A. Cao

Genotype-phenotype correlations in β-thalassemias

Abstract In this paper we review the molecular basis of the marked heterogeneity of the thalassemia syndromes as well as the relative implications for carrier screening and prenatal diagnosis. The classical phenotype of heterozygous β-thalassemia may be modified by a number of evironmental and genetic interacting factors—among which the most relevant are: (1) coinheritance of α-thalassemia, which may normalize the red blood cell indices; (2) the presence of a mild β-thalassemia mutation; (3) cotrasmission of δ-thalassemia which may reduce the increase of HbA2 typical of heterozygous β-thalassemia to normal values and (4) the presence of a silent mutation which can be defined only by imbalanced β-globin chain synthesis. A number of molecular mechanisms are able to produce the non transfusion dependent attenuated forms of thalassemia syndromes referred to as thalassemia intermedia. The most common are homozygosity for mild β-thalassemia mutations, coinheritance with homozygous β-thalassemia of α-thalassemia or genetic determinants able to substain a continuous production of HbF in adult life or the presence of heterozygosity for hyperunstable globin variants.

Beta thalassaemia mutations in Sardinians: implications for prenatal diagnosis.

In this study we have characterised by oligonucleotide hybridisation and direct restriction endonuclease analysis the beta thalassaemia mutation in 494 Sardinian beta thalassaemia heterozygotes. The most prevalent mutation, accounting for 95.4% of the cases, was the nonsense mutation at codon 39. The remainder, in decreasing order of frequency, were a frameshift at codon 6 (2.2%), beta + IVS-1, nt 110 (0.4%), and beta + IVS-2, nt 745 (0.4%). This information allows prenatal diagnosis by DNA analysis to be made in the great majority of Sardinian couples at risk for beta thalassaemia.

A novel silent beta-thalassemia mutation in the distal CACCC box affects the binding and responsiveness to EKLF

The silent β‐thalassemia mutation, β+‐101C→T, is the only mutation currently described in the distal β‐globin CACCC box. We present a novel mutation, a C→G transversion, in the same position. Expression analysis in heterozygous subjects demonstrated that the mutation determines a 20% reduction in the output of the β‐globin gene. DNA–protein interaction and transactivation analysis correlated the decrease in the β‐globin synthesis with the reduced binding and transactivation of EKLF to the mutant promoter. These data predict that the β‐101C→G mutation will display a silent thalassemia phenotype similar to that of the β‐101C→T mutation.

Neonatal onset of nephrogenic syndrome of inappropriate antidiuresis

This paper describes the manifestaton in a child of a new syndrome characterized by unusual, severe, persistent hyponatremia associated with hyposmolarity, euvolemia, inappropriately concentrated urine and elevated natriuresis. This is the fourth case of this syndrome reported to date, and the first to be reported in a neonate. The clinical features resemble those typically observed in patients with inappropriate antidiuretic hormone secretion, although high arginine vasopressin (AVP) levels are lacking. The findings led the authors to hypothesise a nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The previously described R137C gain-of-function mutation was detected by means of mutation analysis of the V2R gene. Our results indicate that NSIAD is already present during the neonatal period and that molecular analysis of the V2R receptor should therefore be carried out, in all newborns with prolonged euvolemic hyponatremia with hypo-osmolarity, high urinary sodium and normal/low or undetectable AVP levels.

Chronic inflammatory demyelinating polyneuropathy as a possible novel component of autoimmune poly-endocrine-candidiasis-ectodermal dystrophy

We describe two unrelated boys with autoimmune poly-endocrine-candidiasis-ectodermal dystrophy syndrome (APECED) who, in addition to manifesting the most common symptoms (chronic mucocutaneous candidiasis, hypoparathyroidism and Addison’s disease), developed progressive muscular weakness in both the proximal and distal limbs, sensory loss and absent tendon reflexes. Electrophysiological studies disclosed a reduction of nerve conduction velocity in both patients that was consistent with the diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP).This diagnosis was supported by histological demyelination in nerve biopsy specimens with patchy CD4, CD8 and CD68-positive cell infiltration in the first patient and increased protein content in the cerebrospinal fluid in the second patient. Our cases represent the first report of an association between APECED and CIDP, in which peripheral nerve demyelination may represent a novel disease component in APECED. Our findings highlight the need to explore apparently rare manifestations in patients with APECED.

