An exon-skipping mutation in the btk gene of a patient with X-linked agammaglobulinemia and isolated growth hormone deficiency

Novel Insights Into the Genetic Causes of Short Stature in Children

Short stature is a common reason for consulting a growth specialist during childhood. Normal height is a polygenic trait involving a complex interaction between hormonal, nutritional and psychosocial components. Genetic factors are becoming very important in the understanding of short stature. After exclusion of the most frequent causes of growth failure, clinicians need to evaluate whether a genetic cause might be taken into consideration. In fact, genetic causes of short stature are probably misdiagnosed during clinical practice and the underlying cause of short stature frequently remains unknown, thus classifying children as having idiopathic short stature (ISS). However, over the past decade, novel genetic techniques have led to the discovery of novel genes associated with linear growth and thus to the ability to define new possible aetiologies of short stature. In fact, thanks to the newer genetic advances, it is possible to properly re-classify about 25–40% of children previously diagnosed with ISS. The purpose of this article is to describe the main monogenic causes of short stature, which, thanks to advances in molecular genetics, are assuming an increasingly important role in the clinical approach to short children.

Delayed diagnosis of X-linked agammaglobulinaemia in a boy with recurrent meningitis

Background

X-linked agammaglobulinaemia (XLA) is a rare inherited primary immunodeficiency disease characterized by the B cell developmental defect, caused by mutations in the gene coding for Bruton’s tyrosine kinase (BTK), which may cause serious recurrent infections. The diagnosis of XLA is sometimes challenging because a few number of patients have higher levels of serum immunoglobulins than expected. In this study, we reported an atypical case with recurrent meningitis, delayed diagnosis with XLA by genetic analysis at the second episode of meningitis at the age of 8 years.

Case report

An 8-year-old Chinese boy presented with fever, dizziness and recurrent vomiting for 3 days. The cerebrospinal fluid (CSF) and magnetic resonance imaging (MRI) results were suggestive of bacterial meningoencephalitis, despite the negative gram staining and cultures of the CSF. The patient was treated with broad-spectrum antibiotics and responded well to the treatment. He had history of another episode of acute pneumococci meningitis 4 years before. The respective level of Immunoglobulin G (IgG), Immunoglobulin A (IgA) and Immunoglobulin M (IgM) was 4.85 g/L, 0.93 g/L and 0.1 g/L at 1^st episode, whereas 1.9 g/L, 0.27 g/L and 0 g/L at second episode. The B lymphocytes were 0.21 and 0.06% of peripheral blood lymphocytes at first and second episode respectively. Sequencing of the BTK coding regions showed that the patient had a point mutation in the intron 14, hemizyous c.1349 + 5G > A, while his mother had a heterozygous mutation. It was a splice site mutation predicted to lead to exon skipping and cause a truncated BTK protein.

Conclusion

Immunity function should be routinely checked in patients with severe intracranial bacterial infection. Absence of B cells even with normal level of serum immunoglobulin suggests the possibility of XLA, although this happens only in rare instances. Mutational analysis of BTK gene is crucial for accurate diagnosis to atypical patients with XLA.

Genetic screening of male patients with primary hypogammaglobulinemia can guide diagnosis and clinical management

The precise diagnosis of an immunodeficiency is sometimes difficult to assess, especially due to the large spectrum of phenotypic variation reported among patients. Common variable immunodeficiency disorders (CVID) do not have, for a large part, an identified genetic cause. The identification of a causal genetic mutation is important to confirm, or in some cases correct, the diagnosis. We screened >150 male patients with hypogammaglobulinemia for mutations in three genes involved in pediatric X-linked primary immunoglobulin deficiency: CD40LG, SH2D1A and BTK. The SH2D1A screening allowed to reclassify two individuals with an initial CVID presentation as XLP after mutations identification. All these mutations were associated with a lack of protein expression. In addition, 4 patients with a primary diagnosis of CVID and one with a primary IgG subclass deficiency were requalified as XLA after identifying BTK mutations. Interestingly, two out of these 5 patients carried a damaging coding BTK mutation associated with a lower, but detectable, BTK expression in monocytes, suggesting that a dysfunctional protein explains the disease phenotype in these patients. In conclusion, our results advocate to include SH2D1A and BTK in newly developed targeted NGS genetic testing, to contribute to providing the most appropriate medical treatment and genetic counselling.

