Mutation of MYH9, encoding non-muscle myosin heavy chain A, in May-Hegglin anomaly.

May-Hegglin anomaly (MHA) is an autosomal dominant macrothrombocytopenia of unclear pathogenesis characterized by thrombocytopenia, giant platelets and leukocyte inclusions. Studies have indicated that platelet structure and function are normal, suggesting a defect in megakaryocyte fragmentation. The disorder has been linked to chromosome 22q12-13. Here we screen a candidate gene in this region, encoding non-muscle myosin heavy chain A (MYH9), for mutations in ten families. In each family, we identified one of three sequence variants within either the -helical coiled coil or the tailpiece domain that co-segregated with disease status. The E1841K mutation was found in 5 families and occurs at a conserved site in the rod domain. This mutation was not found in 40 normal individuals. Four families had a nonsense mutation that resulted in truncation of most of the tailpiece. One family had a T1155I mutation present in an affected mother and daughter, but not in the mother's parents, thus representing a new mutation. Among the 30 affected individuals, 21 unaffected individuals and 13 spouses in the 10 families, there was correlation of a variant of MYH9 with the presence of MHA. The identification of MYH9 as the disease gene for MHA establishes the pathogenesis of the disorder, should provide further insight into the processes of normal platelet formation and may facilitate identification of the genetic basis of related disorders.

Nevoid basal cell carcinoma syndrome with medulloblastoma in an African-American boy: a rare case illustrating gene-environment interaction.

We present an 8-year-old African-American boy with medulloblastoma and nevoid basal cell carcinoma syndrome (NBCCS) who exhibited the radiosensitive response of basal cell carcinoma (BCC) formation in the area irradiated for medulloblastoma. Such a response is well-documented in Caucasian NBCCS patients with medulloblastoma. The propositus was diagnosed with medulloblastoma at the age of 2 years and underwent surgery, chemotherapy, and craniospinal irradiation. At the age of 6 years, he was diagnosed with NBCCS following his presentation with a large odontogenic keratocyst of the mandible, pits of the palms and soles and numerous BCCs in the area of the back and neck that had been irradiated previously for medulloblastoma. Examination of other relatives showed that the propositus' mother also had NBCCS but was more mildly affected; in particular, she had no BCCs. This case illustrates complex gene-environment interaction, in that increased skin pigmentation in African-Americans is presumably protective against ultraviolet, but not ionizing, radiation. This case and other similar cases in the literature show the importance of considering NBCCS in the differential diagnosis of any patient who presents with a medulloblastoma, especially before the age of 5 years, and of examining other close relatives for signs of NBCCS to determine the patient's at-risk status. Finally, for individuals who are radiosensitive, protocols that utilize chemotherapy in lieu of radiotherapy should be considered.

Sib-pair linkage analyses of nuclear family data: quantitative versus dichotomous disease classification.

Model-free sib-pair linkage analysis was used to screen 367 highly polymorphic markers for evidence of linkage to a disease, defined either quantitatively (Q1) or dichotomously (AF). Five individual replicates, plus a case family data set containing all families in these replicates with at least one individual with AF, were analyzed. Sib-pair linkage results for Q1 and AF varied considerably among the five replicates and did not consistently detect any of the three underlying major loci, MG1, MG2, and MG3. For the pooled case families, linkage analyses of Q1, but not AF, detected the flanking markers for MG1 and MG2 at the 0.05 and 0.01 levels, respectively. Overall, type 1 error rates were not elevated. The ability to analyze the disease quantitatively (Q1) and construct a data set more appropriate for linkage analysis (case families) enhanced the power to detect at least some of the major loci underlying the disease.

A genome-wide search for loci contributing to smoking and alcoholism.

Using the Collaborative Study on the Genetics of Alcoholism (COGA) data, we performed a sib-pair linkage analysis of two smoking-related traits and one alcoholism phenotype. The first trait, EVRNVR, was a dichotomous one we constructed based on epidemiological definitions of smoking. The second trait, PKYRS, used the quantitative pack-year history provided, and the third trait was the COGA alcoholism classification, ALDX1. There was some evidence for linkage of the EVRNVR trait to regions-on chromosomes 6, 9, and 14. Smaller numbers of loci provided nominal evidence for linkage to PKYRS, although some candidate gene regions were identified. The number of loci identified using EVRNVR suggests that a threshold-based phenotype may better identify loci affecting smoking history. Approximately one-third of the loci that showed evidence for linkage to EVRNVR at a nominal significance level (p < 0.01) also showed evidence for linkage to ALDX1. Some of these regions may represent loci increasing vulnerability to both smoking and alcoholism.

Sib-pair linkage analyses of alcoholism: dichotomous and quantitative measures.

