Estudios funcionales de variantes de DKK1 presentes en la población general / Functional studies of DKK1 variants present in the general population

Objetivo: En las últimas décadas se han identificado genes asociados a la masa ósea y al riesgo de fractura osteoporótica, varios de los cuales pertenecen a la vía de Wnt. En este proyecto se estudió la funcionalidad de 7 mutaciones de cambio de sentido del gen DKK1 -un inhibidor de la vía de Wnt- presentes en la población general. Material y métodos: Se realizaron estudios in vitro del gen reportero luciferasa para medir la actividad de la vía de Wnt en presencia o ausencia de DKK1 silvestre o mutada, y estudios de western blot, para evaluar si las distintas mutaciones afectan a su síntesis y/o a su estabilidad. Resultados: La proteína DKK1 con la variante p.Ala41Thr presenta menor actividad inhibidora de la vía en comparación con la proteína silvestre. También se observaron diferencias significativas entre los experimentos realizados en ausencia de DKK1 y los que incluyen DKK1 con la mutación p.Ala41Thr. Los western blots mostraron que la cantidad de proteína era similar para todas las variantes, tanto las mutadas como la silvestre, por lo que la pérdida de actividad de p.Ala41Thr no parecía deberse a falta de proteína. El resto de las mutaciones no presentaron un comportamiento diferente al de la proteína DKK1 silvestre. Conclusiones: La variante de cambio de sentido p.Ala41Thr de la proteína DKK1, con una frecuencia poblacional de 0,013%, presenta una pérdida parcial de su función inhibidora, que no es debida a la falta de expresión de ésta. Esta variante génica podría conllevar un aumento de la densidad mineral ósea en las personas de la población general portadoras de esta mutación

Objective: In recent decades, genes associated with bone mass and osteoporotic fracture risk have been identified, several of which belong to the Wnt pathway. In this project, the functionality of 7 missense mutations of the gene DKK1-an inhibitor of the Wnt pathway- present in the general population was studied. Material and methods: In vitro studies of the luciferase reporter gene were carried out to measure Wnt pathway activity in the presence or absence of wild-type or mutated DKK1, and western blot studies, to evaluate if the different mutations affect its synthesis and/or stability. Results: The DKK1 protein with the p.Ala41Thr variant shows lower pathway inhibitory activity compared to the wild-type protein. Significant differences were also observed between the experiments performed in the absence of DKK1 and those that include DKK1 with the p.Ala41Thr mutation. Western blots showed that the amount of protein was similar for all variants, both mutated and "wild-type, so the loss of p.Ala41Thr activity did not seem to be due to a lack of protein. The rest of the mutations did not show different behavior from that of the wild DKK1 protein. Conclusions: The missense variant p.Ala41Thr of the DKK1 protein, with a population frequency of 0.013%, shows a partial loss of its inhibitory function, which is not due to the lack of expression. This gene variant could lead to an increase in bone mineral density in those people in the general population who carry this mutation

Identificación de variantes genéticas asociadas con la densidad mineral ósea (DMO) en el gen FLJ42280 / Identification of genetic variants associated with bone mineral density (BMD) in the FLJ42280 gene

