Antoniettina Rinaldi

A Novel Approach to Search for Identity by Descent in Small Samples of Patients and Controls from the Same Mendelian Breeding Unit: A Pilot Study on Myopia

Autosomal dominant high myopia, a genetic disorder already mapped to region 18p11.31, is common in Carloforte (Sardinia, Italy), an isolated village of 8,000 inhabitants descending from a founder group of 300 in the early 1700s. Fifteen myopic propositi and 36 normal controls were selected for not having ancestors in common at least up to the grandparental generation, although still descendants of the original founders. All subjects were genotyped for 14 markers located on autosome 18 at a resolution of about 10 cM. Allelic distributions were found to be similar at all tested loci in propositi and controls, except for the candidate marker D18S63 known to segregate in close linkage association with high myopia. In particular, the frequency of allele 85 among the propositi was almost double that of the controls (Fisher’s exact test, p = 0.037). The association is more striking when the frequency of the genotype 85/85 in the two groups is compared (Fisher’s exact test, p = 0.005). This conclusion was further evaluated through a bootstrap analysis by computing the overall probability of the observed data under the null hypothesis (i.e. no difference between the two groups in frequency distributions for the chromosome 18 markers). Again, marker D18S63 was found to have a sample probability lower than 0.004, which is significant at the 0.05 level after correcting for simultaneous testing of multiple loci. The study demonstrates the efficiency of our novel strategy to detect identity by descent (IBD) in small numbers of patients and controls when they are both part of well-defined Mendelian breeding units (MBUs). The iterative application of our strategy in separate MBUs is expected to become the method of choice to evaluate the ever-growing number of reported associations between candidate genes and multifactorial traits and diseases.

The myoblast defect identified in Duchenne muscular dystrophy is not a primary expression of the DMD mutation

SummaryWe previously proposed the hypothesis that the primary expression of the defect in X-linked Duchenne muscular dystrophy (DMD) occurred in the myoblast, or muscle precursor cell. This was based on the observation that the number of viable myoblasts obtained per gram DMD muscle tissue was greatly reduced and those that grew in culture had decreased proliferative capacity and an aberrant distended flat morphology. Here we test that hypothesis by determining whether the expression of the myoblast defect is X-linked. Muscle cells were obtained from five doubly heterozygous carriers of two X-linked loci, DMD and glucose-6-phosphate dehydrogenase (G6PD), and compared with those from five sex-and age-matched controls heterozygous for G6PD only. A total of 1,355 individual clones were determined to be muscle and evaluated at the single cell level for proliferative capacity, morphology, and G6PD isozyme expression. The results demonstrate that the proportion of defective myoblast clones is significantly increased in DMD carriers. However, since this cellular defect does not consistently segregate with a single G6PD phenotype in the myoblast clones derived from any of the carriers, it is unlikely to be the primary expression of the DMD mutant allele.

Further linkage evidence for localization of mutational sites for nonsyndromic types of X‐linked mental retardation at the pericentromeric region

We used several microsatellite markers scattered along the X chromosome to search for linkage relationships in a large Sardinian pedigree segregating for nonspecific X-linked mental retardation (MRX). Markers DXS573 and AR, located at chromosomal subregions Xp11.4-p11.22 and Xq11.2-q12, respectively, were found to segregate in full concordance with the disease, leading to a LOD score of 4.21 at zero recombination value. Recombination with the disease was found with markers MAOB and DXS454 located at Xp11.4-p11.3 and Xq21.1-q22, respectively; accordingly, markers distal to Xp11.4 and Xq22 also segregated independently of the disease. These findings provide strong linkage evidence in favor of the localization of one MRX mutational site in the pericentromeric region of the human X chromosome, justifying the assignment of a new symbol (MRX26) to our pedigree. Finally, on the basis of the recombinational events observed in the Xq21-q22 region, we have been able to refine the assignment of marker DXS456 to Xq21.33-q22.

Old and new genetics help ordering loci at the telomere of the human X-chromosome long arm.

