Genetic Diseases Related with Osteoporosis

Primary osteoporosis is the disease’s most common form and results from the progressive loss of bone mass related to aging and unassociated with other illness, a natural process in adult life; its etiology is considered multifactorial and polygenic. This form currently represents a growing worldwide health problem due in part, to the contemporary environmental condi‐ tions of modern civilization. Risk factors that are considered as “modifiable” also play an important role and include physical activity, dietary habits and eating disorders. Furthermore, there is another group of associated risk factors that are considered “non-modifiable”, including gender, age, race, a personal and/or family history of fractures that in turn, indirectly reflect the degree of genetic susceptibility to this disease [2-4]. Secondary osteoporosis encompasses a large heterogeneous group of primary conditions favoring osteoporosis development. Table 1 summarizes some of the disease entities associated to primary and secondary osteoporosis.


Introduction
Osteoporosis is a disease entity characterized by the progressive loss of bone mineral density (BMD) and the deterioration of bone microarchitecture, leading to the development of fractures.Its classification encompasses two large groups, primary and secondary osteoporosis [1].
Primary osteoporosis is the disease's most common form and results from the progressive loss of bone mass related to aging and unassociated with other illness, a natural process in adult life; its etiology is considered multifactorial and polygenic.This form currently represents a growing worldwide health problem due in part, to the contemporary environmental conditions of modern civilization.Risk factors that are considered as "modifiable" also play an important role and include physical activity, dietary habits and eating disorders.Furthermore, there is another group of associated risk factors that are considered "non-modifiable", including gender, age, race, a personal and/or family history of fractures that in turn, indirectly reflect the degree of genetic susceptibility to this disease [2][3][4].Secondary osteoporosis encompasses a large heterogeneous group of primary conditions favoring osteoporosis development.Table 1 summarizes some of the disease entities associated to primary and secondary osteoporosis.

Genetic aspects of primary osteoporosis
This form of osteoporosis results from the interaction of several environmental and genetic factors, leading to difficulties in its study.It is not easy to define the magnitude of the effect of genetic susceptibility since it is a trait determined by multiple genes whose products affect the bone phenotype; moreover, the environmental factors compromising bone mineral density are also difficult to analyze.However, in spite of these barriers, research suggests that inherited factors affect BMD in ranges between 40 -70% in the spine, 70 -85% in the hip and 50 -60% in the wrist.Bone density studies in monozygotic (MZ) and dizygotic (DZ) twins suggest that spinal and femoral neck BMD concordance is higher (6-8:1) in MZ versus DZ twins.Other studies have estimated that fracture predisposition heritability per se ranges between 25 -35% and up to 40% of patients with osteoporotic fractures have a positive family history of fractures, thus reflecting the great influence of genetic factors in this disease.On the other hand, the geometry and length of the femoral neck, the bone's properties on ultrasound, growth speed and bone remodeling variations are also dependent on genetic factors.The genes associated with the bone phenotype are distributed throughout the human genome and located in practically all chromosomes; their products fulfill specific functions and contribute in different manners to the genetic control of the bone tissue phenotype [5][6][7][8][9][10][11][12].Some of these genes and their products are presented in Table 2 [13][14][15][16][17][18][19][20][21][22][23].
It is important to mention that the mechanisms conditioning the hereditary susceptibility to osteoporosis are determined, among other factors, by the presence of mutations or genetic polymorphisms (natural genomic variations) in one or several genes involved in bone phenotype genetic control.These polymorphisms follow a well-defined inheritance pattern and their distribution is different among racial groups and populations.There are several reports in the world literature, of associations between specific genetic variants and

Topics in Osteoporosis
osteoporosis development or the risk of fractures; these risks may vary according to the fractures' anatomic location [3,4,[24][25][26][27][28][29][30] due to its clinical characteristics, lacking in idiopathic osteoporosis.The genetic basis of this disease has of yet, not been established but it is possible that genetic mutations with preferential tissue expression in bone and with great impact on the tissue's phenotype, may explain some of these cases [31,32].

Osteogenesis imperfecta
Osteogenesis imperfecta, also known as "brittle bone disease", has an estimated incidence of approximately 1 in 20 000 births.It has great phenotypic variability, different patterns of inheritance and a wide clinical spectrum ranging from very mild forms of the disease to severe cases with an unfavorable prognosis.It is caused by the defective synthesis of one of the two alpha chains of type I collagen (COL1A1 and COL1A2), leading to anomalies in these protein's structure; it is normally constituted by 3 coiled sub-units, two α1 chains and one α2 chain.This type of collagen is considered the most abundant component of structural protein in bone as well as in ligaments, tendons, sclerae and skin.Quantitative or qualitative defects in this protein lead to bone fragility and hence, to an increased risk of fractures.
The genes encoding the α1 and α2 chains are located in the 17q21.31-q22and 7q22.1 chromosomes, respectively.Aside from brittle bones, these patients may also present long bones with no curvatures, severe deformities preventing appropriate gait and even standing, conductive deafness due to malformations of the auditory canal, dentinogenesis imperfecta, joint hyperlaxity and intervertebral disc herniation.Patients with severe forms of the disease have a long history of fractures on mild impact and variable bone deformities.The most severe variants may even lead to fractures in utero and pre or perinatal death.Tables 3 and 4 shows different forms of the disease [33][34][35].