FLJ42280 es un posible gen de susceptibilidad a la osteoporosis. Distintos estudios de GWAs han identificado 4 SNPs no-codificantes en este gen que se asocian a la densidad mineral ósea (DMO) y el riesgo de fractura. Para descubrir la causa de la asociación entre estos SNPs y la osteoporosis, se realizó una búsqueda de variantes genéticas mediante resecuenciación de 28 kb que contienen el gen, en una selección truncada de mujeres con DMO muy baja (n=50) o muy alta (n=50) de la cohorte BARCOS (Barcelona Cohorte Osteoporosis, cohorte de mujeres postmenopáusicas de Barcelona). Las variantes encontradas se filtraron y se analizó su frecuencia en cada grupo. Se analizó el solapamiento de las variantes con elementos funcionales del proyecto ENCODE y también se calculó el desequilibrio de ligamiento entre los SNPs de la región. Finalmente, se hizo un análisis de eQTL de los 4 SNPs no-codificantes respecto a los niveles de expresión de genes cercanos a FLJ42280 en linfoblastos. Se seleccionaron 110 variantes. Las diferencias de sus frecuencias entre los dos grupos estuvieron por debajo del poder estadístico del diseño experimental. Sin embargo, 3 variantes solaparon con posibles enhancers y una solapó con un enhancer activo en osteoblastos (rs4613908). Se observó un fuerte desequilibrio de ligamiento entre los 4 SNPs no-codificantes y el SNP rs4613908, que pertenecen a un bloque que abarca el gen casi por completo. Ninguno de los SNPs no-codificantes mostró asociación con los niveles de expresión de genes cercanos a FLJ42280. En conclusión, el SNP rs4613908 podría estar implicado funcionalmente en la determinación de la DMO. Serán necesarios experimentos concretos para confirmarlo (AU)

FLJ42280 is a possible gene for susceptibility to osteoporosis. Different studies of GWAs have identified 4 non-coding SNPs in this gene associated with bone mineral density (BMD) and fracture risk. In order to ascertain the cause of the association between these SNPs and osteoporosis, we searched for genetic variants by resequencing the 28-kb gene, in a truncated selection of women with very low (n=50) or very high BMD (N=50) of the BARCOS cohort (Barcelona Cohort Osteoporosis, cohort of postmenopausal women in Barcelona). The variants found were filtered and their frequency analyzed in each group. The overlap of the variants with functional elements of the ENCODE project was calculated. Finally, an eQTL analysis of the 4 SNPs-coding was performed on the expression levels of FLJ42280 neighbor genes in lymphoblasts. In all, 110 variants were selected. The differences in their frequencies between the two groups were below the statistical power of the experimental design. However, three variants overlapped with possible enhancers and one overlapped with an active enhancer in osteoblasts (rs4613908). A strong linkage disequilibrium was observed between the 4 non-coding SNPs and the SNP rs4613908, which belong to a block spanning the gene almost completely. None of the non-coding SNPs showed association with the expression levels of FLJ42280 neighbor genes. In conclusion, the SNP rs4613908 could be involved functionally in determining BMD. Tangible experiments will be required to confirm this (AU)

A broad spectrum of genomic changes in latinamerican patients with EXT1/EXT2-CDG.

Multiple osteochondromatosis (MO), or EXT1/EXT2-CDG, is an autosomal dominant O-linked glycosylation disorder characterized by the formation of multiple cartilage-capped tumors (osteochondromas). In contrast, solitary osteochondroma (SO) is a non-hereditary condition. EXT1 and EXT2, are tumor suppressor genes that encode glycosyltransferases involved in heparan sulfate elongation. We present the clinical and molecular analysis of 33 unrelated Latin American patients (27 MO and 6 SO). Sixty-three percent of all MO cases presented severe phenotype and two malignant transformations to chondrosarcoma (7%). We found the mutant allele in 78% of MO patients. Ten mutations were novel. The disease-causing mutations remained unknown in 22% of the MO patients and in all SO patients. No second mutational hit was detected in the DNA of the secondary chondrosarcoma from a patient who carried a nonsense EXT1 mutation. Neither EXT1 nor EXT2 protein could be detected in this sample. This is the first Latin American research program on EXT1/EXT2-CDG.

Mutations in the EXT1 and EXT2 genes in Spanish patients with multiple osteochondromas.

Multiple osteochondromas is an autosomal dominant skeletal disorder characterized by the formation of multiple cartilage-capped tumours. Two causal genes have been identified, EXT1 and EXT2, which account for 65% and 30% of cases, respectively. We have undertaken a mutation analysis of the EXT1 and EXT2 genes in 39 unrelated Spanish patients, most of them with moderate phenotype, and looked for genotype-phenotype correlations. We found the mutant allele in 37 patients, 29 in EXT1 and 8 in EXT2. Five of the EXT1 mutations were deletions identified by MLPA. Two cases of mosaicism were documented. We detected a lower number of exostoses in patients with missense mutation versus other kinds of mutations. In conclusion, we found a mutation in EXT1 or in EXT2 in 95% of the Spanish patients. Eighteen of the mutations were novel.

