Philip D. Cotter

Paternal uniparental disomy for chromosome 14 : a case report and review

Uniparental disomy (UPD) for several chromosomes has been associated with disease phenotypes. Maternal UPD for chromosome 14 has been described and has a characteristic abnormal phenotype. Paternal UPD14 is rare and only three previous cases have been reported. We describe a new case of paternal UPD for chromosome 14 in an infant with a 45,XX,der(13q;14q) karyotype, which was confirmed by molecular analysis. The proposita had findings similar to those of the previous cases of patUPD14 and we conclude that there is a characteristic patUPD14 syndrome most likely due to imprinting effects. Couples with Robertsonian translocations involving chromosome 14 should be counseled as to the possibility of UPD14 and the option of prenatal diagnosis when indicated.

Molecular Defects of Erythroid 5-Aminolevulinate Synthase in X-Linked Sideroblastic Anemia

The erythroid-specific isozyme of 5-aminolevulinate synthase (ALAS2), the first and ratelimiting enzyme of heme biosynthesis, is expressed concomitantly with the differentiation and maturation of the erythroid cell in order to accommodate generation of the large amounts of heme required for hemoglobin production. During the past few years the ALAS2 gene and its transcript have been characterized and the amino acid sequence of the enzyme deduced. The human genetic disorder X-linked sideroblastic anemia, previously postulated to be caused by defects of ALAS, has now been analyzed at the molecular and tissue-specific level. A heterogeneous group of point mutations in the catalytic domain of the ALAS2 enzyme has been found to cause the disorder. Impaired activity of recombinant mutant ALAS2 enzymes has also been demonstrated. Characterization of molecular defects in individuals with X-linked sideroblastic anemia has provided improved diagnosis for at-risk family members.

Assignment of human erythroid δ-aminolevulinate synthase (ALAS2) to a distal subregion of band Xp11.21 by PCR analysis of somatic cell hybrids containing X ; autosome translocations

The erythroid-specific (ALAS2) and housekeeping (ALAS1) genes encoding delta-aminolevulinate synthase have recently been mapped to chromosomes Xp21.1----q21 and 3p21, respectively. The erythroid-specific gene is a candidate for mutations resulting in X-linked sideroblastic anemia. Analysis of DNA from hybrid clones containing translocations in the region Xp11.21----Xq21.3 permitted the finer localization of the ALAS2 gene with respect to other loci and breakpoints within this region. These studies localized the ALAS2 gene to the distal subregion of Xp11.21 in Interval 5 indicating the following gene order: Xpter-OATL2-[L62-3A, Xp11.21; A62-1A-4b, Xp11.21]-(ALAS2, DXS323)-[B13-3, Xp11.21; C9-5, Xp11.21]-(DXS14, DXS429)-DXS422-(DXZ1, Xcen). Thus, the reported linkage of acquired sideroblastic anemia and sideroblastic anemia with ataxia to Xq13 presumably results from genes other than ALAS2.

Loss of subtelomeric sequence associated with a terminal inversion duplication of the short arm of chromosome 4

We report on a 4(1/2)-year-old girl, who presented with multiple minor anomalies consistent with trisomy for 4p. GTG-banding identified a de novo terminal inversion duplication of distal 4p, dup(4)(p16.3p15.3). Fluorescence in situ hybridization (FISH) with a wcp4 probe confirmed the chromosome 4 origin of the additional material. FISH with a 4p subtelomere probe, D4F26, showed no signal on the dup(4) chromosome identifying a deletion of this region. Molecular analysis of 4p STS loci confirmed the subtelomeric deletion and showed loss of the paternal allele in this region. The paternal origin of the deleted region and homozygosity for one of the two paternal alleles within the region of the duplication suggests that a sister chromatid rearrangement on the paternal chromosome 4 was involved in the formation of the dup(4) chromosome. To date, the best characterized mechanisms of formation of chromosome duplications are terminal inversion duplications of 8p, which were shown to be derived from rearrangements at maternal meiosis-I. Our data show that mechanisms other than a maternal meiosis-I rearrangement can lead to the formation of terminal inversion duplications. FISH analysis with the appropriate subtelomeric probes is warranted in terminal inversion duplications to check for associated deletions.

Assignment of the human housekeeping δ-aminolevulinate synthase gene (ALAS1) to chromosome band 3p21.1 by PCR analysis of somatic cell hybrids

The precise map position of the human house-keeping delta-aminolevulinate synthase (ALAS1) gene has been localized to chromosome band 3p21.1. PCR analysis of somatic cell and radiation hybrids localized ALAS1 to the same distal region of 3p21.1 as the aminoacylase-1 gene.