Immunodeficiencies Associated with Abnormal Newborn Screening for T Cell and B Cell Lymphopenia

Newborn screening for SCID has revealed the association of low T cells with a number of unexpected syndromes associated with low T cells, some of which were not appreciated to have this feature. This review will discuss diagnostic approaches and the features of some of the syndromes likely to be encountered following newborn screening for immune deficiencies.

Genetic causes of isolated and combined pituitary hormone deficiency

Research over the last 20 years has led to the elucidation of the genetic aetiologies of Isolated Growth Hormone Deficiency (IGHD) and Combined Pituitary Hormone Deficiency (CPHD). The pituitary plays a central role in growth regulation, coordinating the multitude of central and peripheral signals to maintain the body's internal balance. Naturally occurring mutation in humans and in mice have demonstrated a role for several factors in the aetiology of IGHD/CPHD. Mutations in the GH1 and GHRHR genes shed light on the phenotype and pathogenesis of IGHD whereas mutations in transcription factors such as HESX1, PROP1, POU1F1, LHX3, LHX4, GLI2 and SOX3 contributed to the understanding of CPHD. Depending upon the expression patterns of these molecules, the phenotype may consist of isolated hypopituitarism, or more complex disorders such as septo-optic dysplasia (SOD) and holoprosencephaly. Although numerous monogenic causes of growth disorders have been identified, most of the patients with IGHD/CPHD remain with an explained aetiology as shown by the relatively low mutation detection rate. The introduction of novel diagnostic approaches is now leading to the disclosure of novel genetic causes in disorders characterized by pituitary hormone defects.

X-linked Agammaglobulinemia

X-linked agammaglobulinemia (XLA) is one of the commonest primary immune deficiencies encountered in pediatric clinical practice. In adults, common variable immunodeficiency (CVID) is the most common primary immunodeficiency disease (PID). It is an X-linked disorder characterized by increased susceptibility to encapsulated bacteria, severe hypergammaglobulinemia and absent circulating B cells in the peripheral blood. Replacement immunoglobulin therapy is the main cornerstone of treatment. Aggressive management of intercurrent infections and prophylactic antimicrobials are needed. This review attempts to highlight varied clinical manifestations and management of XLA, especially in the context of developing country.

MECHANISMS IN ENDOCRINOLOGY: Novel genetic causes of short stature

The fast technological development, particularly single nucleotide polymorphism array, array-comparative genomic hybridization, and whole exome sequencing, has led to the discovery of many novel genetic causes of growth failure. In this review we discuss a selection of these, according to a diagnostic classification centred on the epiphyseal growth plate. We successively discuss disorders in hormone signalling, paracrine factors, matrix molecules, intracellular pathways, and fundamental cellular processes, followed by chromosomal aberrations including copy number variants (CNVs) and imprinting disorders associated with short stature. Many novel causes of GH deficiency (GHD) as part of combined pituitary hormone deficiency have been uncovered. The most frequent genetic causes of isolated GHD are GH1 and GHRHR defects, but several novel causes have recently been found, such as GHSR, RNPC3, and IFT172 mutations. Besides well-defined causes of GH insensitivity (GHR, STAT5B, IGFALS, IGF1 defects), disorders of NFκB signalling, STAT3 and IGF2 have recently been discovered. Heterozygous IGF1R defects are a relatively frequent cause of prenatal and postnatal growth retardation. TRHA mutations cause a syndromic form of short stature with elevated T3/T4 ratio. Disorders of signalling of various paracrine factors (FGFs, BMPs, WNTs, PTHrP/IHH, and CNP/NPR2) or genetic defects affecting cartilage extracellular matrix usually cause disproportionate short stature. Heterozygous NPR2 or SHOX defects may be found in ∼3% of short children, and also rasopathies (e.g., Noonan syndrome) can be found in children without clear syndromic appearance. Numerous other syndromes associated with short stature are caused by genetic defects in fundamental cellular processes, chromosomal abnormalities, CNVs, and imprinting disorders.

