Elizabeth C. Melvin

Genetic studies in neural tube defects

Neural tube defects (NTD) are one of the most common birth defects and are caused by both environmental and genetic factors. The approach to identifying the genes predisposing to NTD, through linkage analysis and candidate gene analysis, is reviewed along with characteristics of a large, nationally ascertained cohort of families. Results from specific assessments of p53, PAX3 and MTHFR failed to suggest that these genes play a major role in NTD development in these families. Advances in genetic laboratory and statistical techniques have made this a prime opportunity for investigation into the causes of complex disorders, such as NTD. However, traditional approaches may prove to be challenging due to the difficulty of ascertaining samplable multiplex families.

The thermolabile variant of methylenetetrahydrofolate reductase (MTHFR) is not a major risk factor for neural tube defect in American Caucasians

ABSTRACTMutations in the gene for methylenetetrahydrofolate reductase (MTHFR) have been implicated as a risk factor in the formation of neural tube defects. We investigated this gene in a series of 65 sporadic American Caucasians patients with lumbosacral NTD and their unaffected parents, using both case-control design and assessment of linkage disequilibrium. We found no evidence to support mutations in MTHFR as a significant risk factor for NTD in this population.

Possible interaction of genotypes at cystathionine β‐synthase and methylenetetrahydrofolate reductase (MTHFR) in neural tube defects

Neural tube defects are a common, complex disorder with genetic and environmental components to risk. We investigated the previously reported interaction between homozygosity for the thermolabile variant at the methylenetetrahydrofolate reductase and heterozygosity for the 844ins68 allele at the cystathionine β‐synthase loci in cases with lumbosacral myelomeningocele and their parents. Using control allele frequencies from our sample pooled with those published in the literature, we confirm a marginally significant interaction at these two loci. This finding suggests that additional, larger studies are warranted to investigate this possible interaction in more detail.

Updated investigations of the role of methylenetetrahydrofolate reductase in human neural tube defects

Folate supplementation appears to reduce the risk for neural tube defects (NTDs). Methylenetetrahydrofolate reductase (MTHFR) is a candidate gene in the folate metabolism pathway that has been extensively studied in different human populations. We examined the risk associated with having the thermolabile variant (TT) of MTHFR in a study of 175 American Caucasians with NTDs and their families. We found a significant association in patients compared with 195 unrelated controls [odds ratio (OR) = 2.13, 95% confidence interval (95% CI) = 1.11–4.09)], but not in mothers (OR = 1.29, 95% CI = 0.622–2.67) or in fathers (OR = 1.45, 95% CI = 0.681–3.09). We found no evidence for unequal transmission from parents to an affected child (p > 0.10). We failed to find a previously reported association for a combined haplotype for MTHFR and cystathionine β‐synthase, except in subjects with NTDs compared with 559 pooled controls (OR = 2.87, 95% CI = 1.03–8.03). We found no evidence for an association for a novel CA‐repeat polymorphism identified in a gene closely linked to MTHFR (p > 0.10). Our studies continue to suggest that additional candidate genes other than MTHFR may be responsible for an increased risk to NTD in some American Caucasian families. 

Whole genomewide linkage screen for neural tube defects reveals regions of interest on chromosomes 7 and 10

Neural tube defects (NTDs) are the second most common birth defects (1 in 1000 live births) in the world. Periconceptional maternal folate supplementation reduces NTD risk by 50–70%; however, studies of folate related and other developmental genes in humans have failed to definitively identify a major causal gene for NTD. The aetiology of NTDs remains unknown and both genetic and environmental factors are implicated. We present findings from a microsatellite based screen of 44 multiplex pedigrees ascertained through the NTD Collaborative Group. For the linkage analysis, we defined our phenotype narrowly by considering individuals with a lumbosacral level myelomeningocele as affected, then we expanded the phenotype to include all types of NTDs. Two point parametric analyses were performed using VITESSE and HOMOG. Multipoint parametric and nonparametric analyses were performed using ALLEGRO. Initial results identified chromosomes 7 and 10, both with maximum parametric multipoint lod scores (Mlod) >2.0. Chromosome 7 produced the highest score in the 24 cM interval between D7S3056 and D7S3051 (parametric Mlod 2.45; nonparametric Mlod 1.89). Further investigation demonstrated that results on chromosome 7 were being primarily driven by a single large pedigree (parametric Mlod 2.40). When this family was removed from analysis, chromosome 10 was the most interesting region, with a peak Mlod of 2.25 at D10S1731. Based on mouse human synteny, two candidate genes (Meox2, Twist1) were identified on chromosome 7. A review of public databases revealed three biologically plausible candidates (FGFR2, GFRA1, Pax2) on chromosome 10. The results from this screen provide valuable positional data for prioritisation of candidate gene assessment in future studies of NTDs.

