RESUMO
Heterozygosity for certain mutations of the GH receptor (GHR) gene has been proposed as the cause of partial resistance to GH, and there has been a recent demonstration of a dominant-negative effect of such a mutation in a mother and child. To examine the effect of heterozygosity in a large genetically homogeneous population with GHR deficiency, in which a substantial number of heterozygous (carrier) subjects and homozygous normal individuals can be compared, we studied a population in Ecuador in which 70 individuals with GHR deficiency were homozygous for the E180 splice mutation. We found that 58 heterozygous relatives of probands were not significantly shorter than 37 homozygous normal relatives [SD score (SDS) for height -1.85 +/- 1.04 (SD) vs. -1.55 +/- 0.96, P > 0.10]. When only those families with both homozygous normals and carriers were compared, the 33 heterozygous and 29 normal relatives did not differ significantly in height SDS (-1.98 +/- 1.07 vs. -1.77 +/- 0.91, P > 0.3). If heterozygosity for the E180 splice mutation were to influence stature, heights of heterozygous parents of probands would be expected to correlate with those of probands and of carriers who are their offspring and not with heights of their homozygous normal children. Parental height SDS did not correlate with height SDS of affected offspring (r = 0.24). For unaffected siblings as a group or analyzed separately as normals or carriers, there was a strong correlation between parental and offspring SDS for height (P < 0.01 for all comparisons). Thus, the effect of homozygosity for the GHR mutation was so profound as to abolish parental influence on height, and there was no difference in the influence of parental stature between carrier and noncarrier offspring. These findings demonstrate no meaningful effect on stature of heterozygosity for the E180 splice mutation of the GHR, which is a functional null mutation and, in the homozygous state, results in profound short stature from severe insulin-like growth factor-I deficiency.
Assuntos
Estatura/fisiologia , DNA Recombinante , Saúde da Família , Heterozigoto , Homozigoto , Receptores da Somatotropina/genética , Adolescente , Adulto , Criança , Pré-Escolar , Equador , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Mutação , Valores de ReferênciaRESUMO
Eight different mutations were detected in the growth hormone (GH) receptor gene of patients with inherited GH receptor deficiency (GHRD; Laron syndrome) from five continents. All the mutations are located in the extracellular domain of the receptor and are predicted to cause gross structural abnormalities and non-functional receptor molecules. They include three nucleotide changes in the coding region causing translational stop signals, including the newly identified E183X mutation; two nucleotide changes in introns that affect splice junctions; two dinucleotide deletions that result in stop codons downstream; and one single nucleotide change that activates a donor splice site within an exon and results in a transcript missing 24 nucleotides. This latter mutation (E180splice) was first identified in a cohort of patients with GHRD from southern Ecuador. Based on the fact that the E180splice mutation generates a new cleavage site for the restriction enzyme MnlI, a simple diagnostic test has been developed that can be carried out on dried blood spots collected on filter paper. A total of 55 affected individuals from Ecuador has been found to be homozygous for this mutation. Asymptomatic carriers can also be detected, and 104 of 150 individuals screened were found to be carriers. Using this test, the E180splice mutation has recently been detected in one of two oriental Jewish patients from Israel.
Assuntos
Receptores da Somatotropina/deficiência , Receptores da Somatotropina/genética , Sequência de Bases , Equador , Éxons , Haplótipos , Humanos , Judeus , Dados de Sequência Molecular , Mutação , FenótipoRESUMO
Laron syndrome is an autosomal recessive condition characterized by resistance to growth hormone. We sought to determine the molecular basis of this condition in an Ecuadorean population with a high incidence of affected individuals. Growth hormone receptor gene sequences from an obligate heterozygote were amplified by the polymerase chain reaction and screened for mutations using denaturing gradient gel electrophoresis. Only exon 6 revealed homo- and heteroduplexes on denaturing gradient gels. Sequencing revealed a substitution of guanine for adenine in the third position of codon 180 that did not change the amino acid encoded. Sequencing of the exon 6-exon 7 splice junction from RNA-polymerase chain reaction amplified cellular RNA of an affected individual revealed that the substitution activates a 5' splice site 24 nucleotides upstream from the normal exon 6-intron 6 boundary. Splicing in two probands' lymphoblasts occurred virtually exclusively at the abnormal 5' splice site created by the codon 180 substitution. Exon 6 sequences from 38 patients and 47 relatives were amplified and analyzed by sequencing or dot-blot hybridization with allele-specific oligonucleotides. The substitution was detected in 74 of 76 Laron syndrome patients' GH-receptor alleles. All 26 parents and 12 of 21 unaffected siblings were heterozygous for this mutation. It was absent in 61 unrelated unaffected control individuals. We conclude that the codon 180 nucleotide substitution probably causes Laron syndrome as translation of the observed, abnormally spliced growth hormone receptor transcript would lead to the synthesis of a receptor protein with an 8 amino acid deletion from the extracellular domain.