TBX1 is responsible for cardiovascular defects in velo-cardio-facial/DiGeorge syndrome

Velo-cardio-facial syndrome (VCFS)/DiGeorge syndrome (DGS) is a human disorder characterized by a number of phenotypic features including cardiovascular defects. Most VCFS/DGS patients are hemizygous for a 1.5-3.0 Mb region of 22q11. To investigate the etiology of this disorder, we used a cre-loxP strategy to generate mice that are hemizygous for a 1.5 Mb deletion corresponding to that on 22q11. These mice exhibit significant perinatal lethality and have conotruncal and parathyroid defects. The conotruncal defects can be partially rescued by a human BAC containing the TBX1 gene. Mice heterozygous for a null mutation in Tbx1 develop conotruncal defects. These results together with the expression patterns of Tbx1 suggest a major role for this gene in the molecular etiology of VCFS/DGS.

Regionally restricted developmental defects resulting from targeted disruption of the mouse homeobox gene hox-1.5

Gene targeting in mouse embryo-derived stem cells has been used to disrupt the homeobox gene hox-1.5. Mice heterozygous at the hox-1.5 locus appear normal, whereas hox-1.5-/hox-1.5- mice die at or shortly after birth. These homozygotes are athymic, aparathyroid, have reduced thyroid and submaxillary tissue and exhibit a wide range of throat abnormalities. In addition, they often feature defects of the heart and arteries as well as craniofacial abnormalities. These deficiencies are remarkably similar to the pathology of the human congenital disorder DiGeorge's syndrome.

Genetic ablation of parathyroid glands reveals another source of parathyroid hormone

The parathyroid glands are the only known source of circulating parathyroid hormone (PTH), which initiates an endocrine cascade that regulates serum calcium concentration. Glial cells missing2 (Gcm2), a mouse homologue of Drosophila Gcm, is the only transcription factor whose expression is restricted to the parathyroid glands. Here we show that Gcm2-deficient mice lack parathyroid glands and exhibit a biological hypoparathyroidism, identifying Gcm2 as a master regulatory gene of parathyroid gland development. Unlike PTH receptor-deficient mice, however, Gcm2-deficient mice are viable and fertile, and have only a mildly abnormal bone phenotype. Despite their lack of parathyroid glands, Gcm2-deficient mice have PTH serum levels identical to those of wild-type mice, as do parathyroidectomized wild-type animals. Expression and ablation studies identified the thymus, where Gcm1, another Gcm homologue, is expressed, as the additional, downregulatable source of PTH. Thus, Gcm2 deletion uncovers an auxiliary mechanism for the regulation of calcium homeostasis in the absence of parathyroid glands. We propose that this backup mechanism may be a general feature of endocrine regulation.

AnFgf8mouse mutant phenocopies human 22q11 deletion syndrome

Deletion of chromosome 22q11, the most common microdeletion detected in humans, is associated with a life-threatening array of birth defects. Although 90% of affected individuals share the same three megabase deletion, their phenotype is highly variable and includes craniofacial and cardiovascular anomalies, hypoplasia or aplasia of the thymus with associated deficiency of T cells, hypocalcemia with hypoplasia or aplasia of the parathyroids, and a variety of central nervous system abnormalities. Because ablation of neural crest in chicks produces many features of the deletion 22q11 syndrome, it has been proposed that haploinsufficiency in this region impacts neural crest function during cardiac and pharyngeal arch development. Few factors required for migration, survival, proliferation and subsequent differentiation of pharyngeal arch neural crest and mesoderm-derived mesenchyme into their respective cardiovascular, musculoskeletal, and glandular derivatives have been identified. However, the importance of epithelial-mesenchymal interactions and pharyngeal endoderm function is becoming increasingly clear.

Fibroblast growth factor 8 is a signaling molecule expressed in the ectoderm and endoderm of the developing pharyngeal arches and known to play an important role in survival and patterning of first arch tissues. We demonstrate a dosage-sensitive requirement for FGF8 during development of pharyngeal arch, pharyngeal pouch and neural crest-derived tissues. We show that FGF8 deficient embryos have lethal malformations of the cardiac outflow tract, great vessels and heart due, at least in part, to failure to form the fourth pharyngeal arch arteries, altered expression of Fgf10 in the pharyngeal mesenchyme, and abnormal apoptosis in pharyngeal and cardiac neural crest.

The Fgf8 mutants described herein display the complete array of cardiovascular, glandular and craniofacial phenotypes seen in human deletion 22q11 syndromes. This represents the first single gene disruption outside the typically deleted region of human chromosome 22 to fully recapitulate the deletion 22q11 phenotype. FGF8 may operate directly in molecular pathways affected by deletions in 22q11 or function in parallel pathways required for normal development of pharyngeal arch and neural crest-derived tissues. In either case, Fgf8 may function as a modifier of the 22q11 deletion and contribute to the phenotypic variability of this syndrome.

