A Case of Wiskott-Aldrich Syndrome With Dual Mutations in Exon 10 of the WASP Gene: An Additional De Novo One-Base Insertion, Which Restores Frame Shift Due to an Inherent One-Base Deletion, Detected in the Major Population of the Patient's Peripheral Blood Lymphocytes

Somatic reversion in Wiskott-Aldrich syndrome: Case reports and mechanistic insights

This report describes two brothers from India and a Chinese patient with somatic reversion of an inherited deleterious mutation in the WAS gene. Both the Indian siblings had inherited a single nucleotide deletion causing a frameshift mutation (c.1190del, p.Pro397Argfs*48) (variant 1: marked in blue) from the mother. Another variant (variant 2: marked in red), a 12-nucleotide deletion at position 1188-1199 (c.1188_1199del, p.P401_P404del) was also found, which resulted in restoration of the frame and subsequent rescue of the protein sequence. DNA sequencing from buccal mucosal cells revealed only the inherited variant (variant 1), while no reversion mutation was identified in the mucosal cells. Similarly, the Chinese patient was found to have a novel germline 14-base duplication (ACGAAAATGCTTGG) c.120_132 + 1dup (variant 1). This resulted in abolishment of the original splice junction coupled with the creation of a new junction 14 bases 3' and a frameshift mutation with predicted protein truncation p. Thr45Aspfs*. DNA from the patient's PBMC showed co-existence of wild-type and mutated sequences, but only the mutant was present in the buccal cells. Genomic and mRNA analysis of the isolated CD3+ T lymphocytes, CD3- mononuclear cells, and EBV-transformed B lymphocytes indicated that the reverant variant (germline variant was restored to wild-type sequence) were selectively found in CD3+ T lymphocytes.

Revertant Mosaicism in Genodermatoses: Natural Gene Therapy Right before Your Eyes

Revertant mosaicism (RM) is the intriguing phenomenon in which nature itself has successfully done what medical science is so eagerly trying to achieve: correcting the effect of disease-causing germline variants and thereby reversing the disease phenotype back to normal. RM was molecularly confirmed for the first time in a genodermatosis in 1997, the genetic skin condition junctional epidermolysis bullosa (EB). At that time, RM was considered an extraordinary phenomenon. However, several important discoveries have changed this conception in the past few decades. First, RM has now been identified in all major subtypes of EB. Second, RM has also been identified in many other genodermatoses. Third, a theoretical mathematical exercise concluded that reverse mutations should be expected in all patients with a recessive subtype of EB or any other genodermatosis. This has shifted the paradigm from RM being an extraordinary phenomenon to it being something that every physician working in the field of genodermatoses should be looking for in every patient. It has also raised hope for new treatment options in patients with genodermatoses. In this review, we summarize the current knowledge on RM and discuss the perspectives of RM for the future treatment of patients with genodermatoses.

Reversion Mosaicism in Primary Immunodeficiency Diseases

Reversion mosaicism has been reported in an increasing number of genetic disorders including primary immunodeficiency diseases. Several mechanisms can mediate somatic reversion of inherited mutations. Back mutations restore wild-type sequences, whereas second-site mutations result in compensatory changes. In addition, intragenic recombination, chromosomal deletions, and copy-neutral loss of heterozygosity have been demonstrated in mosaic individuals. Revertant cells that have regained wild-type function may be associated with milder disease phenotypes in some immunodeficient patients with reversion mosaicism. Revertant cells can also be responsible for immune dysregulation. Studies identifying a large variety of genetic changes in the same individual further support a frequent occurrence of reversion mosaicism in primary immunodeficiency diseases. This phenomenon also provides unique opportunities to evaluate the biological effects of restored gene expression in different cell lineages. In this paper, we review the recent findings of reversion mosaicism in primary immunodeficiency diseases and discuss its clinical implications.

The impact of clonal diversity and mosaicism on haematopoietic function in Fanconi anaemia

Recent advances have facilitated studies of the clonal architecture of the aging haematopoietic system, and provided clues to the mechanisms underlying the origins of hematopoietic malignancy. Much less is known about the clonal composition of haematopoiesis and its impact in bone marrow failure (BMF) disorders, including Fanconi anaemia (FA). Understanding clonality in FA is likely to inform both the marked predisposition to cancer and the rapid erosion of regenerative reserve seen with this disease. This may also hold broader lessons for haematopoietic stem cell biology in other diseases with a clonal restriction. In this review, we focus on the conceptual basis and available tools to study clonality, and highlight insights in somatic mosaicism and malignant evolution in FA in the context of haematopoietic failure and gene therapy.