We hypothesized that a quantitative alcoholism trait would have greater power than the Collaborative Study on the Genetics of Alcoholism (COGA) dichotomous alcoholism traits, ALDX1 and ALDX2, to detect putative alcoholism loci. To test this, we performed nonparametric sib-pair linkage analysis to screen 285 polymorphic autosomal markers for evidence of linkage to ALDX1, ALDX2, and a quantitative trait, QUANT, defined from the 11 COGA latent class variables. We also examined the effects on the analyses of including covariates (sex, age, and pack-years of smoking) and of transforming QUANT (log and square root). ALDX1 and ALDX2 showed the greatest evidence for linkage to markers on chromosome 1, by both the affected sib-pair and the Haseman-Elston tests. Regions of interest were also identified on chromosomes 4, 8, 16, and 17. QUANT showed little evidence for linkage to any chromosomal region, having no more significant results than were expected by chance. Including covariates or transforming QUANT had little effect on the analyses. A quantitative trait based on all 37 latent class variables, with each variable appropriately weighted, may have had more power than QUANT to detect genomic regions of relevance to alcoholism.

Detecting gene-environment interactions using a case-control design.

We assessed the sample size required for detecting gene-environment (G x E) interactions in a case-control study of complex diseases. The results suggest that large numbers of cases and controls will be required to detect G x E interaction for some odds ratio and exposure frequency combinations. These and other results suggest that alternative study designs may be needed to detect G x E interaction particularly with rare genes or uncommon environmental exposures.

Effects of marker information on sib-pair linkage analysis of a rare disease.

Model-free sib-pair linkage analysis was used to screen the GAW9-Problem 1 data set for evidence of linkage of a rare disease to any of 360 highly polymorphic marker loci. Negative regressions nominally significant at the alpha = 0.05 level were obtained for 44 markers; however all of these proved to be Type I errors. None of the four disease loci were detected by sib-pair linkage, which was not surprising, given the particular model and sampling scheme used to generate these data. Neither deleting parental marker genotypic information nor misspecifying marker allele frequency estimates substantially increased the Type I error rate. A two-stage testing procedure using a 10 or 20 cM map and a liberal first stage significance level gave the same overall results as a one-stage 2 cM map but required only about 42% or 22% as many markers, respectively.

Autosomal dominant macrothrombocytopenia with leukocyte inclusions (May-Hegglin anomaly) is linked to chromosome 22q12-13.

Macrothrombocytopenia with leukocyte inclusions (May-Hegglin anomaly) is a rare autosomal dominant disorder characterized by thrombocytopenia, giant platelets, and Döhle body-like inclusions in leukocytes. To determine the genetic basis of this disorder, we performed a genome-wide screen for linkage in three families with May-Hegglin anomaly. For the pooled analysis of the three families, three markers on chromosome 22 had two-point logarithm-of-difference (lod) scores greater than 3, with a maximum lod score of 3.91 at a recombination fraction (theta) of 0.076 for marker D22S683. Within the largest family (MHA-1), the maximum lod score was 5.36 at theta=0 at marker D22S445. Fine mapping of recombination events using eight adjacent markers indicated that the minimal disease region of family MHA-1 alone is in the approximately 26 cM region from D22S683 to the telomere. The maximum lod score for the three families combined was 5.84 at theta=0 for marker IL2RB. With the assumption of locus homogeneity, haplotype analysis of family MHA-4 indicated the disease region is centromeric to marker D22S1045. These data best support a minimal disease region from D22S683 to D22S1045, a span of about 1 Mb of DNA that contains 17 known genes and 4 predicted genes. Further analysis of this region will identify the genetic basis of May-Hegglin anomaly, facilitating subsequent characterization of the biochemical role of the disease gene in platelet formation.

Familial chordoma, a tumor of notochordal remnants, is linked to chromosome 7q33.

Chordoma is a rare tumor originating from notochordal remnants that is usually diagnosed during midlife. We performed a genomewide analysis for linkage in a family with 10 individuals affected by chordoma. The maximum two-point LOD score based on only the affected individuals was 2.21, at recombination fraction 0, at marker D7S2195 on chromosome 7q. Combined analysis of additional members of this family (11 affected individuals) and of two unrelated families (one with 2 affected individuals and the other with 3 affected individuals), with 20 markers on 7q, showed a maximum two-point LOD score of 4.05 at marker D7S500. Multipoint analysis based on only the affected individuals gave a maximum LOD score of 4.78, with an approximate 2-LOD support interval from marker D7S512 to marker D7S684. Haplotype analysis of the three families showed a minimal disease-gene region from D7S512 to D7S684, a distance of 11.1 cM and approximately 7.1 Mb. No loss of heterozygosity was found at markers D7S1804, D7S1824, and D7S2195 in four tumor samples from affected family members. These results map a locus for familial chordoma to 7q33. Further analysis of this region, to identify this gene, is ongoing.