Thymosin activity in patients with cellular immunodeficiency

An extract of calf thymus, thymosin, induces an increase in percentage of T-cell rosettes when incubated in vitro with sheep erythrocytes and lymphocytes from patients with primary immunodeficiency diseases or viral illness. Precursor lymphocytes are required for this increase to occur. The percentage of T-cell rosettes, when they are normal, is not increased further upon incubation with thymosin. A patient with thymic hypoplasia and immunoglobulin synthesis was selected to receive thymosin in vivo when her T-cell rosettes had increased from 15 to 48 per cent after in vitro incubation with thymosin. During therapy, she clinically improved, the percentage of T-cell rosettes gradually increased to normal, and positive delayed hypersensitivity skin tests developed. Thymosin may be useful clinically for partial reconstitution of cellular immunity. An increased percentage of T-cell rosettes after incubation with thymosin in vitro may predict which patients will respond to thymosin therapy in vivo.

Full spectrum of malformations in velo-cardio-facial syndrome/DiGeorge syndrome mouse models by altering Tbx1 dosage

Velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS) is associated with de novo hemizygous 22q11.2 deletions and is characterized by malformations attributed to abnormal development of the pharyngeal arches and pouches. The main physical findings include aortic arch and outflow tract heart defects, thymus gland hypoplasia or aplasia and craniofacial anomalies. The disorder varies greatly in expressivity; while some patients are mildly affected with learning disabilities and subtle craniofacial malformations, others die soon after birth with major cardiovascular defects and thymus gland aplasia. In addition to the main clinical features, many other findings are associated with the disorder such as chronic otitis media and hypocalcemia. Tbx1, a gene encoding a T-box transcription factor, which is hemizygously deleted on chromosome 22q11.2, was found to be a strong candidate for the equivalent of human VCFS/DGS in mice. Mice hemizygous for a null allele of Tbx1 had mild malformations, while homozygotes had severe malformations in the affected structures; neither precisely modeling the syndrome. Interestingly, bacterial artificial chromosome (BAC) transgenic mice overexpressing human TBX1 and three other transgenes, had similar malformations as VCFS/DGS patients. By employing genetic complementation studies, we demonstrate that altered TBX1 dosage and not overexpression of the other transgenes is responsible for most of the defects in the BAC transgenic mice. Furthermore, the full spectrum of VCFS/DGS malformations was elicited in a Tbx1 dose dependent manner, thus providing a molecular basis for the pathogenesis and varied expressivity of the syndrome.

The reasons for failure in parathyroid operations

Primary hyperparathyroidism occurs in about 1 in every 700 individuals. We analyzed our experience with 81 patients with persistent or recurrent hyperparathyroidism who were treated at the University of California, San Francisco, and the Veterans Administration Medical Center, San Francisco, from January 1979 through September 1988. In the 89 reoperations performed, the following six reasons or combination of reasons were responsible for the failed initial operation: (1) in 50 patients, there were multiple abnormal glands (30 hyperplastic and 20 second adenoma); (2) in 40 patients, the tumor was located in an ectopic position (22 mediastinal, 9 deep-seated cervical, 7 intrathyroidal, and 2 undescended); (3) in 17 patients, there were supernumerary parathyroid glands; (4) in 12 patients, the abnormal parathyroid glands were found in normal locations and the tumors were missed because of surgeon inexperience; (5) in 4 patients, failure was due to metastatic parathyroid cancer; and (6) in 4 patients, failure was due to errors on frozen section examinations. Preoperative localization studies usually identified the abnormal parathyroid tumor(s) prior to reoperations and were helpful in these patients. Knowledge of the reasons for failed parathyroid operations and the usual and unusual sites where parathyroid tumors are situated as well as a complete exploration should decrease the frequency of failed parathyroid operations. Localization studies are helpful for identifying these often elusive tumors.