[Genetic aspects of growth hormone deficiency]

Congenital growth hormone deficiency (GHD) is a rare cause of growth delay. It should be suspected when other causes of hypopituitarism (sellar tumor, postsurgical or radioinduced hypopituitarism, etc.) have been ruled out. GHD can be isolated (IGHD) or associated with at least one other pituitary hormone deficiency (CPHD) including thyrotroph, lactotroph, corticotroph, or gonadotroph deficiencies. CPHD is caused by mutations of genes coding for pituitary transcription factors involved in pituitary ontogenesis or in the hypothalamic-pituitary axis. Clinical presentation varies, depending on the type and severity of GHD, the age at diagnosis, the association with other pituitary hormone deficiencies, or extrapituitary malformations. Clinical, biological, and radiological work-up is very important to determine for which transcription factor the patient should be screened. There is a wide variety of phenotypes depending on the transcription factor involved: PROP1 (somatolactotroph, thyrotroph, gonadotroph, and sometimes corticotroph deficiencies ; pituitary hyper- or hypoplasia), POU1F1 (somatolactotroph and thyrotroph deficiencies, pituitary hypoplasia), HESX1 (variable pituitary deficiencies, septo-optic dysplasia), and less frequently LHX3 (somatolactotroph, thyrotroph, and gonadotroph deficiencies, deafness, and limited head and neck rotation), LHX4 (variable pituitary deficiencies, ectopic neurohypophysis, cerebral abnormalities), and OTX2 (variable pituitary deficiencies, ectopic neurohypophysis, ocular abnormalities). Mutations of PROP1 remain the first identified cause of CPHD, and as a consequence the first to be sought. POU1F1 mutations should be looked for in the postpubertal population presenting with GH/TSH deficiencies and no extrapituitary malformations. Once genetic diagnosis has been concluded, a strict follow-up is necessary because patients can develop new deficiencies (for example, late-onset corticotroph deficiency in patients with PROP1 mutations). Identification of gene defects allows early treatment of pituitary deficiency and prevention of their potentially lethal consequences. If untreated, the main symptoms include short stature, cognitive alterations, or delayed puberty. An appropriate replacement of hormone deficiencies is therefore required. Depending on the type of transmission (recessive transmission for PROP1 and LHX3, dominant for LHX4, autosomal dominant or recessive for POU1F1 and HESX1), genetic counseling might be proposed. Genotyping appears highly beneficial at an individual and familial level.

Genetics of GHRH, GHRH-receptor, GH and GH-receptor: its impact on pharmacogenetics

When a child is not following the normal, predicted growth curve, an evaluation for underlying illnesses and central nervous system abnormalities is required and, appropriate consideration should be given to genetic defects causing GH deficiency (GHD). Because Insulin-like-Growth Factor-I (IGF-I) plays a pivotal role, GHD could also be considered as a form of IGF-I deficiency (IGFD). Although IGFD can develop at any level of the GHRH-GH-IGF axis, a differentiation should be made between GHD (absent to low GH in circulation) and IGFD (normal to high GH in circulation). The main focus of this review is on the GH-gene, the various gene alterations and their possible impact on the pituitary gland. However, although transcription factors regulating the pituitary gland development may cause multiple pituitary hormone deficiency they may present initially as GHD. These defects are discussed in various different chapters within this book, whereas, the impact of alterations of the GHRH-, GHRH-receptor- --as well as the GH-receptor (GHR) gene--will be discussed here.

Genetic defects causing functional and structural isolated growth hormone deficiency

Normal somatic growth requires the integrated function of many of the hormonal, metabolic, and other growth factors involved in the hypothalamo-pituitary-somatotrope axis. Human growth hormone (hGH) causes a variety of physiological and metabolic effects in humans and its pivotal role in postnatal growth is undisputed. Disturbances that occur during this process often cause subnormal GH secretion and/or subnormal GH sensitivity/responsiveness resulting in short stature. Despite the complexity of this linear growth process, the growth pattern of children, if evaluated in the context of normal standards, is rather predictable. Children presenting with short stature (i.e out of normal standards) are treated with daily injections of recombinant human GH (rhGH), which leads in almost all cases to an increase of height velocity. Although it is becoming more and more evident that many genes are involved in controlling the regulation of growth, the main aim of this review is to focus on the GH-1 gene, the various gene alterations and their important physiological and pathophysiological role in growth.

Genetics of isolated growth hormone deficiency

When a child is not following the normal, predicted growth curve, an evaluation for underlying illnesses and central nervous system abnormalities is required, and appropriate consideration should be given to genetic defects causing growth hormone (GH) deficiency (GHD). Because Insulin-like Growth Factor-I (IGF-I) plays a pivotal role, GHD could also be considered as a form of IGF-I deficiency (IGFD). Although IGFD can develop at any level of the GH-releasing hormone (GHRH)-GH-IGF axis, a differentiation should be made between GHD (absent to low GH in circulation) and IGFD (normal to high GH in circulation). The main focus of this review is on the GH gene, the various gene alterations and their possible impact on the pituitary gland. However, although transcription factors regulating the pituitary gland development may cause multiple pituitary hormone deficiency, they may present initially as GHD.