Mosaicism in Fanconi anemia: concise review and evaluation of published cases with focus on clinical course of blood count normalization

Fanconi anemia (FA) is a DNA repair disorder resulting from mutations in genes encoding for FA DNA repair complex components and is characterized by variable congenital abnormalities, bone marrow failure (BMF), and high incidences of malignancies. FA mosaicism arises from reversion or other compensatory mutations in hematopoietic cells and may be associated with BMF reversal and decreased blood cell sensitivity to DNA-damaging agents (clastogens); this sensitivity is a phenotypic and diagnostic hallmark of FA. Uncertainty regarding the clinical significance of FA mosaicism persists; in some cases, patients have survived multiple decades without BMF or hematologic malignancy, and in others hematologic failure occurred despite the presence of clastogen-resistant cell populations. Assessment of mosaicism is further complicated because clinical evaluation is frequently based on clastogen resistance in lymphocytes, which may arise from reversion events both in lymphoid-specific lineages and in more pluripotent hematopoietic stem/progenitor cells (HSPCs). In this review, we describe diagnostic methods and outcomes in published mosaicism series, including the substantial intervals (1-6 years) over which blood counts normalized, and the relatively favorable clinical course in cases where clastogen resistance was demonstrated in bone marrow progenitors. We also analyzed published FA mosaic cases with emphasis on long-term clinical outcomes when blood count normalization was identified. Blood count normalization in FA mosaicism likely arises from reversion events in long-term primitive HSPCs and is associated with low incidences of BMF or hematologic malignancy. These observations have ramifications for current investigational therapeutic programs in FA intended to enable gene correction in long-term repopulating HSPCs.

A "late-but-fitter revertant cell" explains the high frequency of revertant mosaicism in epidermolysis bullosa

Revertant mosaicism, or "natural gene therapy", is the phenomenon in which germline mutations are corrected by somatic events. In recent years, revertant mosaicism has been identified in all major types of epidermolysis bullosa, the group of heritable blistering disorders caused by mutations in the genes encoding epidermal adhesion proteins. Moreover, revertant mosaicism appears to be present in all patients with a specific subtype of recessive epidermolysis bullosa. We therefore hypothesized that revertant mosaicism should be expected at least in all patients with recessive forms of epidermolysis bullosa. Naturally corrected, patient-own cells are of extreme interest for their promising therapeutic potential, and their presence in all patients would open exciting, new treatment perspectives to those patients. To test our hypothesis, we determined the probability that single nucleotide reversions occur in patients' skin using a mathematical developmental model. According to our model, reverse mutations are expected to occur frequently (estimated 216x) in each patient's skin. Reverse mutations should, however, occur early in embryogenesis to be able to drive the emergence of recognizable revertant patches, which is expected to occur in only one per ~10,000 patients. This underestimate, compared to our clinical observations, can be explained by the "late-but-fitter revertant cell" hypothesis: reverse mutations arise at later stages of development, but provide revertant cells with a selective growth advantage in vivo that drives the development of recognizable healthy skin patches. Our results can be extrapolated to any other organ with stem cell division numbers comparable to skin, which may offer novel future therapeutic options for other genetic conditions if these revertant cells can be identified and isolated.

Novel WASP mutation in a patient with Wiskott-Aldrich syndrome: Case report and review of the literature

BACKGROUND

The Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive immunodeficiency disorder, caused by mutations in the WAS protein (WASP) gene and characterised by thrombocytopenia, small platelets, eczema, and recurrent infections associated with increased risk of autoimmunity and malignancy disorders. The gene for WAS has been mapped to the short arm of the X chromosome at Xp 11.22-23 and early detection of patients and diagnosis of new mutation might reduce related complications and increase their life expectancy.

