Commentary: genes for osteoarthritis: interpreting twin data

The heritability of human disease: estimation, uses and abuses

Relatives provide the basic material for the study of inheritance of human disease. However, the methodologies for the estimation of heritability and the interpretation of the results have been controversial. The debate arises from the plethora of methods used, the validity of the methodological assumptions and the inconsistent and sometimes erroneous genetic interpretations made. We will discuss how to estimate disease heritability, how to interpret it, how biases in heritability estimates arise and how heritability relates to other measures of familial disease aggregation.

Genetics of primary generalized osteoarthritis

Osteoarthritis (OA) is a common joint disease with complex etiology, including biomechanical, biochemical, and genetic components. The genetics of primary idiopathic OA is likely complex as multiple susceptibility loci have been identified from genome-wide scans. The future lies in finer mapping of these chromosomal regions, association studies, and candidate gene analysis and identification, which should collectively give rise to an improved understanding of the genetic mutations and biological pathways which increase the risk of developing this common joint disorder.

Genetics of hand osteoarthritis

There is convincing epidemiological evidence for a strong hereditary component to hand osteoarthritis (OA). For example: (1) greater concordance for hand OA in monozygotic than in dizygotic twins, the estimated proportion of variance explained by genetic factors being as high as 0.59; and (2) a substantially increased risk of hand OA in first-degree relatives (siblings, parents, offspring) of subjects with hand OA. Such evidence clearly justifies a search for the genes involved. However, gene association studies in genetically complex polygenic conditions such as OA present many problems, including case definition, late age of phenotype expression and adjustment for other constitutional and environmental risk factors. Nevertheless, association studies of affected sibling pairs and nuclear families, using candidate gene and genome wide screening and transmission disequilibrium testing, suggest no association with candidates such as COL2A1 (responsible for some rare monogenic syndromes of premature generalized OA) but possible associations, currently not isolated, on chromosome 2q. Such ongoing work and subsequent gene-gene and gene-environment interaction studies are likely to give important, perhaps unexpected, insights into the pathogenesis of hand OA.

Evidence for genetic associations between asthma, atopy, and bronchial hyperresponsiveness: a study of 8- to 18-yr-old twins

We measured asthma in the last 12 mo, diagnosed by a respiratory physician at interview; atopy, defined by a positive skin prick test to any of eight common allergens; and bronchial hyperresponsiveness (BHR) to hypertonic saline, in 381 twin pairs aged 8 to 18 yr selected from the Australian Twin Registry-183 monozygous (MZ) and 198 dizygous (DZ). The associations between twins, as measured by an odds ratio, were greater in MZ pairs compared with DZ pairs for asthma: 25.6 (95% confidence interval 11.3- 57.8) versus 1.9 (1.0-3. 5); atopy: 14.6 (7.1-30.1) versus 2.5 (1.4- 4.5); and BHR: 14.1 (6. 4-31.0) versus 4.2 (2.1-8.6) (all p < 0.002). The associations between each pair of traits within an individual were slightly greater than the association between one trait in a twin and the other trait in the cotwin (cross-trait cross-pair) in MZ pairs. Further, the associations in MZ pairs were greater than in DZ pairs (p < 0.05). Under the assumptions of the classic twin model, these data suggest that the strong cross-sectional associations between these three traits are due to an overlap between the genetic factors involved in each of these three traits.

Hereditary disorders mimicking and/or causing premature osteoarthritis

Osteoarthritis is the most common joint disease, causing considerable disability and impairment of quality of life. Hereditary osteochondrodysplasias and some inborn errors of metabolism may mimic or cause premature osteoarthritis. Osteochondrodysplasias usually cause joint deformities, such as coxa vara or genu varum, which can cause abnormal biomechanics. In most of these disorders, the articular cartilage is originally defective as a result of genetically determined collagen or matrix protein abnormalities, or the deposition of mucopolysaccharides. In the case of inborn errors of metabolism, the pathological process affects healthy articular structures, causing secondary osteoarthritis. In alkaptonuria, the pathological deposition of polymerized homogenistic acid causes defective changes in cartilage, articular capsule and tendons. In Wilson's disease, the premature osteoarthritis might be caused by the copper deposition. It is worth paying attention to these rare disorders, even when they are mild or incomplete, because early diagnosis can lead to prevention and effective treatment. In addition, research is discovering the specific gene defects and molecular abnormalities that are responsible for disease expression. This may in turn lead to opportunities for prenatal diagnosis; thus, genetic counselling and gene replacement therapy may be a realistic possibility in the near future.