Molecular characterization of five patients with homocystinuria due to severe methylenetetrahydrofolate reductase deficiency.

Methylenetetrahydrofolate reductase (MTHFR) plays a major role in folate metabolism. Disturbed function of the enzyme results in hyperhomocysteinemia and causes severe vascular and neurological disorders and developmental delay. Five patients suspected of having non-classical homocystinuria due to MTHFR deficiency were examined with respect to their symptoms, MTHFR enzyme activity and genotypes of the MTHFR gene. All patients presented symptoms of severe central nervous system disease. Two patients died, at the ages of 15 months and 14 years. One patient is currently 32 years old, and is being treated with betaine and folinic acid. The other two patients, with an early diagnosis and a severe course of the disease, are currently improving under treatment. MTHFR enzyme activity in the fibroblasts of four of the patients was practically undetectable. We found four novel mutations, three of which were missense changes c.664G> T (p.V218L), c.1316T> C (p.F435S) and c.1733T> G (p.V574G), and the fourth was the 1-bp deletion c.1780delC (p.L590CfsX72). We also found the previously reported nonsense mutation c.1420G> T (p.E470X). All the patients were homozygous. Molecular modelling of the double mutant allele (p.V218L; p.A222V) revealed that affinity for FAD was not affected in this mutant. For the p.E470X mutation, the evidence pointed to nonsense-mediated mRNA decay. In general, genotype-phenotype analysis predicts milder outcomes for patients with missense changes than for those in which mutations led to severe alterations of the MTHFR protein.

Polymorphisms and haplotypes across the osteoprotegerin gene associated with bone mineral density and osteoporotic fractures.

UNLABELLED: Osteoprotegerin plays a key role in bone remodelling. We studied the association between 24 polymorphisms and haplotypes on the OPG gene and bone mineral density and fractures. After multiple-testing correction, one SNP and two block-haplotypes were significantly associated with FN BMD. Two other block-haplotypes were associated with fracture. INTRODUCTION AND HYPOTHESIS: Osteoprotegerin (OPG) plays a key role in bone remodelling. Here we studied the association between polymorphisms and haplotypes on the OPG gene and bone mineral density (BMD) and fractures. METHODS: Twenty-four single nucleotide polymorphisms (SNPs) were selected to cover six haplotypic blocks and were genotyped in 964 postmenopausal Spanish women. Haplotypes were established with HaploStats. Association was analysed by GLM (for BMD) and logistic regression (for fractures) both at single SNP and haplotype levels. RESULTS: Upon adjustment for multiple testing (p < 0.0073), one of the SNPs (SNP #17, rs1032129) remained significantly associated with FN BMD (p = 0.001). Four block-haplotypes stood multiple-testing correction. Two remained associated with FN BMD and two with fracture. The association of block-4 haplotype "AC" (of SNPs #18 and #17) with FN BMD (p = 0.0002) was stronger than that of SNP#17 alone and was the best result overall. A global assessment of the results indicated that all the alleles and haplotypes with a protective effect, at p < 0.05, belonged to a frequent long-range haplotype. CONCLUSIONS: In conclusion, these results provide a detailed picture of the involvement of common variants and haplotypes of the OPG gene in bone phenotypes.

Two successful pregnancies in pyridoxine-nonresponsive homocystinuria.

Two successful pregnancies are reported in a pyridoxine-nonresponsive woman who was also homozygous for the MTHFR 677C>T polymorphism. Two healthy children were delivered, although there had also been an early miscarriage of an apparently normal fetus in another pregnancy. Management of the patient's homocystine and methionine levels was maintained throughout pregnancy and, in view of the increased thromboembolic risk, anticoagulation therapy was also included in management.