Genetics of McCune-Albright syndrome

Combining Real-Time COLD- and MAMA-PCR TaqMan Techniques to Detect and Quantify R201 GNAS Mutations in the McCune-Albright Syndrome

BACKGROUND/AIM

The McCune-Albright syndrome (MAS) is a potentially severe disorder hallmarked by fibrous bone dysplasia, café-au-lait skin spots, and endocrine hyperfunction. It is caused by postzygotic activating mutations at the R201 codon of the GNAS gene, leading to a state of somatic mosaicism. Our aim was to improve the mutation detection rate and to quantify the presence of R201 GNAS mutations in different DNA samples from MAS patients.

METHODS

Real-time COLD- and MAMA-PCR TaqMan techniques were combined to search for R201 mutations in the DNA of blood or affected tissues from 16 previously molecularly characterized MAS patients, from a further 84 subjects with MAS signs who were R201 negative at RFLP analysis, and from 36 controls. The ability of this new method to provide quantitative data was tested in the serial dilution of wild-type, R201H, or R201C cloned plasmid DNA samples; the mutant abundance was measured by spectrophotometry.

RESULTS

A linear correlation between the true and the relative mutant abundance was observed until 2.5%, indicating a reliable quantification of R201 mutations. The assay's sensitivity was 0.05%, similar to that of previously described molecular methods.

CONCLUSION

The real-time COLD-MAMA-PCR approach is a rapid, efficient, and inexpensive molecular technique for the identification of mutant alleles poorly represented in DNA samples.
.

GNAS sequencing identifies IPMN-specific mutations in a subgroup of diminutive pancreatic cysts referred to as "incipient IPMNs"

Incipient intraductal papillary mucinous neoplasms (IPMNs) are poorly described subcentimeter pancreatic cysts with papillae and mucin similar to IPMNs. They are larger than pancreatic intraepithelial neoplasia but do not meet the cutoff size for IPMNs (≥ 1 cm). GNAS codon 201 mutations are hallmark genetic alterations of IPMNs. Hence, we sought to determine the GNAS status of incipient IPMNs to better classify these lesions. Incipient IPMNs from 3 institutions were histologically reassessed, manually microdissected, and the genomic DNA was extracted. Using a sensitive digital ligation technique, the mutational status of KRAS at codon 12 and GNAS at codon 201 was determined. We included 21 incipient IPMNs from 7 male and 12 female patients with a median age of 63 years (range, 40 to 76 y). Most patients underwent surgery for pancreatic ductal adenocarcinoma (N = 8) or ampullary adenocarcinoma (N = 3). The median incipient IPMN size was 4 mm (range, 2 to 7 mm), and a majority had gastric-foveolar (N = 11) or intestinal (N = 5) differentiation. The maximum dysplasia observed was intermediate, and most of the lesions had intermediate-grade dysplasia. Mutational analysis revealed KRAS codon 12 mutations in all 21 incipient IPMNs, whereas 7 lesions (33%) in 7 individual patients harbored GNAS codon 201 mutations. The presence of GNAS 201 mutations in incipient IPMNs suggests that a fraction of these cysts are in fact small IPMNs. Morphologically, incipient IPMNs do not appear to be high-risk lesions. Additional studies in a larger cohort are needed to define the relationship of incipient IPMNs to larger IPMNs and, more importantly, to determine their clinical significance.

Multilineage somatic activating mutations in HRAS and NRAS cause mosaic cutaneous and skeletal lesions, elevated FGF23 and hypophosphatemia

Pathologically elevated serum levels of fibroblast growth factor-23 (FGF23), a bone-derived hormone that regulates phosphorus homeostasis, result in renal phosphate wasting and lead to rickets or osteomalacia. Rarely, elevated serum FGF23 levels are found in association with mosaic cutaneous disorders that affect large proportions of the skin and appear in patterns corresponding to the migration of ectodermal progenitors. The cause and source of elevated serum FGF23 is unknown. In those conditions, such as epidermal and large congenital melanocytic nevi, skin lesions are variably associated with other abnormalities in the eye, brain and vasculature. The wide distribution of involved tissues and the appearance of multiple segmental skin and bone lesions suggest that these conditions result from early embryonic somatic mutations. We report five such cases with elevated serum FGF23 and bone lesions, four with large epidermal nevi and one with a giant congenital melanocytic nevus. Exome sequencing of blood and affected skin tissue identified somatic activating mutations of HRAS or NRAS in each case without recurrent secondary mutation, and we further found that the same mutation is present in dysplastic bone. Our finding of somatic activating RAS mutation in bone, the endogenous source of FGF23, provides the first evidence that elevated serum FGF23 levels, hypophosphatemia and osteomalacia are associated with pathologic Ras activation and may provide insight in the heretofore limited understanding of the regulation of FGF23.

Genodermatoses caused by genetic mosaicism

Genetic mosaicism is defined as the existence of at least two genetically distinct cell populations within one individual. Mosaic presentation of genetic disorders is common and is often particularly obvious in the skin, because there it will generate recognizable patterns. Recognizing those can frequently assist in establishing a diagnosis. In this review, we discuss the mechanisms that give rise to genetic mosaicism. We describe its most frequent cutaneous manifestations that are relevant to paediatric practice. While most mosaic genetic diseases are rare, it is important to recognize them so that patients and parents may receive appropriate genetic counselling. Moreover, recent developments are now resulting in novel, targeted treatments for such disorders that promise to considerably improve patients' lives.

