Although successfully rescued from SCID by HSCT, our patient suffered 2 severe, uncommon complications of chronic GVHD: recurrent acute myositis and myasthenia gravis (MG). MG has been associated with particular haplotypes of HLA class II, including DR3 [5
], and class I, including A1 and B8 [6
]. However, neither the patient nor his mother, the donor, had these HLA haplotypes. Although each of the patient's complications has been reported in SCID patients post-HSCT, we could find only 2 reports in which both disorders co-existed in a patient with chronic GVHD [7
]. One recovered, a child with aplastic anemia post HSCT from an HLA-matched sibling, but GVHD-associated myositis, like ours, requires prolonged immunosuppression, with glucocorticoids or calcineurin antagonists [9
Persistence of host B cells is a common finding post-HSCT in patients with T-B+ SCID, including those with IL7R
]. Some IL7R-deficient SCID patients who retain only autologous B cells are nontheless able to make specific antibodies and avoid lifelong IVIG treatment, presumably because IL-7 signals are not required for development of functional human B cells as long as allogeneic T helper cells are restored. Whether autoimmune disease in these patients could result from mixed chimerism, as was suggested in a mouse model of chronic GVHD [10
] is as yet unknown. However, our patient's successful B cell engraftment indicates that autoantibodies and immune dysregulation are not limited to patients who have donor T cells but retain autologous B cells.
Over a dozen genes have been implicated in SCID, and the biological effects of their mutations can be grouped into four broad categories: defects in cytokine receptor pathways (γc, Jak3, IL7Ra), accumulation of toxic metabolites (ADA, PNP), T cell receptor signaling defects (CD45 and the CD3 γ, ε, δ, and ξ chains), and defects in DNA recombination (recombinase activating genes RAG1/2, Artemis, Cernunnos, and DNA Ligase 4) [2
]. At least 10% of SCID cases still have no identified mutations. However, most gene sequencing is limited to coding and immediately flanking splice regions from genomic DNA, and this approach would miss intronic mutations such as that in our patient.
Our patient had a previously unreported mutation in intron 3 of IL7R
that introduced into the mRNA a 104 bp insertion containing an early termination codon. This mutation was greatly under-represented in panels of cDNA clones from both patient and mother, indicating that mRNA containing the 104 bp insertion was unstable, consistent with nonsense mediated decay. Intronic mutations that result in incorporation of new exons into transcripts have been described in a number of human genetic diseases, including β thalassemia [12
], ornithine aminotransferase deficiency [13
], cystic fibrosis [14
], β-glucuronidase deficiency [16
], neurofibromatosis type I [18
], ataxia-telangiectasia [19
], and Fabry disease [20
]. Searching within introns could be fruitful in other cases of a suspected compound heterozygous mutation where only one mutant allele has been found. Analysis of cDNA would require banking of mRNA or cells if a molecular diagnosis is not easily made.
The discovery of the mutations in IL7R causing SCID in this patient has also made possible prenatal testing of the parents' subsequent at-risk pregnancy. Fetal DNA obtained from an amniocentesis contained the paternal 353 A>G C118 Y mutation, but not the maternal 379 (+288) g>a mutation, predicting that this infant would not have IL7R SCID. Indeed, postnatal lymphocyte counts and T cell function were normal, a confirmation that the baby is unaffected.