METHOD AND RESULT

We found a novel mutation by sequence analysis of genomic DNA coding of a 9-month old boy suffering from WAS. The mutation was insertion G in exon 10 of WASP gene. The consequence of the G insertion is a premature stop immediately at amino acid 335 (N335X or p.G334GfsX1) and truncated protein.

CONCLUSION

The mutation analysis is helpful for the diagnosis of WAS patients and also expanding the spectrum of WASP mutations for carrier detection and prenatal diagnosis.

In vivo reversion of an inherited mutation in a Chinese patient with Wiskott-Aldrich syndrome

A spontaneous reversion that restores Wiskott-Aldrich syndrome protein (WASP) expression was reported recently. However, the genetic mechanism underlying the reversion event was unclear. In the present study, a WAS patient with a nonsense mutation (155 C>T, R41X) was followed during a three-year period. No expression of WASP was detected in peripheral blood mononucleated cells (PBMCs) in 2009 and a small population of intracellular WASP positive lymphocytes was detected during the following three years. The increasing trend of the revertant genotype was significant. WASP-expressing cells were present mainly CD56+ NK cells and CD8+ T cells. Sorted WASP+ cells were analyzed, indicating that the population of CD3+ T cells increased from 36.81% to 99.8%. Although the revertant cells in vivo may have a growth advantage, the patient presented a persistent autoimmune disease, thrombocytopenia, and died from extensive pulmonary fibrosis. The results suggest that the clinical consequences of T-cell mosaicism in WAS remain difficult to predict and is not sufficient to fully normalize immune functions in patients with WAS.

Patient-specific naturally gene-reverted induced pluripotent stem cells in recessive dystrophic epidermolysis bullosa

Spontaneous reversion of disease-causing mutations has been observed in some genetic disorders. In our clinical observations of severe generalized recessive dystrophic epidermolysis bullosa (RDEB), a currently incurable blistering genodermatosis caused by loss-of-function mutations in COL7A1 that results in a deficit of type VII collagen (C7), we have observed patches of healthy-appearing skin on some individuals. When biopsied, this skin revealed somatic mosaicism resulting from the self-correction of C7 deficiency. We believe this source of cells could represent an opportunity for translational “natural” gene therapy. We show that revertant RDEB keratinocytes expressing functional C7 can be reprogrammed into induced pluripotent stem cells (iPSCs) and that self-corrected RDEB iPSCs can be induced to differentiate into either epidermal or hematopoietic cell populations. Our results give proof in principle that an inexhaustible supply of functional patient-specific revertant cells can be obtained—potentially relevant to local wound therapy and systemic hematopoietic cell transplantation. This technology may also avoid some of the major limitations of other cell therapy strategies, e.g., immune rejection and insertional mutagenesis, which are associated with viral- and nonviral- mediated gene therapy. We believe this approach should be the starting point for autologous cellular therapies using “natural” gene therapy in RDEB and other diseases.

Distribution, clinical features and molecular analysis of primary immunodeficiency diseases in Chinese children: a single-center study from 2005 to 2011

METHODS

Two hundred three children with genetically proven primary immunodeficiency diseases (PIDs) from 197 unrelated families were enrolled from January 2005 to December 2011.

RESULTS

On the basis of criteria developed by the International Union of Immunological Societies, 79 patients were diagnosed as "other well-defined immunodeficiency syndromes" (38.9%), 62 (30.6%) with "predominant antibody deficiencies," 26 (12.8%) with "congenital defects of phagocyte," 25 (12.3%) with "T- and B-cell immunodeficiency" and 11 (5.4%) with "diseases of immune dysregulation." The median time to the diagnosis was 27.9 months and the patients had a wide range of clinical presentations. In addition, a total of 23 pathogenic genes were identified and 213 mutations were detected, including 42 novel mutations.

CONCLUSIONS

With the increase in the awareness of PIDs and diagnostic competence, more PID patients will be diagnosed and we will be able to more accurately identify the frequency and the distribution of PIDs in the most populous country in the world.

Wiskott-Aldrich syndrome; an x-linked primary immunodeficiency disease with unique and characteristic features

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency disease with unique and characteristic features. In 1994, the responsible gene for WAS, the WASP gene on X-chromosome, was identified. Since then, renewed clinical and basic researches of WAS have started and remarkably developed. I will comment on recent progress in the clinical and basic researches of WAS, including some topics reported by our and other groups.