Genetic syndromic immunodeficiencies with antibody defects

This article reviews the major syndromic immunodeficiencies with significant antibody defects, many of which may require intravenous immunogammaglobulin therapy. The authors define syndromic immunodeficiency as an illness associated with a characteristic group of phenotypic abnormalities or laboratory features that comprise a recognizable syndrome. Many are familial with a defined inheritance pattern. Immunodeficiency may not be a major part of the illness and may not be present in all patients; thus, these conditions differ from primary immunodeficiency syndromes, in which immune abnormalities are a consistent and prominent feature of their disease.

Genetics of growth hormone deficiency

When a child is not following the normal, predicted growth curve, an evaluation for underlying illness and central nervous system abnormalities is required and appropriate consideration should be given to genetic defects causing growth hormone (GH) deficiency. This article focuses on the GH gene, the various gene alterations, and their possible impact on the pituitary gland. Transcription factors regulating pituitary gland development may cause multiple pituitary hormone deficiency but may present initially as GH deficiency. The role of two most important transcription factors, POU1F1 (Pit-1) and PROP 1, is discussed.

X-linked lymphoproliferative disease associated with hypogammaglobulinemia and growth-hormone deficiency

X-linked lymphoproliferative disease is a rare immunodeficiency disorder characterized by extreme vulnerability to Epstein-Barr virus, dysgammaglobulinemia, and very high incidence of lymphoma. Growth-hormone deficiency has been described in rare cases to be associated with certain immunodeficiencies, such as X-linked agammaglobulinemia. We report a first case with X-linked lymphoproliferative disease associated with hypogammaglobulinemia and growth-hormone deficiency, which was confirmed by SAP gene mutation. The patient's mutation is novel. He is also the first patient with X-linked lymphoproliferative disease to be reported from Saudi Arabia. The patient's Btk expression and BTK gene were normal. Patients with hypogammaglobulinemia and GH deficiency should be considered to have not only X-linked agammaglobulinemia, but also X-linked lymphoproliferative disease.

Update on X-linked hypogammaglobulinemia with isolated growth hormone deficiency

PURPOSE OF REVIEW

To provide an update on the syndrome X-linked hypogammaglobulinemia with isolated growth hormone deficiency, focusing on the pedigree described originally.

RECENT FINDINGS

An additional case of X-linked hypogammaglobulinemia with isolated growth hormone deficiency and an unaffected male have been born to a female carrier in the family, allowing improved disease locus mapping. Unpublished research has identified a mutation in the transcription factor myeloid elf-1-like factor that may be the cause of the disease.

SUMMARY

X-linked hypogammaglobulinemia with isolated growth hormone deficiency is not caused by Bruton's tyrosine kinase mutations in the family described originally, but may be due to a mutation in myeloid elf-1-like factor.

Xq chromosome duplication in males: Clinical, cytogenetic and array CGH characterization of a new case and review

Males with duplications within the long arm of the X chromosome are rare and most cases are inherited from a maternal heterozygote. We report a male with a de novo Xq duplication and review of the literature. The proband was ascertained prenatally after an abnormal expanded alpha-fetoprotein (AFP) screen and abnormal ultrasound findings. Chromosome analysis on amniocyte and subsequent peripheral blood lymphocyte cultures showed a male karyotype containing additional material on the long arm of the X chromosome. Fluorescence in situ hybridization with an X chromosome whole chromosome paint probe showed that the additional material was derived from the X chromosome, interpreted as a dup(X)(q13.3q24). Further characterization of the duplication by array CGH showed a duplication size between 30-44 Mb as determined by the map position of the flanking clones on the array, and refined the breakpoints of the duplicated region to Xq21.32 --> Xq25. At birth, the proband had multiple craniofacial abnormalities, musculoskeletal anomalies, bilateral cryptorchidism with scrotal hypoplasia, conductive hearing loss, and profound generalized hypotonia despite normal birthweight, length, and head circumference. Although data regarding Xq duplications in males are limited, a clear pattern of characteristic features can be discerned as illustrated in the present case and confirmed in our literature review. Mental, psychomotor and growth retardation, as well as, craniofacial anomalies, muscle hypotonia, hypoplastic genitalia, cryptorchidism, feeding difficulties, and endocrine dysfunction are all significant issues in these individuals.

