New mutations in the human genome occur at a rate of approximately 1 in 100 million base pairs per generation; mutations of cytosine–guanine (CG) to thymine–guanine (TG) occur frequently. When base pairs 154,375,028 to 154,375,029 on chromosome 1 change from CG to TG, the new sequence resembles a splice donor, and the cell's spliceosome deletes 150 nucleotides from the LMNA
mRNA. CAAX is the motif cysteine (C), two alipathic amino acids (AA), and any amino acid (X). Farnesyl groups linked to cysteines of C-terminal CAAX boxes tether the normal and mutant lamin A (progerin, which lacks 50 amino acids near the carboxy terminus) to the inner nuclear membrane.10,30
Although normal lamin A is released by enzymatic cleavage of 15 C-terminal amino acids, progerin, lacking the cleavage site, remains permanently anchored to the membrane, binding other proteins, causing blebbing of the nucleus, disrupting mitosis,31
and altering gene expression. These abnormalities are not due to haploinsufficiency, since mice completely lacking lamin A have normal nuclear morphologic characteristics and phenotypes.8
Rather, progerin acts in a dominant negative fashion, since transfection of a mutant allele into normal cells induces nuclear blebbing.9
The result is Hutchinson–Gilford progeria syndrome, one of 11 laminopathies caused by more than 180 known LMNA
mutations. Our prospective investigation of Hutchinson–Gilford progeria syndrome confirmed the growth impairment, alopecia, sclerotic skin changes, bone-growth abnormalities, cardiovascular and central nervous system complications, abnormal dentition, occasional mild aminoaciduria, and decreased body fat associated with this disorder.1–3,12,32
We also confirmed the normal findings with respect to hematologic values, serum chemical laboratory values, renal tubular and glomerular function, and humoral and cellular immune function. New findings included prolonged prothrombin times, elevated platelet counts and serum phosphorus levels, hyperopia, specific abnormalities of joint motion, a particular low-frequency conductive hearing loss, and oral motor abnormalities such as decreased lingual range of motion, labial weakness, and vertical chewing. In addition, our patients with Hutchinson–Gilford progeria syndrome were surprisingly active and mobile; despite reduced vascular compliance, in these children the mean distance of the 6-minute walk test (approximately 1000 ft [304.8 m]) was associated with high function, especially in view of their musculoskeletal impairments.
Growth in patients with Hutchinson–Gilford progeria syndrome is clearly abnormal. In the patients in our study, weight began to deviate from normal before height (), and the average weight gain between 2 and 10 years of age (0.65 kg per year) was similar to that reported for children with Hutchinson–Gilford progeria syndrome who were studied both retrospectively (0.44 kg per year) and prospectively (0.52 kg per year).13
Muscle volume remained proportional to body mass, as indicated by normal production of creatinine per kilogram of body weight. Despite radiologic evidence of bone resorption, laboratory evidence suggests a normal rate of bone turnover. Although our calculated z scores suggest osteoporosis or osteopenia, consideration must be given to short stature and small bones in children with Hutchinson–Gilford progeria syndrome as compared with age-matched control children.33
When adjusted for these factors, bone density may be higher than that determined in this study.
Several possible causes of impaired growth were ruled out. Inadequate nutrition was not responsible, since energy intake was sufficient for growth and serum prealbumin levels were normal. Growth hormone production appeared to be adequate, since IGF-I levels were normal. Insulin resistance was at worst mild; levels of both serum glucose and plasma free fatty acids decreased in response to endogenously produced insulin.
Cardiovascular complications generally cause death in Hutchinson–Gilford progeria syndrome. Medial smooth-muscle cells are lost, with secondary maladaptive vascular remodeling, intimal thickening, disrupted elastin fibers, and deposition of extracellular matrix; sclerotic plaques that form in the aorta and coronary arteries are associated with stenosis.34,35
A transgenic mouse model recapitulates the vascular pathological features in humans and is useful in the investigation of potential therapies. The mouse model contains the human mutant G608G LMNA
gene as well as the normal complement of Lmna
genes. It shows progressive drop-out of vascular smooth-muscle cells, collagen and proteoglycan deposition with medial-wall fibrosis and thickening, and relative sparing of the endothelial-cell layer.36
Loss of medial cells is associated with a blunted vasodilator response.
Our clinical findings also indicate reduced vascular compliance, with elevated systolic and diastolic blood-pressure levels and an increased arterial augmentation rate. Peripheral vascular disease, with reduced ankle–brachial indexes and vessel occlusion, occurred in two children (). Endothelial function was reasonably preserved; brachial-artery reactivity was normal.
One possible therapy for Hutchinson–Gilford progeria syndrome would involve inhibition of farnesyl transferase activity to prevent the permanent anchoring of progerin to the inner nuclear membrane. This treatment normalizes the nuclear morphologic features of fibroblasts in Hutchinson–Gilford progeria syndrome30,37–39
; in the transgenic mouse model,36
it maintains vascular smooth-muscle cells and decreases proteoglycan deposition in vessel walls.40
In the LmnaHG/+
mouse knock-in model that expresses normal lamin A on one allele and progerin on the other, mutated allele, use of the farnesyl transferase inhibitor ABT100 improved body weight, increased fat tissue and bone mineralization, and reduced the fracture rate.41
These promising results led to an open-label clinical trial of inhibition of farnesyl transferase in Hutchinson–Gilford progeria syndrome (ClinicalTrials.gov number, NCT000425607). The trial, using weight gain as an outcome variable, has just begun. A concept-based trial such as this requires detailed knowledge of the disease process (for safety concerns) and reliable outcome variables (for efficacy). Measurements of blood pressure and the ankle–brachial index () could help to evaluate vascular function, and serum lipid levels could be followed. Long-term measures of improvement could include normalization of low-frequency hearing, bone density, body fat, and range of motion in the wrists and ankles.
Recent molecular evidence indicates that the wild-type cryptic splice site in exon 11 of LMNA
is recognized occasionally and used by the splicing machinery in normal cells. Indeed, fibroblasts from an older person contain more progerin than fibroblasts from a younger person,42
and individual cells with increased progerin show nuclear blebbing and other membrane abnormalities.31
Hence, Hutchinson–Gilford progeria syndrome may serve as a model for the normal aging process.