Revertant T lymphocytes in a patient with Wiskott-Aldrich syndrome: analysis of function and distribution in lymphoid organs

BACKGROUND

The Wiskott-Aldrich syndrome (WAS) is a rare genetic disease characterized by thrombocytopenia, immunodeficiency, autoimmunity, and hematologic malignancies. Secondary mutations leading to re-expression of WAS protein (WASP) are relatively frequent in patients with WAS.

OBJECTIVE

The tissue distribution and function of revertant cells were investigated in a novel case of WAS gene secondary mutation.

METHODS

A vast combination of approaches was used to characterize the second-site mutation, to investigate revertant cell function, and to track their distribution over a 18-year clinical follow-up.

RESULTS

The WAS gene secondary mutation was a 4-nucleotide insertion, 4 nucleotides downstream of the original deletion. This somatic mutation allowed the T-cell-restricted expression of a stable, full-length WASP with a 3-amino acid change compared with the wild-type protein. WASP(+) T cells appeared early in the spleen (age 10 years) and were highly enriched in a mesenteric lymph node at a later time (age 23 years). Revertant T cells had a diversified T-cell-receptor repertoire and displayed in vitro and in vivo selective advantage. They proliferated and produced cytokines normally on T-cell-receptor stimulation. Consistently, the revertant WASP correctly localized to the immunologic synapse and to the leading edge of migrating T cells.

CONCLUSION

Despite the high proportion of functional revertant T cells, the patient still has severe infections and autoimmune disorders, suggesting that re-expression of WASP in T cells is not sufficient to normalize immune functions fully in patients with WAS.

Evolution of highly polymorphic T cell populations in siblings with the Wiskott-Aldrich Syndrome

Population level evolutionary processes can occur within a single organism when the germ line contains a mutation that confers a cost at the level of the cell. Here we describe how multiple compensatory mutations arose through a within-individual evolutionary process in two brothers with the immune deficiency Wiskott-Aldrich Syndrome (WAS). As a result, both brothers have T lymphocyte populations that are highly polymorphic at the locus of the germ line defect, and no single allele achieves fixation. WASP, the gene product affected in this disease, is specific to white blood cells where it is responsible for regulating actin cytoskeleton dynamics in a wide range of cellular responses. The brothers inherited a rare allele predicted to result in truncated WASP lacking the carboxy-terminal VCA domains, the region that directly catalyzes actin filament generation. Although the brothers' T cell populations are highly polymorphic, all share a corrective effect relative to the inherited allele in that they restore the VCA domain. This indicates massive selection against the truncated germ line allele. No single somatic allele becomes fixed in the circulating T cell population of either brother, indicating that a regulated step in maturation of the affected cell lineage is severely compromised by the germ line allele. Based on the finding of multiple somatic mutations, the known maturation pathway for T-lineage cells and the known defects of T cells and precursor thymocytes in mice with truncated WASP, we hypothesize that the presence of truncated WASP (WASPΔVCA) confers an extreme disadvantage in early developing thymocytes, above and beyond the known cost of absence of full-length WASP, and that the disadvantage likely occurs through dominant negative competition of WASPΔVCA with N-WASP, a protein that otherwise partially compensates for WASP absence in developing thymocytes.

Unprecedented diversity of genotypic revertants in lymphocytes of a patient with Wiskott-Aldrich syndrome

Spontaneous somatic reversions of inherited mutations are poorly understood phenomena that are thought to occur uncommonly in a variety of genetic disorders. When molecularly characterized, revertant cells have rarely exhibited more than one revertant genotype per patient. We analyzed individual allospecific T-cell clones derived from a Wiskott-Aldrich syndrome (WAS) patient identified by flow cytometry to have 10% to 15% revertant, WAS protein-expressing lymphocytes in his blood. Genotypic analysis of the clones revealed a remarkable diversity of deletions and base substitutions resulting in at least 34 different revertant genotypes that restored expression of WASp. A large fraction of these revertant genotypes were also identified in primary T cells purified from peripheral blood. These data suggest that the use of sensitive methods may reveal the presence of wide arrays of individual genotypic revertants in WAS patients and offer opportunities for further understanding of their occurrence.

