Expression of restricted immunoglobulin isotypes in plasmacellular hyperplasia after allogeneic bone marrow transplantation

The hypervariable DXS255 locus contains a LINE-1 repetitive element with a CpG island that is extensively methylated only on the active X chromosome

The DXS255 locus at Xp11.22 is highly polymorphic due to a 26-bp variable number of tandem repeats (VNTR) motif. In previous studies, one of the MspI sites flanking the VNTR manifested a correlation between methylation and X chromosome inactivation. Here we show, by DNA sequence analysis, that this MspI site is located within the CpG island at the 5' end of a LINE-1 element, which is 2.5 kb from the VNTR. The methylation status of the CpG island was assessed in Southern blotting experiments using the methylation-sensitive enzymes HpaII, HhaI, and BssHII. All these sites were completely methylated on active X chromosomes, consistent with previously reported findings of full methylation of LINE-1 elements throughout the genome. However, on inactive X chromosomes these sites were predominantly unmethylated, although patterns were found to be heterogeneous. The results suggest that LINE-1 elements on the inactive X chromosome are not suppressed by full methylation of their CpG islands. The differential methylation of the DXS255 CpG island provides the basis for a highly informative X inactivation analysis system.

X chromosome inactivation patterns in haematopoietic cells of female carriers of X-linked severe combined immunodeficiency determined by methylation analysis at the hypervariable DXS255 locus

The patterns of X chromosome inactivation were determined in 14 females from three unrelated X-linked severe combined immunodeficiency (XSCID) pedigrees. All the females were found to be heterozygous for the hypervariable DXS255 locus, enabling analysis of differential methylation of this locus in peripheral blood haematopoietic cells. All six obligate carriers manifested a unilateral X chromosome inactivation in the T lymphocyte population. Differential methylation analysis of T lymphocytes was subsequently applied to establish the carrier status of females at risk in the XSCID pedigrees. In the B lymphocyte population of four XSCID carriers a unilateral X chromosome inactivation was observed. Four other carriers had minor fractions and one carrier had a substantial fraction of B lymphocytes with the XSCID gene defect on the active X chromosome. Within single XSCID pedigrees the carriers manifested different patterns. In two pedigrees the granulocyte populations of all carriers showed a random distribution of X chromosome inactivation. In the third pedigree the granulocytes of the three carriers analyzed manifested complete inactivation of the X chromosome that carried the XSCID mutation, exposing a selective disadvantage of granulocytes that express the XSCID defect. The pedigree-dependent differences in the involvement of the granulocyte population suggest the existence of two distinct XSCID defects.

Differential methylation at the 5' and the 3' CCGG sites flanking the X chromosomal hypervariable DXS255 locus

The degree of methylation at the 5' and 3' CCGG sequences flanking the variable number of tandem repeat (VNTR) region of the DXS255 locus at Xp11.22 was analysed separately in several haematopoietic cell lineages. The 5' CCGG site on active chromosomes was found to be completely methylated in B and T lymphocytes and granulocytes. Methylation of the 5' site on inactive X chromosomes differed between females (0%-60%), but was consistent in different cell lineages obtained from individual females. In contrast, methylation at the 3' CCGG site on active chromosomes was found to vary in B lymphocytes (40%-100%), whereas complete methylation was found in T lymphocytes and granulocytes. The extent of methylation on inactive X chromosomes was found to differ significantly between B lymphocytes (17%), T lymphocytes (54%) and granulocytes (82%). Thus, methylation at the 5' CCGG site seems to be primarily related to the status of X chromosome inactivation, whereas methylation at the 3' CCGG site is mainly subject to cell-lineage-specific influences.

Inheritance of a large deletion within the human immunoglobulin heavy chain constant region gene complex and immunological implications

A deletion of the immunoglobulin heavy chain (IgH) pseudo-gamma, gamma-2, gamma-4, epsilon, and alpha-2 constant region gene segments was found to segregate unchanged in three generations of a family. The IgG1 locus on the IgH allele carrying the deletion was expressed to the same extent as its normal counterpart. One individual who was heterozygous for the deletion had an IgG2 deficiency, whereas the four other heterozygous individuals had serum levels of IgG2 and IgG4 within the normal ranges. IgA2 levels were low or below the normal range in all heterozygous individuals. The data indicate that the expression of some Ig isotypes can be decreased by hemizygous deletions, possibly due to a lower probability for switching.

Polyclonal hyper-immunoglobulin G1(A1) syndrome. Evidence for a dominant immunoglobulin production regulator within the human immunoglobulin heavy chain gene complex

The hyper-IgG1(A1) syndrome entails a polyclonal selective increase of the serum levels of immunoglobulin (Ig) G1 and to a lesser extent of IgA1; this is not mediated by malignancy, infectious or autoimmune diseases or environmental agents. In three generations of a family, all the affected individuals carried an immunoglobulin heavy chain (IgH) allele distinguished by restriction fragment length polymorphism analysis; the IgH allele was not present in non-affected family members. A 32:1 chance for the linkage of this rare IgH haplotype with the hyper-IgG1(A1) syndrome in the family argues for a dominant regulator located at the human IgH locus having a selective influence on the production of IgG1 and IgA1.

Predominant immunoglobulin gene rearrangements in two patients with immunodeficiency: restricted use of V gene segments and DNA hypermethylation

The majority of common variable immunodeficiencies (CVID) is caused by intrinsic B cell defects which impede distinct stages of B cell differentiation. B cell differentiation is accompanied by the rearrangement of immunoglobulin (Ig) genes. The first step in the rearrangement process is the assembly of IgH genes, and subsequently, IgL genes are rearranged. During B cell maturation, Ig genes are demethylated in a stepwise, locus-specific manner. Here, we examined the Ig gene rearrangements of four patients with classical CVID and of one child suffering from an unusual immunodeficiency associated with CD5+ B cell lymphocytosis. In one of the four adult patients with CVID, we observed a predominant type of VHDJH-gene rearrangement. In the child, different polyclonal VHDJH-gene rearrangements were found together with a predominant type of kappa light chain gene rearrangement. The rearranged kappa chain genes were methylated (as in the pre-B cell stage). These findings together with the cell phenotype analysis and the clinical course of the disease in the child suggests that in some patients with primary immunodeficiency a maturation arrest may occur in B cells leading to a predominant Ig V gene rearrangement.