Prevalence of hip osteoarthritis in Iceland

OBJECTIVE

To assess the prevalence of primary hip osteoarthritis (OA) in Iceland. To compare the prevalence of primary hip OA in Iceland with published rates of primary hip OA for related Scandinavian populations.

METHODS

Roentgenographs were examined of 1530 Icelandic people 35 years or older (653 males, 877 females) subjected to colon radiography during the years 1990-1996. The radiographs examined represent approximately 40% of all colon radiographs taken in Iceland during this period. After exclusion of non-primary hip OA cases, the minimum hip joint space was measured with a mm ruler. Presence of hip OA was defined as a minimum joint space of 2.5 mm or less on an anteroposterior radiograph. Intraclass correlation coefficients for inter and intraobserver variability of assessment of mm joint space were 0.91 and 0.95, respectively.

RESULTS

Of the 1517 people included, 227 hips in 165 patients (77 men, 88 women) were diagnosed as having radiological primary hip OA. The mean age at colon examination for these patients was 68 (35-89) years. The overall prevalence of coxarthrosis among all examined patients 35 years and older was 10.8% (12% for men, 10% for women), rising from 2% at 35-39 years to 35.4% for those 85 years or older. If the population structure (age and sex distribution) for those older than 35 years in Iceland was used to standardise prevalence for both Iceland and south Sweden (using previously published data for south Sweden), the age and sex standardised prevalence of hip OA for those older than 35 years in Iceland was 8%, compared with 1.2% for south Sweden.

CONCLUSIONS

The prevalence of radiological primary hip OA is very high in Iceland, and in excess of fivefold higher than the prevalence found by using similar techniques in studies on related populations in southern Scandinavia. The rate difference is particularly notable for those younger than 70 years.

Epilepsies in twins: genetics of the major epilepsy syndromes

We studied twins to examine the genetics of epilepsy syndromes. We ascertained 358 twin pairs in whom one or both reported seizures. After evaluation, 253 of 358 (71%) had seizure disorders and 105 pairs were false positives. Among the monozygous (MZ) pairs, more were concordant for seizures (48 of 108; casewise concordance = 0.62 +/- 0.05) than among the dizygous (DZ) pairs (14 of 145; casewise concordance = 0.18 +/- 0.04). In 94% of concordant MZ pairs, and 71% of concordant DZ pairs, both twins had the same major epilepsy syndrome. When analyzed according to major epilepsy syndrome, the casewise concordances for generalized epilepsies (MZ = 0.82; DZ = 0.26), both idiopathic (MZ = 0.76; DZ = 0.33) and symptomatic (MZ = 0.83; DZ = 0), were greater than those for partial epilepsies (MZ = 0.36; DZ = 0.05), with intermediate values seen for febrile seizures (MZ = 0.58; DZ = 0.14) and unclassified epilepsies (MZ = 0.53; DZ = 0.18). We conclude that genetic factors are particularly important in the generalized epilepsies but also play a role in the partial epilepsies. The high frequency of concordant MZ pairs with the same major syndrome strongly suggests there are syndrome-specific genetic determinants rather than a broad genetic predisposition to seizures.

A twin-pronged attack on complex traits

Before one starts the hunt for quantitative trait loci (QTLs) for a complex trait, it is necessary to show that the trait is genetically influenced. This evidence is most likely to come from the classical twin study--the demonstration that monozygotic twins are more similar for the trait than dizygotic twins. The strengths and weaknesses of twin studies are discussed, and it is suggested that, far from becoming irrelevant with advances in molecular biology, they can improve the efficiency of QTL detection and play an important role in unravelling developmental genetic mechanisms.

Genetic and environmental components of the population variance in bone density

Understanding of the genetic and environmental factors contributing to the differences in fracture rates between young and old, between women and men, and between races is limited. The reasons for this may partly relate to the lack of a detailed understanding of the structural basis of bone fragility. This information, although difficult to obtain because of the invasiveness of bone biopsy, will be required if advances are to occur. aBMD cannot be relied upon as an endpoint in the study of the pathophysiology of osteoporosis. Advances will require the description of the age-, gender and race-specific means and variances in trabecular number, thickness, spacing and orientation, cortical thickness, and bone size and shape in women and men of different racial groups. Subsequent comparison of the structures in young versus old, female versus male, and in racial groups may reveal the structural differences from which inferences may be made concerning the differences in fracture rates between these groups. By defining the structural basis for bone fragility, the genetic and modifiable environmental determinants of these structures may then be identified, providing hypotheses to be tested in randomized trials aimed at reducing the incidence of fractures in these groups.