CM, and CC = 0.30; CT = 0.52 and TT = 0.18 in MDS. Allelic frequencies were C = 0.47 and T = 0.53 in CM and C = 0.56 and T = 0.44 in MDS. No deviation of HWE was observed, and both DNMT 3B rs2424913 genotype (χ2 = 4.53; DF = 1; P = 0.03) and allelic (χ2 = 4.90; DF = 1; P = 0.03) frequencies show significant differences between MDS and CM. The presence of the mutant DNMT 3B T allele decreases 30% the risk of bearing a DS child (OR = 0.69; 95% CI: 0.50-0.96; P = 0.03), and the risk is diminished up to 45% in association with the homozygous genotype (OR = 0.54; 95% CI: 0.31-0.96; P = 0.04). Our results suggest that women harboring the single nucleotide polymorphism DNMT 3B rs2424913 have a decreased risk of a DS pregnancy, and further studies are necessary to confirm this protective effect.

Dynamic mutation and human disorders: the spinocerebellar ataxias (review).

A completely new mutational event associated with human diseases - the dynamic mutation - was discovered in the last decade. The molecular mechanism underlying dynamic mutation involves the expansion and intergenerational instability of a tandem-arrayed nucleotide sequence that acquire a pathological size, despite its polymorphic occurrence in normal individuals. To date, at least fourteen neurological disorders are associated with this phenomenon, including Huntington's disease (HD), dentatorubral and palidoluysian atrophy (DRPLA), spinobulbar and muscular atrophy (SBMA), myotonic dystrophy (DM), fragile X syndrome, FRAXE mental retardation and spinocerebellar ataxias (SCA) types 1-3, 6-8, 12 and 17. The spinocerebellar ataxias comprise a heterogeneous group of severe neurodegenerative-late onset disorders characterized by loss of balance and coordination. Most of the spinocerebellar ataxias exhibit an autosomal dominant pattern of inheritance and are promoted by the intergenerational expansion of a trinucleotide repeat (CAG)n inside the coding region of the respective gene. The expanded segment is translated into an abnormal polyglutamine tract in the protein, leading to the formation of nuclear aggregates that have been considered the basis of the pathogenesis in most of SCA types. One striking characteristic of these diseases is that the gene is expressed throughout the brain and also in other tissues but no pathological consequences are observed, despite the specific cellular degeneration. The characterization of the mutational event has led to the development of specific and sensitive molecular tests for direct DNA analysis, which allow confirmation of clinical diagnostic and an adequate therapeutic indication as well as genetic counseling.

Beare-Stevenson cutis gyrata syndrome: A new case of a c.1124C↷G (Y375C) mutation in the FGFR2 gene.

Beare-Stevenson syndrome (BSS) (MIM#123790) is a rare disorder characterized by craniofacial anomalies and cutis gyrata associated with anogenital anomalies and prominent umbilical stump. There are few reports on the syndrome, and molecular analysis has revealed the involvement of two closely spaced mutations within the FGFR2 gene: c.1115C↷G (p.S372C) and c.1124C↷G (p.Y375C). We herein describe a new case of a c.1124C↷G mutation in a BSS patient.