Genetic control of growth

The application of the powerful tool molecular biology has made it possible to ask questions not only about hormone production and action but also to characterize many of the receptor molecules that initiate responses to the hormones. We are beginning to understand how cells may regulate the expression of genes and how hormones intervene in regulatory processes to adjust the expression of individual genes. In addition, great strides have been made in understanding how individual cells talk to each other through locally released factors to coordinate growth, differentiation, secretion, and other responses within a tissue. In this review I (1) focus on developmental aspects of the pituitary gland, (2) focus on the different components of the growth hormone axis and (3) examine the different altered genes and their related growth factors and/or regulatory systems that play an important physiological and pathophysiological role in growth. Further, as we have already entered the 'post-genomic' area, in which not only a defect at the molecular level becomes important but also its functional impact at the cellular level, I concentrate in the last part on some of the most important aspects of cell biology and secretion.

Syndromic immunodeficiencies: genetic syndromes associated with immune abnormalities

In syndromic immunodeficiencies, clinical features not directly associated with the immune defect are prominent. Patients may present with either infectious complications or extra-immune medical issues. In addition to the immunologic abnormality, a wide range of organ systems may be affected. Patients may present with disturbances in skeletal, neurologic, dermatologic, or gastrointestinal function or development. These conditions can be caused by developmental abnormalities, chromosomal aberrations, metabolic disorders, or teratogens. For a number of these conditions, recent advances have resulted in an enhanced understanding of their genetic basis. The finding of immune deficits in a number of defined syndromes with congenital anomalies suggests that an underlying genetic syndrome should be considered in those patients in whom a significant non-immune feature is present.

Genetic characterization of growth hormone deficiency and resistance: implications for treatment with recombinant growth hormone

Growth failure can be caused by deficient growth hormone production or action. The genes involved in pituitary development, somatotrope function, as well as growth hormone synthesis, secretion, and action have recently been characterized in considerable detail. Familial growth failure has played an important role in identifying these genes, and a large number of mutations adversely affecting the development and function of the growth hormone/insulin-like growth factor axis have been discovered. Inactivating mutations leading to growth retardation in humans have been identified in several pituitary transcription factor genes (HESX1, PITX2, LHX3, PROP1, POU1F1) as well as in genes encoding the growth hormone-releasing hormone receptor (GHRH-R), the G(s) protein alpha subunit (GNAS1), growth hormone itself (GH-1), the growth hormone receptor (GHR), and in a single case each, the insulin-like growth factor I (IGF-I) and the IGF-I receptor. Mutations in pituitary transcription factors cause developmental abnormalities of the pituitary and deficiency of multiple pituitary hormones [growth hormone (GH), prolactin (Prl), thyrotropin (TSH) and lutropin/follitropin (LH/FSH)]. Most of the syndromes respond well to therapy with recombinant GH; exceptions are antibody-mediated resistance in GHD type IA (not all patients) and cases of Laron syndrome (GHR deficiency). Such patients respond to IGF-I therapy. This review summarizes the molecular genetics, functional defects, phenotypes, diagnostic considerations and therapeutic aspects of syndromes associated with mutations in the relevant genes.

Competing modes of self-association in the regulatory domains of Bruton's tyrosine kinase: intramolecular contact versus asymmetric homodimerization

A nuclear magnetic resonance (NMR) investigation of a fragment of the nonreceptor Tec family tyrosine kinase Btk has revealed an intricate set of coupled monomer-dimer equilibria. The Btk fragment studied contains two consecutive proline-rich motifs followed by a single Src homology 3 (SH3) domain. We provide evidence for an asymmetric homodimer in which the amino-terminal proline sequence of one monomer contacts the opposite SH3 binding pocket, whereas the carboxy-terminal proline sequence of the other monomer is engaged by the second SH3 domain across the dimer interface. We show that the asymmetric homodimer structure is mimicked by a heterodimer formed in an equimolar mixture of complimentary mutants: one carrying mutations in the amino-terminal proline stretch; the other, in the carboxy-terminal proline motif. Moreover, a monomeric species characterized by an intramolecular complex between the amino-terminal proline motif and the SH3 domain predominates at low concentration. Association constants were determined for each of the competing equilibria by NMR titration. The similarity of the determined K(a) values reveals a delicate balance between the alternative conformational states available to Btk. Thus, changes in the local concentration of Btk itself, or co-localization with exogenous signaling molecules that have high affinity for either proline sequence or the SH3 domain, can significantly alter species composition and regulate Btk kinase activity.