Histologic parathyroid abnormalities in an autopsy series

The parathyroid glands from 422 subjects without clinical, biochemical, or advanced histologic signs of renal disease were dissected at autopsy. The glands were evaluated histologically in relation to the subjects' age, sex, histologic renal features, other diseases, and medical therapy. Normal parathyroid glands generally had diffuse cellular arrangements, with variable fat cell content. Increased nodularity was observed, especially in enlarged parathyroid glands. Hyperplasia was present in 7 per cent and adenomas in 2.4 per cent. Hyperplastic glands were often nodular and asymmetric, and numbers of oxyphil cells were frequently increased. Some large nodules were histologically indistinguishable from adenomas. Serum calcium levels were elevated in subjects with adenomas or with hyperplastic glands containing large nodules. The findings seem to indicate that nodularity of the parathyroid tissue is a sign of abnormality and that adenomas may arise from such nodular hyperplasia. Hyperplasia and adenomas were more common in elderly subjects and in those with slight or moderate nephrosclerosis. Other diseases or medical therapy were not found to be correlated with parathyroid weight or histologic features.

B and T lymphocytes in primary immunodeficiency disease in man

B- and T-cell populations in 32 patients with different forms of primary immunodeficiency disease were studied. The B-cells in peripheral blood were investigated with respect to surface immunoglobulins by means of immunofluorescence. The T-cell function was studied utilizing quantitation of proliferative response to phytochemagglutinin (PHA)(1) and delayed allergy to various antigens. In 10 patients lymph node lymphocytes were also evaluated 11 male children with infantile x-linked agammaglobulinemia were divided into two subgroups. One did not show immunoglobulin spots on peripheral blood lymphocytes at all, the other contained a very low percentage of IgM- and occasionally IgA bearing lymphocytes. Eight patients with common variable immunodeficiency had moderately decreased percentages of peripheral blood and lymph node lymphocytes with surface immunoglobulins, but these patients lacked immunoglobulin secreting cells. Four cases of isolated IgA deficiency had normal or high percentages, and two cases of ataxia-telangiectasia had high percentages of lymphocytes with IgA in so called receptor distribution in both peripheral blood and lymph nodes. In three patients with infantile combined immunodeficiency that had been corrected by marrow transplantation, the percentages of Ig-bearing lymphocytes increased to normal or high levels together with establishment of functional T-cell population and ultimate secretion of serum immunoglobulins. One case of Di George syndrome reconstituted by fetal thymus transplant showed gradual decrease of B lymphocytes in circulation parallel to restoration of T-cell population.

Hoxa3 and pax1 regulate epithelial cell death and proliferation during thymus and parathyroid organogenesis

The thymus and parathyroid glands in mice develop from a thymus/parathyroid primordium that forms from the endoderm of the third pharyngeal pouch. We investigated the molecular mechanisms that promote this unique process in which two distinct organs form from a single primordium, using mice mutant for Hoxa3 and Pax1. Thymic ectopia in Hoxa3(+/-)Pax1(-/-) compound mutants is due to delayed separation of the thymus/parathyroid primordium from the pharynx. The primordium is hypoplastic at its formation, and has increased levels of apoptosis. The developing third pouch in Hoxa3(+/-)Pax1(-/-) compound mutants initiates normal expression of the parathyroid-specific Gcm2 and thymus-specific Foxn1 genes. However, Gcm2 expression is reduced at E11.5 in Pax1(-/-) single mutants, and further reduced or absent in Hoxa3(+/-)Pax1(-/-) compound mutants. Subsequent to organ-specific differentiation from the shared primordium, both the parathyroids and thymus developed defects. Parathyroids in compound mutants were smaller at their formation, and absent at later stages. Parathyroids were also reduced in Pax1(-/-) mutants, revealing a new function for Pax1 in parathyroid organogenesis. Thymic hypoplasia at later fetal stages in compound mutants was associated with increased death and decreased proliferation of thymic epithelial cells. Our results suggest that a Hoxa3-Pax1 genetic pathway is required for both epithelial cell growth and differentiation throughout thymus and parathyroid organogenesis.

Congenital cardiovascular disease and anomalies of the third and fourth pharyngeal pouch

Patients with the third and fourth pharyngeal pouch syndrome, thymic and parathyroid aplasia or hypoplasia, have a very high incidence of aortic arch anomalies and congenital heart disease. These patients present with a unique syndrome characterized by profound hypocalcemia, defective thymic-mediated cellular immune function, and cardiovascular anomalies. The cardiac abnormalities most frequently are conotruncal malformations of the tetralogy of Fallot or truncus arteriosus types and are often the cause of death. Patients with profound neonatal hypocalcemia should be screened for evidence of normal thymic function and congenital heart disease.

