*Review* **Emerging Gene-Editing Modalities for Osteoarthritis**

**Alekya S. Tanikella 1, Makenna J. Hardy 1,2,3, Stephanie M. Frahs 1,2,3, Aidan G. Cormier 4, Kalin D. Gibbons 4, Clare K. Fitzpatrick <sup>4</sup> and Julia Thom Oxford 1,2,3,\***


Received: 11 July 2020; Accepted: 19 August 2020; Published: 22 August 2020

**Abstract:** Osteoarthritis (OA) is a pathological degenerative condition of the joints that is widely prevalent worldwide, resulting in significant pain, disability, and impaired quality of life. The diverse etiology and pathogenesis of OA can explain the paucity of viable preventive and disease-modifying strategies to counter it. Advances in genome-editing techniques may improve disease-modifying solutions by addressing inherited predisposing risk factors and the activity of inflammatory modulators. Recent progress on technologies such as CRISPR/Cas9 and cell-based genome-editing therapies targeting the genetic and epigenetic alternations in OA offer promising avenues for early diagnosis and the development of personalized therapies. The purpose of this literature review was to concisely summarize the genome-editing options against chronic degenerative joint conditions such as OA with a focus on the more recently emerging modalities, especially CRISPR/Cas9. Future advancements in novel genome-editing therapies may improve the efficacy of such targeted treatments.

**Keywords:** osteoarthritis; CRISPR/Cas9; miRNA; genome editing

#### **1. Introduction**

Osteoarthritis (OA) is a chronic disease affecting the joints of the body, especially the weight-bearing joints, chiefly the knees, hips, shoulders, and spine. OA is the most common type of degenerative arthritis, affecting over 30 million Americans [1]. It is also the second most expensive disease to treat in the US, costing \$139.8 billion in 2013 [2]. As the fifth leading cause of disability in the US, OA has a significant impact on the quality of life.

In 2017, the Centers for Disease Control (CDC) published results on the prevalence of doctor-diagnosed arthritis from 2013 to 2015, including 54.4 million adults, nearly half of whom had activity limitations attributable to arthritis [3]. More alarmingly, as our US population ages, the number of cases is projected to increase to 78.4 million, or 25.9% of the population, by 2040 [4]. According to the World Health Organization (WHO), the United Nations has categorized OA as a priority disease in need of research into potential therapies. Given that between 2015 and 2050, the proportion of the world's population over 60 years old will nearly double from 12% to 22%, an estimated 130 million people will suffer from OA worldwide (WHO, 2018).

The progression of OA includes the degradation of the hyaline articular cartilage at the ends of the articulating bones of the synovial joints. The articular cartilage, when healthy, functions to resist compression, prevent bone–bone contact, and maintain a low-friction surface [5]. Damaged joint articular cartilage, on the other hand, cannot perform the necessary functions [6]. Additionally, the repair of the damaged articular cartilage is a challenging issue, due in part to its avascular nature and its limited ability to heal by itself [5–7]. This often results in the formation of fibrocartilage, which lacks the ideal biomechanical characteristics needed to withstand the compressive stress imposed on the synovial joints during articulation and load-bearing [8,9].

Normally, synovial joints in the body move effortlessly, gliding across each other [10]. This is due in part to the unique properties of the synovial fluid secreted by the cells of the inner lining of the joint capsule and its ability to reduce the friction that is generated by movement at the joint surfaces [5,10,11]. The articular hyaline cartilage at the joint surfaces maintains the proper structure, function, and stability of the synovial joints [10]. Unfortunately, chondrocytes have limited potential for replication, a factor that contributes to the limited intrinsic healing capacity of cartilage in response to injury. Chondrocyte survival depends on an optimal biochemical and mechanical environment [5,12], Localized stress, wear, and tear lead to degeneration of the cartilage and may also lead to the activation of osteoblasts, resulting in new anomalous bone formation [13,14] (Figure 1). Opportunities for prevention or slowing the progression of degradation may exist if the disease is detected during its early stages. If left untreated, however, OA can lead to complete loss of joint cartilage, severe pain, restricted range of joint and limb movement, and altered reshaping of the bones within the joint [15–17].

**Figure 1.** Articular joint structure. (**a**) Bone and cartilage of a healthy tibiofemoral joint (**b**) Simulation of cartilage degeneration and bone spur formation in an osteoarthritic knee.

OA, characterized by joint stiffness, tenderness, swelling, and pain, is not a single disease per se [14]. It is, rather, a complex disease consisting of a group of conditions with multiple pathways that all result ultimately in progressive, irreversible joint failure as the outcome. Many susceptibility factors play a part in the pathogenesis of OA, such as age, gender, and weight [18–20]. Old age, female gender, repetitive joint stress, injury, joint laxity, high body weight, and osteoporosis all contribute to OA [21,22]. With the increasing life span of the population in addition to the epidemic of obesity, the number of people affected by joint degeneration is expected to increase substantially [23–25]. Severe OA can prevent a person from being able to work, especially if their work involves putting stress on a specific joint for an extended duration. OA can result in significant physical, mental, emotional, and social challenges as well [26–28].

Despite nonsurgical and surgical interventions, there is currently neither a cure for this disease nor a means to halt its inevitable progression [22,29]. Autologous chondrocyte transplantation has shown promise in clinical treatment, however, the process involves the harvest, culture, and transplant of cells grown in a monolayer (2D culture) [30–33]. Under these culture conditions, the risk of dedifferentiation of the chondrocytes and an altered phenotype is a major concern of tissue engineering [34].