A recurrent RNA-splicing mutation in the SEDL gene causes X-linked spondyloepiphyseal dysplasia tarda

Spondyloepiphyseal dysplasia tarda (SEDL) is a genetically heterogeneous disorder characterized by mild-to-moderate short stature and early-onset osteoarthritis. Both autosomal and X-linked forms have been described. Elsewhere, we have reported the identification of the gene for the X-linked recessive form, which maps to Xp22.2. We now report characterization of an exon-skipping mutation (IVS3+5G-->A at the intron 3 splice-donor site) in two unrelated families with SEDL. Using reverse transcriptase (RT)-PCR, we demonstrated that the mutation resulted in elimination of the first 31 codons of the open reading frame. The mutation was not detected in 120 control X chromosomes. Articular cartilage from an adult who had SEDL and carried this mutation contained chondrocytes with abundant Golgi complexes and dilated rough endoplasmic reticulum (ER). RT-PCR experiments using mouse/human cell hybrids revealed that the SEDL gene escapes X inactivation. Homologues of the SEDL gene include a transcribed retropseudogene on chromosome 19, as well as expressed genes in mouse, rat, Drosophila melanogaster Caenorhabditis elegans, and Saccharomyces cerevisiae. The latter homologue, p20, has a putative role in vesicular transport from ER to Golgi complex. These data suggest that SEDL mutations may perturb an intracellular pathway that is important for cartilage homeostasis.

Transcriptional Regulatory Elements Within the First Intron of Bruton's Tyrosine Kinase

Defects in the gene for Bruton's tyrosine kinase (Btk) result in the disorder X-linked agammaglobulinemia (XLA). Whereas XLA is characterized by a profound defect in B-cell development, Btk is expressed in both the B lymphocyte and myeloid cell lineages. We evaluated a patient with XLA who had reduced amounts of Btk transcript but no abnormalities in his coding sequence. A single base-pair substitution in the first intron of Btk was identified in this patient, suggesting that this region may contain regulatory elements. Using reporter constructs we identified two transcriptional control elements in the first 500 bp of intron 1. A strong positive regulator, active in both pre-B cells and B cells, was identified within the first 43 bp of the intron. Gel-shift assays identified two Sp1 binding sites within this element. The patient's mutation results in an altered binding specificity of the proximal Sp1 binding site. A negative regulator, active in pre-B cells only, was located between base pairs 281 and 491 of the intron. These findings indicate that regulation of Btk transcription is complex and may involve several transcriptional regulatory factors at the different stages of B-cell differentiation.

Transcriptional Regulatory Elements Within the First Intron of Bruton's Tyrosine Kinase

Defects in the gene for Bruton's tyrosine kinase (Btk) result in the disorder X-linked agammaglobulinemia (XLA). Whereas XLA is characterized by a profound defect in B-cell development, Btk is expressed in both the B lymphocyte and myeloid cell lineages. We evaluated a patient with XLA who had reduced amounts of Btk transcript but no abnormalities in his coding sequence. A single base-pair substitution in the first intron of Btk was identified in this patient, suggesting that this region may contain regulatory elements. Using reporter constructs we identified two transcriptional control elements in the first 500 bp of intron 1. A strong positive regulator, active in both pre-B cells and B cells, was identified within the first 43 bp of the intron. Gel-shift assays identified two Sp1 binding sites within this element. The patient's mutation results in an altered binding specificity of the proximal Sp1 binding site. A negative regulator, active in pre-B cells only, was located between base pairs 281 and 491 of the intron. These findings indicate that regulation of Btk transcription is complex and may involve several transcriptional regulatory factors at the different stages of B-cell differentiation.