Mosaicism for a small supernumerary ring X chromosome in a dysmorphic, growth-retarded male: mos47,XXY/48,XXY, + r(X)

Supernumerary ring X [r(X)] chromosomes are often found in patients with Turner syndrome. The phenotypic effects of the r(X) chromosome are variable, and largely depend on the presence or absence of the X inactivation (XIST) locus. Ring(X) chromosomes in males are rare and have been previously reported in only four cases, with 47,XY, + r(X) or mos47,XY, + r(X)/46,XY karyotypes. These patients all had developmental delay and dysmorphic features. We describe a 2.5‐year‐old male patient with facial dysmorphia, growth retardation, microcephaly, global developmental delay, and microphallus. Cytogenetic analysis from peripheral blood lymphocytes and fibroblasts identified mosaicism for two cell lines: mos48,XXY, + r(?X)/47,XXY. Fluorescence in situ hybridization (FISH) with an X chromosome paint showed the ring chromosome to be X chromosome derived. This is the first case of an r(X) chromosome described in a 47,XXY patient. FISH analysis of the r(X) chromosome with an XIST probe showed that the XIST locus was absent. Functional disomy of genes in the r(X) chromosome most likely accounts for the abnormal phenotype in the proband.

Prenatal detection and molecular characterization of a de novo duplication of the distal long arm of chromosome 19

A tandem duplication of the distal long arm of chromosome 19 was identified in a 10 week fetus by analysis of chorionic villi. The fetal karyotype from two primary cultures was 46,XY,dir dup(19)(q13.2q13.4). The origin of the extra material was confirmed by fluorescence in situ hybridization using a chromosome 19 whole chromosome probe. Parental chromosomes were normal, indicating a de novo origin of the extra chromosome material. This is the first case of dup(19q) detected by prenatal diagnosis. Molecular studies demonstrated that the duplication involved a maternal chromosome 19.

Screening for the mitochondrial DNA A3243G mutation in children with insulin-dependent diabetes mellitus☆

Since recent studies demonstrated the occurrence of the mitochondrial DNA (mtDNA) mutation A3243G in patients with adult-onset diabetes, an investigation was undertaken to determine the frequency of this mutation in a pediatric population with insulin-dependent diabetes mellitus (IDDM). DNA was extracted from peripheral blood of 270 pediatric patients with IDDM. The presence of the mtDNA A3243G mutation was screened for by minisequencing and mutation-specific ApaI endonuclease restriction after polymerase chain reaction (PCR) amplification of mtDNA. The A3243G mtDNA mutation was not found in any IDDM patients examined. This mutation is uncommon in children with IDDM from various ethnic and racial groups. Therefore, the contribution of the mutation to the pathogenesis of IDDM, if any, is minimal.

Prenatal diagnosis of a fetus with two balanced de novo chromosome rearrangements

Two apparently balanced chromosome rearrangements were identified in a 17-week fetus by analysis of cultured amniocytes. The fetal karyotype was 46,XX,t(2;16) (q33;q24), inv(7)(p15q11.23). Parental karyotypes were normal, indicating a de novo origin of both chromosome rearrangements in the fetus. The risk of phenotypic abnormality from a de novo reciprocal translocation of inversion has been estimated at approximately 7% [Warburton, 1991]. The risk of abnormality in this fetus was estimated to be a minimum of 14%, based on the additive risk of each rearrangement, equivalent to 3.5% per chromosome breakpoint. The pregnancy was terminated because of the risk of abnormality and the detection of intrauterine growth retardation by ultrasound. In the absence of additional experience, the minimum presumed risk of phenotypic abnormality for de novo, multiple or complex chromosome rearrangements identified prenatally may be estimated as the additive risk of the number of chromosome breakpoints involved.

Prenatal diagnosis of a familial interchromosomal insertion of Y chromosome heterochromatin

An apparently unbalanced karyotype containing an abnormal chromosome 11 was identified in a 16-week female fetus by analysis of cultured amniocytes. Fluorescence in situ hybridization (FISH) with a chromosome 11 paint identified the presence of an insertion in band 11q24. Parental karyotyping documented an unbalanced karyotype with the same der(11) chromosome in the phenotypically normal father. CBG-banding and FISH identified the insertion to be Yq12 heterochromatin: 46,XY, der(11)ins(11;Y)(q24;q12q12).ish der(11) (wcp11+,DYZ1+). The same der(11) chromosome was also found in the phenotypically normal paternal grandmother, demonstrating this additional Y chromosomal material did not affect normal female sexual development or fertility. The parents elected to continue the pregnancy and a normal girl was born at term, further confirming that this rare familial variant has no clinical significance. This case illustrates the importance of family studies, appropriate banding, and FISH analyses to accurately characterize apparent chromosomal abnormalities.