[Expression anomalies of the CD3-TCR complex expression and immunodeficiencies]

Molecular characterization of immunodeficiencies contributes to a better understanding of the physiological mechanisms of immune function. The T cell receptor is a heterodimer (alpha/beta or gamma/delta) associated with four transmembrane units of the CD3 complex (gamma, delta, epsilon and zeta). We herein summarize the immunodeficiency states resulting from defects in genes encoding the CD3 complex. Such analysis highlights the respective role of each of these chains in T lymphocyte development and underscores differences between T lymphocyte development in man and mouse. Currently, there is a growing body of knowledge on immunodeficiencies specifically involving the four chains of the CD3, namely gamma, delta, epsilon and zeta. Thus, we can compare the phenotypes observed in these patients with those seen in mice knockout for these genes. The main differences observed involve the respective roles of the CD3gamma chain as well as the CD3delta, whose functions seem to be reciprocal between the two species. Indeed, in the mouse, knockout of CD3delta allows some degree of T lymphocyte differentiation since mature CD4 and CD8 as well as TCRgammadelta T lymphocytes are observed in the periphery. In contrast, deleterious mutation of the CD3delta encoding gene in the human leads to a severe combined immunodeficiency characterised by the complete absence of mature T cell subpopulations including TCRalpha/beta and TCRgamma/delta. Reciprocally, in the human, mutation of the CD3gamma encoding gene leads to a moderate immunodeficiency which contrasts with the complete block of T cell differentiation observed in mice knockout for this gene. This article brings into focus the knowledge gained through studies of immunodeficiency mouse models with the pathophysiological state observed in human disease.

A second-site mutation in the initiation codon of WAS (WASP) results in expansion of subsets of lymphocytes in an Wiskott-Aldrich syndrome patient

Wiskott-Aldrich syndrome (WAS) is caused by mutations in the gene encoding WAS protein (WASP ). Recently, somatic mosaicism caused by reversions or second-site mutations has been reported in some inherited disorders including WAS. In this article, we describe somatic mosaicism in a 15-year-old WAS patient due to a second-hit mutation in the initiation codon. The patient originally had a single-base deletion (c.11delG; p.G4fsX40) in the WAS (WASP) gene, which resulted in a frameshift and abrogated protein expression. Subsequently, a fraction of T and natural killer (NK) cells expressed a smaller WASP, which binds to its cellular partner WASP-interacting protein (WIP). The T and NK cells were found to have an additional mutation in the initiation codon (c.1A>T; p.M1_P5del). The results strongly suggest that the smaller WASP is translated from the second ATG downstream of the original mutation, and not only T cells but also NK cells carrying the second mutation acquired a growth advantage over WASP negative counterparts. To our knowledge, this is the first report describing somatic mosaicism due to a second-site mutation in the initiation codon of any inherited disorders.

Inherited and somatic CD3zeta mutations in a patient with T-cell deficiency

A four-month-old boy with primary immunodeficiency was found to have a homozygous germ-line mutation of the gene encoding the CD3zeta subunit of the T-cell receptor-CD3 complex. CD3zeta is necessary for the development and function of T cells. Some of the patient's T cells had low levels of the T-cell receptor-CD3 complex and carried the Q70X mutation in both alleles of CD3zeta, whereas other T cells had normal levels of the complex and bore the Q70X mutation on only one allele of CD3zeta, plus one of three heterozygous somatic mutations of CD3zeta on the other allele, allowing expression of poorly functional T-cell receptor-CD3 complexes.

Mosaicism of NK cells in a patient with Wiskott-Aldrich syndrome

Rare cases of somatic mosaicism resulting from reversion of inherited mutations can lead to the attenuation of blood-cell disorders, including Wiskott-Aldrich syndrome (WAS). The impact of the revertant hematopoietic stem or progenitor cells, particularly their representation in blood-cell populations, is of interest because it predicts the outcome of gene therapy. Here we report an 8-year-old patient with WAS caused by a single nucleotide insertion in the WASP gene that abrogates protein expression. The patient nonetheless had mild disease. We found reversion of the mutation in a fraction of patient lymphocytes. Forty percent of natural killer (NK) cells expressed Wiskott-Aldrich syndrome protein (WASP), and NK cells contained both mutated and revertant (normal) sequences. WASP was not expressed in patient T or B cells; T cells contained only the mutated sequence. The selective advantage of WASP+ NK cells was also demonstrated for carrier females. The enrichment of WASP+-revertant NK cells indicates that WASP provides a selective advantage in this lineage and predicts the success of gene therapy for reconstituting the NK-cell compartment. The importance of reconstituting the NK-cell lineage is discussed.