SummaryA Sardinian pedigree described in 1964 for having been found to segregate at the X-linked loci for the Xga antigen, G6PD deficiency, Protan and Deutan color blindness, with an instance of recombination between the last two loci, was re-examined with respect to four common X-linked DNA polymorphisms detected by molecular probes homologous to critical subregions of the human X chromosome. Two branches of this pedigree-including the one with the Protan-Deutan recombinant-were found to segregate also for the common BamHI polymorphism identified with the cDNA probe pHPT-2 of the HPRT gene (Xq26). The analysis of the chromosome haplotypes in the male offspring of the phase known penta-heterozygous mother suggests that the probable order of the relevant loci is HPRT, Deutan, G6PD, Protan, Xq telomere. Though we are fully aware of the risks of generalizing the significance of observations made on a single exceptional pedigree, we believe that this report outlines the potential of families of the type described as reasearch tools to resolve the linear order of tightly X-linked loci and to investigate the biology of genetic recombination in humans.

Pilot Study on Schizophrenia in Sardinia

Objective: Based on a small sample of cases with schizophrenia and control individuals from an isolated population, a genome-wide association study was undertaken to find variants conferring susceptibility to this disease. Methods: Standard association tests were employed, followed by newer multilocus association methods (genotype patterns). Results: Individually, no variant produced a significant result. However, the best two variants (rs1360382 on chromosome 9 and rs1303 on chromosome 14) showed significantly different genotype pattern distributions between patients and control individuals. The risk genotype pattern AA-TT is highly predictive of schizophrenia, with estimated sensitivity and specificity of 1 and 0.96, respectively. Conclusions: These findings support the hypothesis that schizophrenia is partly due to multiple genetic variants, each with a relatively small effect.

+2.71 LOD score at zero recombination is not sufficient for establishing linkage between X-linked mental retardation and X-chromosome markers.

Nonspecific X-linked mental retardation (MRX) is the denomination attributed to the familial type of mental retardation compatible with X-linked inheritance but lacking specific phenotypic manifestations. It is thus to be expected that families falling under such broad definition are genetically heterogeneous in the sense that they may be due to different types of mutations occurring, most probably, at distinct X-chromosome loci. To facilitate a genetic classification of these conditions, the Nomenclature Committee of the Eleventh Human Gene Mapping Workshop proposed to assign a unique MRX-serial number to each family where evidence of linkage with one or more X-chromosome markers had been established with a LOD score of at least +2 at zero recombination. This letter is meant to emphasize the inadequacy of this criterion for a large pedigree where the segregation of the disease has been evaluated against the haplotype constitution of the entire X-chromosome carrying the mutation in question. 12 refs., 2 figs., 1 tab.

Premutation for the Martin-Bell Syndrome Analyzed in a Large Sardinian Family: III. Molecular Analysis with the StB12.3 Probe

This report complements a series of clinical, cytogenetical, and psychological studies previously reported on a large Sardinian pedigree segregating for premutations and full mutations associated with the Martin-Bell syndrome (MBS). Using the StB12.3 probe, we report now the molecular classification of all of the critical members of the pedigree. These molecular findings are evaluated against the variable phenotypic manifestations of the disease in the course of a six-generation segregation of an MBS premutation allegedly present in a common female progenitor of 14 MBS male patients and 9 female MBS heterozygotes seen in the last two generations. The nature and stepwise progression of MBS-premutations toward the fully manifested Martin-Bell syndrome and the possibility of reverse mutational events toward the normal allele are discussed with respect to the application of the presently available diagnostic tools in genetic counselling.

Further data on a 9.1-kb insertion-deletion polymorphism: survey of Mediterranean populations.

ABSTRACT We report the distribution of a previously described 9.1-kb insertion-deletion polymorphism located on chromosome 22. We analyzed 1,844 individuals sampled from 26 Mediterranean populations in mainland Italy, Sicily, Sardinia, Tunisia, Libya, Morocco, Egypt, Greece, and Albania. The 9.1 kb − allele is the prevalent allele in the North African (range, 0.53–0.56), Greek (0.51), and Albanian (0.66) populations, whereas the 9.1 kb + allele is most frequent in a mainland Italian town (0.55) and in all Sicilian and Sardinian towns and villages thus far tested, with marked fluctuation ranges of 0.53–0.78 and 0.56–0.80, respectively. In tests for Hardy-Weinberg equilibrium the genotype frequencies observed in Athens and in four of the nine towns in Sicily (but in none of the towns in Sardinia) departed highly significantly from the expected values. Identical results were found in the same towns for a second insertion-deletion polymorphism located on chromosome 22q13 at a distance compatible with a low incidence of recombination. The data, which are in good agreement with the different histories of the two islands (Sardinia and Sicily), are consistent with a west-east differentiation in Sicily and support the evidence for ancient gene flow from the Iberian peninsula to Sardinia.