Identification of novel Bruton's tyrosine kinase mutations in 10 unrelated subjects with X linked agammaglobulinaemia

Mutations of the Bruton's tyrosine kinase (Btk) gene cause X linked agammaglobulinaemia (XLA). This inherited immunodeficiency disease causes an arrest in B cell differentiation of pre-B cells to mature B cells. In this study we report the characterisation of mutations in the Btk gene in 10 unrelated XLA families. The screening approach we used was based on reverse transcriptase PCR and direct cycle sequencing of the amplified products followed by analysis of the observed mutations at the level of genomic DNA. The single strand confirmation polymorphism (SSCP) technique was used for assessment of the carriers in some of these families. Various mutations throughout the coding gene were observed, including missense and nonsense mutations, a deletion, and several splicing defects. None of the mutations except one has been previously described. There were three point mutations resulting in a single amino acid substitution. One of these missense mutations was observed in a conserved region of the PH domain, the other two were found in the src homology domain 2 that is involved in phosphotyrosyl peptide binding. Two mutations were single base pair substitutions resulting in premature stop codons. In four patients abnormal Btk transcripts were found that were the result of aberrant splicing. One small deletion was observed causing a frameshift and a secondary premature termination signal. Characterisation of the mutations responsible for XLA allowed us to diagnose the disease conclusively and identify the phenotypically normal female carriers.

Familial X-linked mental retardation and isolated growth hormone deficiency: clinical and molecular findings

We report on several members of a family with varying degrees of X-linked mental retardation (XLMR), isolated growth hormone deficiency (IGHD), and infantile behaviour but without other consistent phenotypic abnormalities. Male patients continued to grow until well into their twenties and reached a height ranging from 135 to 159 cm. Except one, all female carriers were mentally normal; their adult height ranged from 159 to 168 cm. By linkage studies we have assigned the underlying genetic defect to the Xq24-q27.3 region, with a maximum lod score of Z = 3.26 at theta = 0.0 for the DXS294 locus. The XLMR-IGHD phenotype in these patients may be due to pleiotropic effects of a single gene or it may represent a contiguous gene syndrome.

BTKbase, mutation database for X-linked agammaglobulinemia (XLA)

X-linked agammaglobulinemia (XLA) is an immunodeficiency caused by mutations in the gene coding for Bruton's agammaglobulinemia tyrosine kinase (BTK). A database (BTKbase) of BTK mutations has been compiled and the recent update lists 225 entries from 189 unrelated families showing 148 unique molecular events. Each patient is given a unique patient identity number (PIN). Information is included regarding the phenotype including symptoms. Mutations in all the five domains of BTK have been noticed to cause the disease, the most common event being missense mutations. The mutations appear almost uniformly throughout the molecule and frequently affect CpG sites forming arginine residues. A decreased frequency of missense mutations was found in the TH, SH3 and upper lobe of the kinase domain. The putative structural implications of all the missense mutations are given in the database.

Impaired expansion of mouse B cell progenitors lacking Btk

Mutations in the gene encoding the protein tyrosine kinase Btk are associated with the human B cell immunodeficiency X-linked agammaglobulinemia (XLA). In the mouse, a point mutation in the Btk pleckstrin homology domain segregates with a milder X-linked immunodeficiency (xid). To assess the importance of Btk function in murine lymphopoiesis, we generated multiple embryonic stem cell clones bearing a targeted disruption of the btk gene and examined their potential to produce lymphocytes in both C57BL/6 and RAG2-/- host chimeric animals. These mice provide a complementary set of in vivo competition assays that formally establish the genetic basis for the xid phenotype. Although the null mutation yields a phenotype quite similar to that of xid, it also compromises expansion of B cell precursors. Our results suggest that the murine and human consequences of Btk deficiency differ only quantitatively, and represent the same disease process.

B-cell-specific demethylation of BTK, the defective gene in X-linked agammaglobulinemia

BTK, the gene that is defective in X-linked agammaglobulinemia, encodes a cytoplasmic tyrosine kinase that is critical for B-cell proliferation, or survival. To identify regulatory elements that control the expression of BTK we evaluated the methylation pattern of this gene in cell lines and in freshly isolated cells. An Hpa II site that was specifically demethylated in mature B cells but not in pre-B cells, T cells, neutrophils, or nonhematopoietic cells was identified in the tenth intron of BTK. In a 40 kilobase (kb) segment of DNA spanning the entire coding region of BTK plus 3 kb upstream of the first exon there were no other sites that demonstrated lineage-specific demethylation. The B-cell-specific demethylation site in intron 10, which falls within the SH2 domain, 26 kb distal to the first exon, occurs in a region rich in regulatory elements including two E2 boxes, two AP-2 sites, and a cAMP response element. It is likely that this site plays a role in maintaining BTK transcription in mature B cells.