Lentiviral Vector-Mediated Gene Transfer in T Cells from Wiskott–Aldrich Syndrome Patients Leads to Functional Correction

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency with a median survival below the age of 20 due to infections, severe hemorrhage, and lymphomas. Transplantation of hematopoietic stem cells from HLA-identical sibling donors is a resolutive treatment, but is available for a minority of patients. Transplantation of genetically corrected autologous hematopoietic stem cells or T cells could represent an alternative treatment applicable to all patients. We investigated whether WAS gene transfer with MMLV-based oncoretroviral and HIV-based lentiviral vectors could restore normal functions of patients' T cells. T cells transduced either with lentiviral vectors expressing the WAS protein (WASP) from the ubiquitous PGK promoter or the tissue-specific WASP promoter or with an oncoretroviral vector expressing WASP from the LTR, reached normal levels of WASP with correction of functional defects, including proliferation, IL-2 production, and lipid raft upregulation. Lentiviral vectors transduced T cells from WAS patients at higher rates, compared to oncoretroviral vectors, and efficiently transduced both activated and naive WAS T cells. Furthermore, a selective growth advantage of T cells corrected with the lentiviral vectors was demonstrated. The observation that lentiviral vector-mediated gene transfer results in correction of T cell defects in vitro supports their application for gene therapy in WAS patients.

Multiple patients with revertant mosaicism in a single Wiskott-Aldrich syndrome family

We previously reported on a 43-year-old patient with Wiskott-Aldrich syndrome (WAS) who experienced progressive clinical improvement and revertant T-cell mosaicism. Deletion of the disease-causing 6-bp insertion was hypothesized to have occurred by DNA polymerase slippage. We now describe 2 additional patients from the same family who also had revertant T lymphocytes that showed selective in vivo advantage. Somatic mosaicism was demonstrated on leukocytes cryopreserved in the first patient when he was 22 years old, 11 years before his death from kidney failure. The second patient is now 16 years old, has a moderate clinical phenotype, and developed revertant cells after the age of 14 years. These results support DNA polymerase slippage as a common underlying mechanism, and they indicate that T-cell mosaicism may have different clinical effects in WAS.

Spontaneous in vivo reversion of an inherited mutation in the Wiskott-Aldrich syndrome

The Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency disease, arising from mutations of the WAS-protein (WASP) gene. Previously, we have reported that mononuclear cells from WAS patients showed lack/reduced of the intracellular WASP (WASP(dim)) by flow cytometric analysis, and analysis of WASP by flow cytometry (FCM-WASP) was useful for WAS diagnosis. In this study, we report a WAS patient who showed the unique pattern of FCM-WASP. The patient had the small population of normal expression of WASP (WASP(bright)) mononuclear cells together with the major WASP(dim) population. The WASP(bright) cells were detected in T cells, not in B cells or in monocytes. Surprisingly, the molecular studies of the WASP(bright) cells revealed that the inherited mutation of WASP gene was reversed to normal. His mother was proved as a WAS carrier, and HLA studies and microsatellite polymorphic studies proved that the WASP(bright) cells were derived from the patient himself. Therefore, we concluded that the WASP(bright) cells were resulted from spontaneous in vivo reversion of the inherited mutation. Furthermore, the scanning electron microscopic studies indicated that WASP-positive cells from the patient restored the dense microvillus surface projections that were hardly observed in the WASP(dim) cells. This case might have significant implications regarding the prospects of the future gene therapy for WAS patients.