Recurrence risks for neural tube defects in siblings of patients with lipomyelomeningocele.

Purpose: Neural tube defects (NTDs) are a group of widely varying congenital malformations resulting from incomplete or improper fusion of the neural tube during embryonic development. NTDs are traditionally classified by the presence or absence of a layer of skin covering the spinal defect. Although a genetic component has been well established in the etiology of open NTDs, studies examining the genetics of closed NTDs such as lipomyelomeningocele are rare. We and others have previously observed families in which multiple members were affected with a broad spectrum of NTDs, suggesting the possibility of a common genetic etiology.Methods: We calculated the sibling recurrence risk in 52 pedigrees in which the proband was diagnosed with lipomyelomeningocele (LMM), defining recurrence broadly to include both closed and open neural tube defects.Results: Although no recurrences of LMM were observed among younger siblings, one younger sibling had myelomeningocele, yielding an estimate of recurrence risk of 0.04 (95% CI 0.01–0.20). When all siblings of the proband were included, two additional affected siblings were identified, one with anencephaly and another with fatty filum, yielding an estimate of recurrence risk of 0.043 (95% CI 0.01–0.12).Conclusions: Although the sample size is small, these data are not inconsistent with recurrence risks for myelomeningocele, ranging from 2% to 5% in siblings. These data suggest the underlying genetic basis for closed defects may be the same or closely related to that for myelomeningocele in some families, although a larger sample will be necessary before these data are appropriate for use in a clinical setting. Further characterizations, including whether risk for recurrence of NTDs or LMM in families in which the proband is affected with LMM are altered by folate supplementation, may shed light on the underlying genetics.

SNPs in the neural cell adhesion molecule 1 gene (NCAM1) may be associated with human neural tube defects

Neural tube defects (NTDs) are common birth defects, occurring in approximately 1/1,000 births; both genetic and environmental factors are implicated. To date, no major genetic risk factors have been identified. Throughout development, cell adhesion molecules are strongly implicated in cell–cell interactions, and may play a role in the formation and closure of the neural tube. To evaluate the role of neural cell adhesion molecule 1 (NCAM1) in risk of human NTDs, we screened for novel single-nucleotide polymorphisms (SNPs) within the gene. Eleven SNPs across NCAM1 were genotyped using TaqMan. We utilized a family-based approach to evaluate evidence for association and/or linkage disequilibrium. We evaluated American Caucasian simplex lumbosacral myelomeningocele families (n=132 families) using the family based association test (FBAT) and the pedigree disequilibrium test (PDT). Association analysis revealed a significant association between risk for NTDs and intronic SNP rs2298526 using both the FBAT test (P=0.0018) and the PDT (P=0.0025). Using the HBAT version of the FBAT to look for haplotype association, all pairwise comparisons with SNP rs2298526 were also significant. A replication study set, consisting of 72 additional families showed no significant association; however, the overall trend for overtransmission of the less common allele of SNP rs2298526 remained significant in the combined sample set. In addition, we analyzed the expression pattern of the NCAM1 protein in human embryos, and while NCAM1 is not expressed within the neural tube at the time of closure, it is expressed in the surrounding and later in differentiated neurons of the CNS. These results suggest variations in NCAM1 may influence risk for human NTDs.

Clinical experience with array CGH: Case presentations from nine months of practice

A total of 124 individuals were tested in the initial 9 months that array CGH technology was offered to clinical genetics patients. In 11 of these patients array CGH identified a previously unsuspected diagnosis. A suspected diagnosis was confirmed in three patients. A single case in this series proved to be a polymorphic copy number variant. This paper describes five of the patients with previously unsuspected diagnoses in detail. We suggest that array CGH is an improved tool ready for routine use in clinical genetics.

Complex Genetic Disorders: Evaluating When Genetic Research Findings Are Applicable for Genetic Counseling Practice

Traditional genetic counseling processes and principles will be extended to a new realm—complex disorders. Although it may seem like a daunting task, understanding the methodologies used to study complex genetic disorders will enable genetic counselors to critically analyze research studies involving complex disorders. In this article, we explain newly evolving methodologies for genetic research, including case-control studies and transmission disequilibrium testing (TDT). Additionally, a framework is provided for evaluating original research findings and replication studies.

No evidence for heterozygote advantage at MTHFR in patients with lumbosacral myelomeningocele or their relatives.

No evidence for heterozygote advantage at MTHFR in patients with lumbosacral myelomeningocele or their relatives