Mazabraud's syndrome. A case with multiple myxomas

Mazabraud's syndrome is defined as the combination of one or more intramuscular myxomas and fibrous dysplasia of bone. The diagnosis is important given the increased risk of malignant transformation of the bone lesions. We report a case in a 56-year-old patient with a 14-year follow-up during which multiple surgical procedures were required to remove myxomas (present at more than 15 sites). The resected myxomas were large and progressive. Unique features in this case include the long follow-up and the number of myxomas considerably above the average for this disease. Eighty other cases of Mazabraud's syndrome have been reported. The condition predominantly affects middle-aged women (mean age, 44 years). The bone lesions may be monostotic or polyostotic. Mazabraud's syndrome may be difficult to distinguish from soft-tissue sarcoma or neurofibromatosis. Identification of the underlying genetic abnormality provides diagnostic confirmation, as shown in our patient. The management consists in surgery to remove the myxomas and magnetic resonance imaging at regular intervals to monitor the lesions.

Combined treatment with bicalutamide and anastrozole in a young boy with peripheral precocious puberty due to McCune-Albright Syndrome

McCune-Albright Syndrome (MAS) is a congenital endocrine disorder due to mosaic tissutal hyper-function. We describe a boy with a molecularly confirmed MAS, clinically evident with congenital café-au-lait spots, bone fibrous dysplasia, hyperthyroidism, and renal phosphate wasting syndrome. At 4.6 years of age he disclosed a rapid progression of peripheral puberty, so we decided to treat him with bicalutamide 25 mg/day and anastrozole 1 mg/day. Combined third generation aromatase inhibitors - competitive androgen receptor blockers were employed in familial male precocious puberty (FMPP). Combined treatment was performed for 49 months from the age of 4.6 to 6.7 years. The patient underwent clinical, laboratory, and instrumental evaluation twice a year from the first admission to the current age. This treatment caused a rapid normalization of growth velocity, subsequent reduction of penile androgenization, and stabilization of testicular volume. The therapy was well tolerated for all its duration and neither side effects, nor secondary hypothalamic activation were noted. This report provides further evidence of effectiveness and safety of combined third generation aromatase inhibitors - competitive androgen receptor blockers in male precocious peripheral puberty, firstly employed in male MAS, and contributes to expand the spectrum of disorders in which their employment may reveal promising.

Benign fibro-osseous lesions of the craniofacial complex. A review

Benign fibro-osseous lesions of the craniofacial complex are represented by a variety of disease processes that are characterized by pathologic ossifications and calcifications in association with a hypercellular fibroblastic marrow element. The current classification includes neoplasms, developmental dysplastic lesions and inflammatory/reactive processes. The definitive diagnosis can rarely be rendered on the basis of histopathologic features alone; rather, procurement of a final diagnosis is usually dependent upon assessment of microscopic, clinical and imaging features together. Fibrous dysplasia and osteitis deformans constitute two dysplastic lesions in which mutations have been uncovered. Other dysplastic bone diseases of the craniofacial complex include florid osseous dysplasia, focal cemento-osseous dysplasia and periapical cemental dysplasia, all showing a predilection for African descent individuals; although no specific genetic alterations in DNA coding have yet to be uncovered and most studies have been derived from predominant high African descent populations. Ossifying fibromas are neoplastic lesions with four subtypes varying with regard to behavior and propensity for recurrence after surgical excision. The clinicopathologic and molecular features of this unique yet heterogeneous group of diseases are reviewed.

Mazabraud's syndrome: a new case and review of the literature

The association between muscular myxomas and fibrous dysplasia is a rare condition known as Mazabraud's syndrome, as reported by Henschen (Verh Dtsch Ges Pathol 21:93-97, 1926) and Mazabraud A and Girard (Rev Rhum Mal Osteoartic 24(9-10):652-659, 1957). We report a case of a 32-year-old woman with multiple myxomas in her right thigh and monomelic fibrous dysplasia. A review of the international literature referring to 67 cases to date was carried out. The syndrome is characterised by the following features: females are twice as likely to be affected as males; the lower limbs are the most frequently affected, fibrous dysplasia is more common in the femur and the pelvis and myxomas in the quadriceps muscle; myxoma is multiple in more than 70% of cases. Although there has never been any continuity between tumours and bone lesions, a significant correlation between dysplastic bone and myxoma has been revealed.

Androgen resistance

Androgen resistance causes the androgen insensitivity syndrome in its variant forms and is a paradigm of clinical syndromes associated with hormone resistance. In its complete form, the syndrome causes XY sex reversal and a female phenotype. Partial resistance to androgens is a common cause of ambiguous genitalia of the newborn, but a similar phenotype may result from several other conditions, including defects in testis determination and androgen biosynthesis. The biological actions of androgens are mediated by a single intracellular androgen receptor encoded by a gene on the long arm of the X chromosome. Mutations in this gene result in varying degrees of androgen receptor dysfunction and phenotypes that often show poor concordance with the genotype. Functional characterization and three-dimensional modelling of novel mutant receptors has been informative in understanding the mechanism of androgen action. Management issues in syndromes of androgen insensitivity include decisions on sex assignment, timing of gonadectomy in relation to tumour risk, and genetic and psychological counselling.