Novel mutations in the Wiskott-Aldrich syndrome protein gene and their effects on transcriptional, translational, and clinical phenotypes

Wiskott-Aldrich syndrome (WAS) is an X-linked recessive immunodeficiency characterized by thrombocytopenia, eczema, and recurrent infections, and caused by mutations in the WAS protein (WASP) gene. WASP contains several functional domains through which it interacts with proteins involved in intracellular signaling and regulation of the actin cytoskeleton. In this report, 17 WASP gene mutations were identified, 12 of which are novel. DNA of affected males and obligate carriers was PCR amplified and analyzed by SSCA, heteroduplex analysis, and direct sequencing. The effects of the mutations at the mRNA and protein level were ascertained by RT-PCR and Western blot analyses. All missense mutations were located in exons 1-4. Most of the nonsense, frameshift and splice site mutations were found in exons 6-11. Mutations that alter splice sites led to the synthesis of several types of mRNAs, a fraction of which represented the normally spliced product. The presence of normally spliced transcripts was correlated with a milder phenotype. When one such case was studied by Western blotting, reduced amounts of normal-size WASP were present. In other cases as well, a correlation was found between the amount of normal or mutant WASP present and the phenotypes of the affected individuals. No protein was detected in two individuals with severe WAS. Reduced levels of a normal-size WASP with a missense mutation were seen in two individuals with XLT. It is concluded that mutation analysis at the DNA level is not sufficient for predicting clinical course. Studies at the transcript and protein level are needed for a better assessment.

In vivo suppressor mutations correct a murine model of hereditary tyrosinemia type I

Hereditary tyrosinemia type I and alkaptonuria are disorders of tyrosine catabolism caused by deficiency of fumarylacetoacetate hydrolase (FAH) and homogentisic acid dioxygenase (HGD), respectively. Tyrosinemia is a severe childhood disease that affects the liver and kidneys, but alkaptonuria is a more benign adult disorder in comparison. Because HGD is upstream of FAH in the tyrosine pathway, mice doubly mutant in both enzymes were found to be protected from the liver and renal damage of tyrosinemia as hypothesized. Mice mutant at the tyrosinemic locus but heterozygous for alkaptonuria spontaneously developed clonal nodules of functionally normal hepatocytes that were able to rescue the livers of some mice with this genotype. This phenotypic rescue was a result of an inactivating mutation of the wild-type homogentisic acid dioxygenase gene, thus presenting an example of an in vivo suppressor mutation in a mammalian model.

Spontaneous functional correction of homozygous fanconi anaemia alleles reveals novel mechanistic basis for reverse mosaicism

Somatic mosaicism due to reversion of a pathogenic allele to wild type has been described in several autosomal recessive disorders. The best known mechanism involves intragenic mitotic recombination or gene conversion in compound heterozygous patients, whereby one allele serves to restore the wild-type sequence in the other. Here we document for the first time functional correction of a pathogenic microdeletion, microinsertion and missense mutation in homozygous Fanconi anaemia (FA) patients resulting from compensatory secondary sequence alterations in cis. The frameshift mutation 1615delG in FANCA was compensated by two additional single base-pair deletions (1637delA and 1641delT); another FANCA frameshift mutation, 3559insG, was compensated by 3580insCGCTG; and a missense mutation in FANCC(1749T-->G, Leu496Arg) was altered by 1748C-->T, creating a cysteine codon. Although in all three cases the predicted proteins were different from wild type, their cDNAs complemented the characteristic hypersensitivity of FA cells to crosslinking agents, thus establishing a functional correction to wild type.

Detection of lymphocytes and granulocytes expressing the mutant WASP message in carriers of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked recessive disease caused by mutations in the recently identified WAS protein gene (WASP). In some X-linked genetic disorders skewed X-inactivation has been observed in all cell populations or some specific cell lineages of female carriers. Recently, female carriers of WAS were also revealed to present skewed X-inactivation patterns at the haemopoietic stem cell level. However, it is not clear if all haematological cells expressing the mutant WASP allele are eliminated in WAS carriers. By reverse transcription PCR methods, we studied 14 WAS carriers from 10 different families to assess whether blood cells expressing the mutant WASP message were present in their peripheral blood. The mutations of each WAS patient were known and carrier diagnosis of their female family members was performed using specific mutation analysis. We detected circulating lymphocytes and granulocytes expressing the mutant WASP message in most of the WAS carriers, nevertheless they showed skewed X-chromosomal inactivation patterns. Interestingly, the presence of blood cells expressing the mutant WASP message seemed to correlate to the WASP genotype and the age of the carriers.