A novel splicing defect (IVS6+1G>T) in a patient with pseudovitamin D deficiency rickets

A 15-month-old boy with severe rickets, that by clinical analysis was diagnosed as affected by hereditary pseudovitamin D deficiency rickets (PDDR), was evaluated for mutations in the 25OHD3 1α-hydroxylase gene. Molecular analysis showed a double heterozygous state for a novel splicing mutation in the invariant dinucleotide of the donor site of IVS6 and a 7 nucleotide insertion in the exon 8, which is common in different ethnical backgrounds.

Prenatal diagnosis of beta thalassaemia by oligonucleotide analysis in Mediterranean populations.

We have used four oligonucleotide probes and two restriction enzymes to detect the beta thalassaemia mutation in a group of 61 couples of Italian descent who were prospective parents. We have been able to define the beta thalassaemia mutation in both parents in 47 couples and in only one parent in 12 couples. Prenatal diagnosis was accomplished successfully either by amniocyte (two) or trophoblast (26) DNA analysis in 28 couples in which the pregnancy was in progress. These results indicate that direct identification of the mutation by oligonucleotide or restriction endonuclease analysis is a practical and useful method for prenatal diagnosis of beta thalassaemia in childless couples.

A Novel Missense Mutation (C84R) in a Patient with Type II Vitamin D-dependent Rickets

A 7-year-old boy with severe rickets that by clinical analysis was diagnosed as affected by type II vitamin D-dependent rickets, was evaluated for mutations in the vitamin D receptor gene (VDR). The molecular analysis showed a homozygous state for a novel missense mutation (C84R) in a highly conserved nucleotide in the second Zn finger of the DNA binding domain.

Reliability of prenatal diagnosis of genetic diseases by analysis of amplified trophoblast DNA.

Dot blot analysis on enzymatically amplified trophoblast DNA with allele specific oligonucleotide probes is currently used for the prenatal diagnosis of single gene disorders characterised at the molecular level, such as the beta thalassaemias, phenylketonuria, sickle cell anaemia, and alpha 1-anti-trypsin deficiency. A potential problem with the use of this procedure is the co-amplification of maternal sequences, which may obscure the diagnosis in the fetus. To address this question, we carried out prenatal diagnosis of beta thalassaemia in 300 couples at risk by dot blot analysis on enzymatically amplified DNA with 32P or horseradish peroxidase labelled allele specific oligonucleotide probes. We verified the diagnosis obtained by this procedure with oligonucleotide hybridisation on electrophoretically separated non-amplified trophoblast DNA fragments. We detected no co-amplified maternal sequences, even with a faint signal, in the dot blot of trophoblast DNA from those fetuses diagnosed as normal or homozygotes, nor in those diagnosed as heterozygotes, who were born to parents carrying different mutations and had inherited the paternal mutation. These results indicate that, when careful dissection of trophoblast tissue from maternal decidua is carried out, amplification of chorionic villi DNA is not associated with amplification of maternal DNA sequences. We may thus conclude that dot blot analysis of trophoblast DNA is a very reliable procedure for prenatal diagnosis.

HB Puttelange [P140(HlS)ALAàVAL] in an Italian Man with Polycythemia

Hb Puttelange [beta 140(H18)Ala-->Val] was found in a 51-year-old Italian man who had mild polycythemia. The variant eluted from ion exchange high performance liquid chromatography at a position between Hb A and Hb A2. It comprised approximately 34% of the total hemoglobin, was weakly unstable and exhibited an increased oxygen affinity. Amplification of the beta-globin exons and nucleotide sequencing revealed a heterozygosity for a GCC-->GTC mutation in codon 140 corresponding to an Ala-->Val replacement. This substitution accounts for the altered functional properties, probably by producing indirect perturbation of the 2 3-diphosphoglycerate pocket through the nearby lysine residue at beta 82(EF6).