Activity of the immunoglobulin heavy and kappa light chain gene enhancers depends on a complex interplay of ubiquitous and developmentally regulated proteins. Two complementary DNAs were isolated that encode proteins, denoted ITF-1 and ITF-2, that are expressed in a variety of cell types and bind the microE5/kappa 2 motif found in both heavy and kappa light chain enhancers. The complementary DNAs are the products of distinct genes, yet both ITF-1 and ITF-2 are structurally and functionally similar. The two proteins interact with one another through their putative helix-loop-helix motifs and each possesses a distinct domain that dictates transcription activation.

Hypophosphatasia is a heritable form of rickets/osteomalacia with extremely variable clinical expression.Severe forms are inherited in an autosomal recessive fashion; the mode of transmission of mild forms is uncertain. The biochemical hallmark of hypophosphatasia is deficient activity of the tissue-nonspecific isozyme of alkaline phosphatase (TNSALP). Previously, we demonstrated in one inbred infant that an identical missense mutation in both alleles of the gene encoding TNSALP caused lethal disease. We have now examined TNSALP cDNAs from four unrelated patients with the severe perinatal or infantile forms of hypophosphatasia. Each of the eight TNSALP alleles from these four individuals contains a different point mutation that causes an amino acid substitution. These base changes were not detected in at least 63 normal individuals and, thus, appear to be causes of hypophosphatasia in the four patients. (Two additional base substitutions, found in one allele from each of the four patients, are linked polymorphisms.) Twenty-three unrelated patients (of 50 screened), who reflect the entire clinical spectrum of hypophosphatasia, possess one of four of the above eight mutations. In two of these additional patients, mild forms ofthe disease are also inherited in an autosomal recessive fashion. Our findings indicate that hypophosphatasia can be caused by a number of different missense mutations and that the specific interactions of different TNSALP mutant alleles are probably important for determining clinical expression. Severe forms, perinatal and infantile disease, are largely the result of compound heterozygosity for different hypophosphatasia alleles. At least some cases of childhood and adult hypophosphatasia are inherited as autosomal recessive traits.

Alkaline phosphatases (ALPs) [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] isolated from human liver, bone, and kidney (L/B/K) exhibit very similar biochemical and immunologic properties that differentiate them from other human ALPs, such as those characteristically found in placenta and intestine. Despite their similarities, the L/B/K ALPs produced in different tissues show slight physical differences. To examine structural and evolutionary relationships between the various ALPs, a cDNA corresponding to L/B/K ALP mRNA has been isolated. A AgtlI cDNA expression library was constructed using poly(A)RNA from the osteosarcoma cell line Saos-2 and screened with anti-liver ALP antiserum. The 2553-base-pair cDNA contains an open reading frame that encodes a 524 amino acid polypeptide with a predicted molecular mass of 57.2 kDa. This ALP precursor protein contains a presumed signal peptide of 17 amino acids followed by 37 amino acids that are identical to the amino-terminal sequence determined from purified liver ALP.

The gene encoding the gamma chain of the lymphocyte interleukin-2 receptor has been cloned and shown to be required to associate with the beta chain in order for IL-2 internalization and cell activation to occur (1). We considered this gene, IL2RG, a candidate for the X-linked form of severe combined immunodeficiency at the SCIDX1 locus, in which affected males have impaired lymphocyte development. Using fluorescence in situ hybridization and PCR amplification of somatic cell hybrid DNAs, we mapped IL2RG to human Xq13.1, a location within the SCIDX1 critical region established by linkage analysis. The 4.2 kb IL2RG gene was sequenced, and its genomic organization was elucidated. Seven of 19 transformed B-lymphocyte cell lines with independent SCIDX1 mutations had absent or minimal IL2RG mRNA. Unique point mutations were documented to be specifically associated with the disease and the carrier state in four unrelated affected males and their family members: one in a boy with no detectable IL2RG mRNA, in which the mutation ablated a splice donor site; one causing premature chain termination; and two causing distinct amino acid changes. The demonstration of impaired IL2RG mRNA expression in males with X-linked SCID and of unique point mutations in SCIDX1 pedigrees constitutes powerful evidence that the SCIDX1 gene is IL2RG. Noguchi et al. (2) have independently published IL2RG mapping to Xq13 and discovery of mutations in three affected males. The specific pathogenesis of IL2RG mutations and approaches to gene therapy can now be addressed in the X-linked form of SCID.