Surgical significance of supernumerary parathyroid glands in renal hyperparathyroidism

In secondary hyperparathyroidism (2HPT) fundamentally all parathyroid glands, including supernumerary glands, become hyperplastic, and stimulation of parathyroid glands continues after parathyroidectomy (PTx). Therefore supernumerary glands have special significance during surgery for 2HPT, whether persistent or recurrent HPT. In the present study 570 patients underwent initial total PTx with a forearm autograft. The frequency, type, location, histopathology, and clinical significance of the supernumerary glands were evaluated. At the initial operation 90 supernumerary glands were removed from 82 to 570 patients (14.4%); 12 patients (2.1%) required extirpation of supernumerary glands for persistent/recurrent HPT. Altogether 104 supernumerary glands were identified at operation in 94 of the 570 patients (16.5%). Among these 104 glands, 25 (24.0%) were of the rudimentary, or split, type and 79 (76.0%) of the proper type. Supernumerary glands were most frequently identified in the thymic tongue (53/104, 51.0%); 32 (60.4%) of these 53 glands were identified only microscopically. In 6 of the 570 cases (1.1%), reoperation was required for persistent HPT due to supernumerary glands located in the mediastinum, and 6 patients underwent neck reexploration for recurrence. Histopathologically, 61 of 104 (58.7%) supernumerary glands, including 36 glands recognized only microscopically, showed diffuse hyperplasia, and 43 (41.3%) displayed nodular hyperplasia. Residual small supernumerary glands with diffuse hyperplasia have the potential to be transformed to nodular hyperplasia during long-term hemodialysis. Therefore all parathyroid glands including supernumerary glands should, if possible, be removed at the initial operation. Routine removal of the thymic tongue and careful examination of the regions surrounding the lower poles of the thyroid, especially on the left side, are important steps in the surgical treatment.

The undescended "parathymus". An occasional cause of failed neck exploration for hyperparathyroidism

Arrested descent of the inferior parathyroid gland ("parathymus") during embryologic development may leave the gland stranded high in the neck. Adenomas of such undescended glands are a rare cause of primary hyperparathyroidism, but they may not be uncommon among patients who have already had a failed cervical exploration. Unless the surgeon is aware of this entity, he will probably overlook it. If one is to find these tumors, it is necessary to extend the dissection upward above the superior pole of the thyroid gland, sometimes as far as the angle of the jaw. Adenomas of undescended "parathymus" glands were encountered in seven patients during a recent 20 month (May 1975 through December 1976) surgical experience encompassing 414 primary operations and 27 reoperations for hyperparathyroidism. These tumors were found at initial exploration in one patient and at reoperation in six patients. The individual case histories are presented to illustrate the difficulties posed by this anatomic variant.

Congenital aplasia of parathyroid glands and thymus

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PTH regulates fetal blood calcium and skeletal mineralization independently of PTHrP

PTH and PTHrP both act in the regulation of fetal mineral metabolism. PTHrP regulates placental calcium transfer, fetal blood calcium, and differentiation of the cartilaginous growth plate into endochondral bone. PTH has been shown to influence fetal blood calcium, but its role in skeletal formation remains undefined. We compared skeletal morphology, mineralization characteristics, and gene expression in growth plates of fetal mice that lack parathyroids and PTH (Hoxa3 null) with the effects of loss of PTHrP (Pthrp null), loss of PTH/PTHrP receptor (Pthr1 null), and loss of both PTH and PTHrP (Hoxa3 null x Pthrp null). Loss of PTH alone does not affect morphology or gene expression in the skeletal growth plates, but skeletal mineralization and blood calcium are significantly reduced. In double-mutant fetuses (Hoxa3 null/Pthrp null), combined loss of PTH and PTHrP caused fetal growth restriction, limb shortening, greater reduction of fetal blood calcium, and reduced mineralization. These findings suggest that 1) PTH may play a more dominant role than PTHrP in regulating fetal blood calcium; 2) blood calcium and PTH levels are rate-limiting determinants of skeletal mineral accretion; and 3) lack of both PTH and PTHrP will cause fetal growth restriction.

A new method with high sensitivity and specificity for localization of abnormal parathyroid glands

A novel method for localization of abnormal parathyroid glands involving color-processing of nuclear scintigrams of the neck after injection of Thallium-201 and Technetium pertechnetate is presented with surgical correlation. Preoperative localization of single parathyroid adenomas was successful in 88% of previously unoperated patients and in 85.7% of those with adenomas not located at previous surgery. Eighty-three per cent of glands with secondary hyperplasia, 66% of glands with primary hyperplasia, and one carcinoma were localized. No abnormal studies were seen in non-hyperparathyroid hypercalcemia, and no false positive studies were seen. Localization appeared related to larger adenomas (300-5000 mg), although one of 60 mg was localized. Color-comparison dual-isotype scintigraphy was useful for localization of parathyroid adenomas and hyperplastic glands and exceeded the reported sensitivity of either ultrasonography or computerized tomography. It deserves wider evaluation in preoperative management of at least hyperparathyroidism of the primary or persistent types.