Osteoporosis -Pseudoglioma Syndrome (OPPG)
This syndrome is an autosomal recessive disease characterized by bone and visual abnormalities including short stature, osteoporosis development during infancy, spontaneous fractures, scoliosis, platyspondyly and long bone deformities.A crucial associated finding is the presence of pseudoglioma that may be associated to microcephaly, blindness during childhood, cataracts and iris atrophy.Occasionally, some patients present interventricular septal defects and mental retardation.This disease is conditioned by mutations of the LRP5 gene, located on chromosome 11q13.4and that encodes the lowdensity lipoprotein receptor-related protein 5 (LRP5).It was initially believed that this entity was another variant of osteogenesis imperfecta (OI) but the study of collagen in patients with OPPG established that this protein was normal and the hypothesis was discarded; however, this is still the most relevant differential diagnosis [36][37][38][39][40][41].

Neuromuscular disorders
Muscular dystrophies, peripheral neuropathies and muscle atrophies of hereditary origin, represent broad groups of diseases that aside from their characteristic clinical stigmata, can be associated with osteoporosis as one of their complications.As the disease progresses in these patients, there is increased difficulty and limitation in walking and periods of immobility become progressively more prolonged leading to the gradual loss of the mechanical stimuli that bone needs to maintain its strength and hence, favoring the development of osteoporosis.As all Mendelian diseases, these neuromuscular abnormalities follow different inheritance patterns and present phenotypic variability [42][43][44].

Inborn errors of metabolism
This group of genetic diseases encompasses a great number of inborn defects with repercussions in several aspects of carbohydrate, amino acid, protein, vitamin, mineral, complex molecule, neurotransmitter and energy metabolism.The genetic basis of most of these entities hinges on gene mutations encoding proteins, particularly enzymes, leading to partial or complete blockade of one or several metabolic processes.In these diseases, symptoms arise for different reasons, including: a deficit of the products generated by the compromised enzymatic reaction, accumulation of the precursor immediate to the defect, an increase in alternative products due to increased activation of alternate metabolic pathways or inhibition of these alternate pathways due to the accumulated substrate.In most cases, inheritance of these diseases is autosomal recessive and less frequently, X-linked recessive.
In cases of metabolic errors, osteoporosis tends to develop for different reasons: in some cases, it is secondary to nutritional deficiencies, progressive neurologic or muscular impairment or as a consequence of the therapeutic measures taken in the management of the primary disease: their secondary effects directly compromise bone quality (steroids, antiseizure drugs, etc.).The number of monogenic diseases whose phenotype may include osteoporosis is large and are shown in Tables 3-5, according to their Mendelian inheritance pattern [45][46][47][48][49][50][51][52][53][54][55][56].

Genetic diseases of chromosomal origin and osteoporosis
Within the different categories of genetic diseases, we can include numeric or structural chromosomal abnormalities.Two of the most common chromosomal diseases are Turner's syndrome and Klinefelter's syndrome, both associated to X chromosome aneuploidy; in the first case, there is complete or partial absence of an X chromosome and less frequently, it can be caused by structural anomalies in the short arms of the X chromosome.In Klinefelter's syndrome, there is an additional X chromosome and occasionally, there may be more than one extra X chromosome.In both syndromes, the phenotypic spectrum includes gonadal dysgenesis, in Turner's syndrome there are fibrous bands instead of ovaries and in Klinefelter's, the testicles are hypoplastic, leading in both cases to hypogonadism and a partial or complete deficit in the sex hormones that would normally be produced by the ovaries and testicles.Due to their lack, the development of normal secondary sexual characteristics is stunted and the various metabolic processes dependent on the hormones are also compromised.One of these metabolic processes occurs in bone [259][260][261][262].
Undoubtedly, bone metabolism is complex and the processes of osteoblastogenesis, osteoclastogenesis and remodeling must occur in a balanced manner; it is important to mention that the entire family of steroid hormone receptors (estrogen, androgen, vitamin D and retinoids), are expressed in bone, both in osteoblasts and osteoclasts as well as in chondrocytes.Within this microenvironment, the action of these hormones on their receptors is key to appropriate skeletal development; as a matter of fact, individuals with genetic mutations encoding any of these receptors develop, among other manifestations, bad quality bone mass.These hormones and their receptors play a pivotal role in female and male bone growth and may also favor epiphyseal closure at the end of the growth period.It is known that one of effects of steroid hormones on bone metabolism is resorption inhibition since they promote osteoclast apoptosis and decrease the frequency of remodeling unit activation.Therefore, the integral treatment of both entities includes hormone replacement that to a certain extent, will improve bone mass and will prevent or delay the development of osteoporosis [263,264].

Conclusion
Bone metabolism and the large amount of processes that it involves, such as osteoblastogenesis, osteoclastogenesis and bone remodeling, must be kept in constant balance.Each one of these aspects of the physiology of bone shows a particular gene expression patterns, which may even differ according to conditions and tissue needs.As previously mentioned the number of genes involved is very large and sometimes their expression might be modified by multiple environmental conditions.It is important to mention that the expression of these genes is ubiquitous and is not restricted to the bone tissue, which explains why the phenotypic characteristics of a large number of monogenic and some polygenic entities include alterations on bone mineral density and on the microarchitecture of this tissue; this includes several degrees of osteopenia,osteoporosis or increased bone mineral density.Even a good number of these genes have been identified through the study of human disease whose phenotype includes altered bone mineral density.Without a doubt, the investigation of several processes that regulate bone metabolism will continue generating new knowledge that will allow better understanding of bone physiology and physiopathology of multiple diseases and possibly new therapeutic options in diseases which compromise the quality and function of the bone.

Table 3 .
Autosomal dominant diseases with bone mineral density loss.

Table 4 .
Autosomal recessive diseases with bone mineral density loss.

Table 5 .
X-linked recessive diseases with bone mineral density loss.