Last month, Murphy and colleagues (Cell, 2015) published a fascinating report about a patient with an immunodeficiency syndrome that underwent spontaneous resolution. The mechanism for this remarkable outcome points to the importance of somatic cell selection and evolution in the origins, pathogenesis, and most dramatically in this case, elimination of disease.
Patient WHIM-09, a white female, came to the National Institutes of Health at the age of 58 along with two of her three daughters, 23 and 21 years old at the time, who had been afflicted by recurrent infections, warts, panleukopenia (reduced numbers of multiple types of white blood cells in the blood), and hypogammaglobulinemia (reduced concentrations of antibodies in the blood). She asked that the doctors evaluate her as well as her two affected daughters.
WHIM-09 herself had suffered from repeated infections and warts up to the age of 38 and not since. The constellation of features originally exhibited by the patient and two of her three daughters were consistent with WHIM syndrome, an immunodeficiency condition associated with warts (W), hypogammaglobulinemia (H), infection (I), and myelokathexis (M). This latter feature of WHIM syndrome refers to a relative inability of mature neutrophils to exit the bone marrow in normal numbers, resulting in neutropenia. In addition, as was true for both of WHIM-09’s daughters, affected individuals can exhibit less than normal numbers of hematopoietic cell types other than neutrophils in the peripheral circulation, particularly B lymphocytes.
The cause of WHIM syndrome was shown in 2003 (Hernandez et al.) to be due to a dominant mutation in the gene encoding the chemokine receptor, CXCR4, which is expressed by multiple hematopoietic cell types and is activated by the chemokine, CXCL12, also known as SDF-1. SDF-1 is produced by bone marrow stromal cells. When SDF-1 binds to and activates CXCR4 on hematopoietic stem cells, it causes the retention of these cells in the bone marrow.
The disease-associated CXCR4 mutation truncates the intracytoplasmic portion of the receptor and causes increased signaling. Therefore, this mutation is classified as a gain-of-function mutation due to the increase in receptor function at the molecular level, but the effect at the cellular level is loss of function, i.e. reduced immune activity leading to increased incidence of infectious complications, such as warts due to papilloma virus. WHIM-09 had this CXCR4-truncating mutation, and since her parents and siblings exhibited no WHIM syndrome-related symptoms or signs, it was likely a de novo mutation arising anew in either the sperm or egg that combined to form the zygote that would become WHIM-09.
So what mechanism could account for WHIM-09 no longer suffering from myelokathexis, warts, or other recurrent infections? How can a putatively ‘genetic’ disease, caused by a mutation presumably inherited in the germline genome simply vanish?
Murphy and colleagues began evaluating the three patients, but I will focus on the index patient, WHIM-09. The investigators found that WHIM-09’s absolute peripheral neutrophil count, which had been abnormally low earlier in her life, as is typical for WHIM syndrome, was now actually above normal, as was the peripheral monocyte count. Whereas WHIM-09’s bone marrow, earlier in life, had been characterized by a number of cellular abnormalities, it was now free of these histopathological indicators of dysfunction.
Although the total peripheral lymphocyte count for WHIM-09 was normal, the numbers of naïve T cells and B cells were below normal. Consistent with a modest decrease in naïve B lymphocytes, WHIM-09 exhibited reduced concentrations of serum immunoglobulins (about 85% of the lower limit of normal). Thus, WHIM-09 was not entirely normal for all laboratory tests.
The authors then tested WHIM-09, her two affected daughters, her third and unaffected daughter, and her unaffected husband for the specific mutation in CXCR4. WHIM-10 and WHIM-11, the two affected daughters, both had the mutation and neither the third daughter nor the husband possessed the mutation.The results for WHIM-09 were more interesting. In samples dominated by neutrophils and monocytes, the mutation, was not present. However, in samples dominated by lymphocytes the CXCR4 mutation was present. Samples from the lining of the cheek and the skin of WHIM-09 also had the CXCR4 mutation in heterozygous form. Thus, WHIM-09 had become a chimera in the hematopoietic compartment with myeloid lineages free of the disease-causing CXCR4 allele but lymphoid lineages still carrying it. So, by what mechanism or mechanisms could such an outcome be spontaneously realized?
The investigators studied the chromosomes of bone marrow cells from WHIM-09. In all 20 metaphase spreads analyzed, the only abnormal chromosome identified in each cell was one copy of chromosome 2, on which the CXCR4 locus resides. Further chromosomal investigation by fluorescence in situ hybridization, chromosomal banding, microarray analysis, and whole genome sequencing convincingly established that the abnormal instances of chromosome 2 were shorter than the normal chromosomes 2 and had suffered seven major deletions constituting about 35 megabases of DNA. Crucially, one of these deletions encompassed the CXCR4 locus. An additional 163 protein-coding loci were also deleted in one lost segment or another. There were also inversions.
Retained portions of the normal chromosome 2 in the abnormal chromosomes were in what the authors regarded as random orientations and random order. Synthesizing this substantial accumulation of information the authors proposed that in one hematopoietic stem cell one chromosome 2 had undergone chromothripsis (Stephens et al., 2011), or chromosome shattering, a spontaneous dissolution and restructuring of the genetic material. While such a major genetic disruption can lead to cell death, in this amazing instance, the loss of one copy of at least one gene resulted in an apparent selective advantage for the this one stem cell, at least for producing cells of the myeloid lineage.
While haploinsufficiency for CXCR4 provides a selective advantage for cells developing in the myeloid lineage, the same was not true for the lymphoid lineage. The loss of the mutant CXCR4 or another of the 163 genes deleted from the post-chromothripsis chromosome 2 (or perhaps some other unappreciated genetic change) apparently prevented the progenitors containing the abnormal chromosome 2 from competing with other progenitors destined to differentiate along the lymphoid lineage.
As the frosting on this therapeutic cake, the authors then assessed the hypothesis that reduced CXCR4 signaling might promote hematopoietic stem cell (HSC) engraftment in the context of clinical blood and marrow transplantation. Their results in a murine bone marrow transplant model suggest that inhibiting CXCR4 function might be of use in clinical hematopoietic cell transplantation.
Evolution is generally described as changes in proportions of genotypes in populations of organisms, cells, or molecules. What this fascinating study emphasizes is that despite the necessary evolutionary focus on populations, an alteration in one gene, on one chromosome, in one cell, in one patient can be the engine of astonishing medical, and perhaps, evolutionary consequences. WHIM-09 suffered an unfortunate sort of ‘genetic lightning’ strike in the form of a de novo mutation causing her constellation of WHIM-related symptoms and signs. And then, she was again struck by ‘genetic lightning,’ in the form of what is typically regarded as a genetic catastrophe, even if of single-cell scope. Astonishingly, what might have been a cellular calamity turned out to be the source of organismal resurrection (physiologically speaking). Somatic cell evolution righted the wrong wrought by genetic drift, a causal component of organismal evolution.
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